DWARF Extensions For Heterogeneous Debugging¶

Warning

This document describes provisional extensions to DWARF Version 5 [DWARF] to support heterogeneous debugging. It is not currently fully implemented and is subject to change.

1. Introduction ¶

AMD [AMD] has been working on supporting heterogeneous computing. A heterogeneous computing program can be written in a high level language such as C++ or Fortran with OpenMP pragmas, OpenCL, or HIP (a portable C++ programming environment for heterogeneous computing [HIP]). A heterogeneous compiler and runtime allows a program to execute on multiple devices within the same native process. Devices could include CPUs, GPUs, DSPs, FPGAs, or other special purpose accelerators. Currently HIP programs execute on systems with CPUs and GPUs.

The AMD [AMD] ROCm platform [AMD-ROCm] is an implementation of the industry standard for heterogeneous computing devices defined by the Heterogeneous System Architecture (HSA) Foundation [HSA]. It is open sourced and includes contributions to open source projects such as LLVM [LLVM] for compilation and GDB for debugging [GDB].

The LLVM compiler has upstream support for commercially available AMD GPU hardware (AMDGPU) [AMDGPU-LLVM]. The open source ROCgdb [AMD-ROCgdb] GDB based debugger also has support for AMDGPU which is being upstreamed. Support for AMDGPU is also being added by third parties to the GCC [GCC] compiler and the Perforce TotalView HPC Debugger [Perforce-TotalView].

To support debugging heterogeneous programs several features that are not provided by current DWARF Version 5 [DWARF] have been identified. The 2. Extensions section gives an overview of the extensions devised to address the missing features. The extensions seek to be general in nature and backwards compatible with DWARF Version 5. Their goal is to be applicable to meeting the needs of any heterogeneous system and not be vendor or architecture specific. That is followed by appendix A. Changes Relative to DWARF Version 5 which contains the textual changes for the extensions relative to the DWARF Version 5 standard. There are a number of notes included that raise open questions, or provide alternative approaches that may be worth considering. Then appendix C. Further Examples links to the AMD GPU specific usage of the extensions that includes an example. Finally, appendix D. References provides references to further information.

2. Extensions ¶

The extensions continue to evolve through collaboration with many individuals and active prototyping within the GDB debugger and LLVM compiler. Input has also been very much appreciated from the developers working on the Perforce TotalView HPC Debugger and GCC compiler.

The inputs provided and insights gained so far have been incorporated into this current version. The plan is to participate in upstreaming the work and addressing any feedback. If there is general interest then some or all of these extensions could be submitted as future DWARF standard proposals.

The general principles in designing the extensions have been:

Be backwards compatible with the DWARF Version 5 [DWARF] standard.
Be vendor and architecture neutral. They are intended to apply to other heterogeneous hardware devices including GPUs, DSPs, FPGAs, and other specialized hardware. These collectively include similar characteristics and requirements as AMDGPU devices.
Provide improved optimization support for non-GPU code. For example, some extensions apply to traditional CPU hardware that supports large vector registers. Compilers can map source languages, and source language extensions, that describe large scale parallel execution, onto the lanes of the vector registers. This is common in programming languages used in ML and HPC.
Fully define well-formed DWARF in a consistent style based on the DWARF Version 5 specification.

It is possible that some of the generalizations may also benefit other DWARF issues that have been raised.

The remainder of this section enumerates the extensions and provides motivation for each in terms of heterogeneous debugging.

2.1 Allow Location Description on the DWARF Expression Stack ¶

DWARF Version 5 does not allow location descriptions to be entries on the DWARF expression stack. They can only be the final result of the evaluation of a DWARF expression. However, by allowing a location description to be a first-class entry on the DWARF expression stack it becomes possible to compose expressions containing both values and location descriptions naturally. It allows objects to be located in any kind of memory address space, in registers, be implicit values, be undefined, or a composite of any of these.

By extending DWARF carefully, all existing DWARF expressions can retain their current semantic meaning. DWARF has implicit conversions that convert from a value that represents an address in the default address space to a memory location description. This can be extended to allow a default address space memory location description to be implicitly converted back to its address value. This allows all DWARF Version 5 expressions to retain their same meaning, while enabling the ability to explicitly create memory location descriptions in non-default address spaces and generalizing the power of composite location descriptions to any kind of location description.

For those familiar with the definition of location descriptions in DWARF Version 5, the definitions in these extensions are presented differently, but does in fact define the same concept with the same fundamental semantics. However, it does so in a way that allows the concept to extend to support address spaces, bit addressing, the ability for composite location descriptions to be composed of any kind of location description, and the ability to support objects located at multiple places. Collectively these changes expand the set of architectures that can be supported and improves support for optimized code.

Several approaches were considered, and the one presented, together with the extensions it enables, appears to be the simplest and cleanest one that offers the greatest improvement of DWARF’s ability to support debugging optimized GPU and non-GPU code. Examining the GDB debugger and LLVM compiler, it appears only to require modest changes as they both already have to support general use of location descriptions. It is anticipated that will also be the case for other debuggers and compilers.

GDB has been modified to evaluate DWARF Version 5 expressions with location descriptions as stack entries and with implicit conversions. All GDB tests have passed, except one that turned out to be an invalid test case by DWARF Version 5 rules. The code in GDB actually became simpler as all evaluation is done on a single stack and there was no longer a need to maintain a separate structure for the location description results. This gives confidence in backwards compatibility.

See A.2.5 DWARF Expressions and nested sections.

This extension is separately described at Allow Location Descriptions on the DWARF Expression Stack [AMDGPU-DWARF-LOC].

2.2 Generalize CFI to Allow Any Location Description Kind ¶

CFI describes restoring callee saved registers that are spilled. Currently CFI only allows a location description that is a register, memory address, or implicit location description. AMDGPU optimized code may spill scalar registers into portions of vector registers. This requires extending CFI to allow any location description kind to be supported.

See A.6.4 Call Frame Information.

2.3 Generalize DWARF Operation Expressions to Support Multiple Places ¶

In DWARF Version 5 a location description is defined as a single location description or a location list. A location list is defined as either effectively an undefined location description or as one or more single location descriptions to describe an object with multiple places.

With 2.1 Allow Location Description on the DWARF Expression Stack, the DW_OP_push_object_address and DW_OP_call* operations can put a location description on the stack. Furthermore, debugger information entry attributes such as DW_AT_data_member_location, DW_AT_use_location, and DW_AT_vtable_elem_location are defined as pushing a location description on the expression stack before evaluating the expression.

DWARF Version 5 only allows the stack to contain values and so only a single memory address can be on the stack. This makes these operations and attributes incapable of handling location descriptions with multiple places, or places other than memory.

Since 2.1 Allow Location Description on the DWARF Expression Stack allows the stack to contain location descriptions, the operations are generalized to support location descriptions that can have multiple places. This is backwards compatible with DWARF Version 5 and allows objects with multiple places to be supported. For example, the expression that describes how to access the field of an object can be evaluated with a location description that has multiple places and will result in a location description with multiple places.

With this change, the separate DWARF Version 5 sections that described DWARF expressions and location lists are unified into a single section that describes DWARF expressions in general. This unification is a natural consequence of, and a necessity of, allowing location descriptions to be part of the evaluation stack.

See A.2.5.3 DWARF Location Description.

2.4 Generalize Offsetting of Location Descriptions ¶

The DW_OP_plus and DW_OP_minus operations can be defined to operate on a memory location description in the default target architecture specific address space and a generic type value to produce an updated memory location description. This allows them to continue to be used to offset an address.

To generalize offsetting to any location description, including location descriptions that describe when bytes are in registers, are implicit, or a composite of these, the DW_OP_LLVM_offset, DW_OP_LLVM_offset_uconst, and DW_OP_LLVM_bit_offset offset operations are added.

The offset operations can operate on location storage of any size. For example, implicit location storage could be any number of bits in size. It is simpler to define offsets that exceed the size of the location storage as being an evaluation error, than having to force an implementation to support potentially infinite precision offsets to allow it to correctly track a series of positive and negative offsets that may transiently overflow or underflow, but end up in range. This is simple for the arithmetic operations as they are defined in terms of two’s complement arithmetic on a base type of a fixed size. Therefore, the offset operation define that integer overflow is ill-formed. This is in contrast to the DW_OP_plus, DW_OP_plus_uconst, and DW_OP_minus arithmetic operations which define that it causes wrap-around.

Having the offset operations allows DW_OP_push_object_address to push a location description that may be in a register, or be an implicit value. The DWARF expression of DW_TAG_ptr_to_member_type can use the offset operations without regard to what kind of location description was pushed.

Since 2.1 Allow Location Description on the DWARF Expression Stack has generalized location storage to be bit indexable, DW_OP_LLVM_bit_offset generalizes DWARF to work with bit fields. This is generally not possible in DWARF Version 5.

The DW_OP_*piece operations only allow literal indices. A way to use a computed offset of an arbitrary location description (such as a vector register) is required. The offset operations provide this ability since they can be used to compute a location description on the stack.

It could be possible to define DW_OP_plus, DW_OP_plus_uconst, and DW_OP_minus to operate on location descriptions to avoid needing DW_OP_LLVM_offset and DW_OP_LLVM_offset_uconst. However, this is not proposed since currently the arithmetic operations are defined to require values of the same base type and produces a result with the same base type. Allowing these operations to act on location descriptions would permit the first operand to be a location description and the second operand to be an integral value type, or vice versa, and return a location description. This complicates the rules for implicit conversions between default address space memory location descriptions and generic base type values. Currently the rules would convert such a location description to the memory address value and then perform two’s compliment wrap around arithmetic. If the result was used as a location description, it would be implicitly converted back to a default address space memory location description. This is different to the overflow rules on location descriptions. To allow control, an operation that converts a memory location description to an address integral type value would be required. Keeping a separation of location description operations and arithmetic operations avoids this semantic complexity.

See DW_OP_LLVM_offset, DW_OP_LLVM_offset_uconst, and DW_OP_LLVM_bit_offset in A.2.5.4.4.1 General Location Description Operations.

2.5 Generalize Creation of Undefined Location Descriptions ¶

Current DWARF uses an empty expression to indicate an undefined location description. Since 2.1 Allow Location Description on the DWARF Expression Stack allows location descriptions to be created on the stack, it is necessary to have an explicit way to specify an undefined location description.

For example, the DW_OP_LLVM_select_bit_piece (see 2.13 Support for Divergent Control Flow of SIMT Hardware) operation takes more than one location description on the stack. Without this ability, it is not possible to specify that a particular one of the input location descriptions is undefined.

See the DW_OP_LLVM_undefined operation in A.2.5.4.4.2 Undefined Location Description Operations.

2.6 Generalize Creation of Composite Location Descriptions ¶

To allow composition of composite location descriptions, an explicit operation that indicates the end of the definition of a composite location description is required. This can be implied if the end of a DWARF expression is reached, allowing current DWARF expressions to remain legal.

See DW_OP_LLVM_piece_end in A.2.5.4.4.6 Composite Location Description Operations.

2.7 Generalize DWARF Base Objects to Allow Any Location Description Kind ¶

The number of registers and the cost of memory operations is much higher for AMDGPU than a typical CPU. The compiler attempts to optimize whole variables and arrays into registers.

Currently DWARF only allows DW_OP_push_object_address and related operations to work with a global memory location. To support AMDGPU optimized code it is required to generalize DWARF to allow any location description to be used. This allows registers, or composite location descriptions that may be a mixture of memory, registers, or even implicit values.

See DW_OP_push_object_address in A.2.5.4.4.1 General Location Description Operations.

2.8 General Support for Address Spaces ¶

AMDGPU needs to be able to describe addresses that are in different kinds of memory. Optimized code may need to describe a variable that resides in pieces that are in different kinds of storage which may include parts of registers, memory that is in a mixture of memory kinds, implicit values, or be undefined.

DWARF has the concept of segment addresses. However, the segment cannot be specified within a DWARF expression, which is only able to specify the offset portion of a segment address. The segment index is only provided by the entity that specifies the DWARF expression. Therefore, the segment index is a property that can only be put on complete objects, such as a variable. That makes it only suitable for describing an entity (such as variable or subprogram code) that is in a single kind of memory.

AMDGPU uses multiple address spaces. For example, a variable may be allocated in a register that is partially spilled to the call stack which is in the private address space, and partially spilled to the local address space. DWARF mentions address spaces, for example as an argument to the DW_OP_xderef* operations. A new section that defines address spaces is added (see A.2.13 Address Spaces).

A new attribute DW_AT_LLVM_address_space is added to pointer and reference types (see A.5.3 Type Modifier Entries). This allows the compiler to specify which address space is being used to represent the pointer or reference type.

DWARF uses the concept of an address in many expression operations but does not define how it relates to address spaces. For example, DW_OP_push_object_address pushes the address of an object. Other contexts implicitly push an address on the stack before evaluating an expression. For example, the DW_AT_use_location attribute of the DW_TAG_ptr_to_member_type. The expression belongs to a source language type which may apply to objects allocated in different kinds of storage. Therefore, it is desirable that the expression that uses the address can do so without regard to what kind of storage it specifies, including the address space of a memory location description. For example, a pointer to member value may want to be applied to an object that may reside in any address space.

The DWARF DW_OP_xderef* operations allow a value to be converted into an address of a specified address space which is then read. But it provides no way to create a memory location description for an address in the non-default address space. For example, AMDGPU variables can be allocated in the local address space at a fixed address.

The DW_OP_LLVM_form_aspace_address (see A.2.5.4.4.3 Memory Location Description Operations) operation is defined to create a memory location description from an address and address space. If can be used to specify the location of a variable that is allocated in a specific address space. This allows the size of addresses in an address space to be larger than the generic type. It also allows a consumer great implementation freedom. It allows the implicit conversion back to a value to be limited only to the default address space to maintain compatibility with DWARF Version 5. For other address spaces the producer can use the new operations that explicitly specify the address space.

In contrast, if the DW_OP_LLVM_form_aspace_address operation had been defined to produce a value, and an implicit conversion to a memory location description was defined, then it would be limited to the size of the generic type (which matches the size of the default address space). An implementation would likely have to use reserved ranges of value to represent different address spaces. Such a value would likely not match any address value in the actual hardware. That would require the consumer to have special treatment for such values.

DW_OP_breg* treats the register as containing an address in the default address space. A DW_OP_LLVM_aspace_bregx (see A.2.5.4.4.3 Memory Location Description Operations) operation is added to allow the address space of the address held in a register to be specified.

Similarly, DW_OP_implicit_pointer treats its implicit pointer value as being in the default address space. A DW_OP_LLVM_aspace_implicit_pointer (A.2.5.4.4.5 Implicit Location Description Operations) operation is added to allow the address space to be specified.

Almost all uses of addresses in DWARF are limited to defining location descriptions, or to be dereferenced to read memory. The exception is DW_CFA_val_offset which uses the address to set the value of a register. In order to support address spaces, the CFA DWARF expression is defined to be a memory location description. This allows it to specify an address space which is used to convert the offset address back to an address in that address space. See A.6.4 Call Frame Information.

This approach of extending memory location descriptions to support address spaces, allows all existing DWARF Version 5 expressions to have the identical semantics. It allows the compiler to explicitly specify the address space it is using. For example, a compiler could choose to access private memory in a swizzled manner when mapping a source language thread to the lane of a wavefront in a SIMT manner. Or a compiler could choose to access it in an unswizzled manner if mapping the same language with the wavefront being the thread.

It also allows the compiler to mix the address space it uses to access private memory. For example, for SIMT it can still spill entire vector registers in an unswizzled manner, while using a swizzled private memory for SIMT variable access.

This approach also allows memory location descriptions for different address spaces to be combined using the regular DW_OP_*piece operations.

Location descriptions are an abstraction of storage. They give freedom to the consumer on how to implement them. They allow the address space to encode lane information so they can be used to read memory with only the memory location description and no extra information. The same set of operations can operate on locations independent of their kind of storage. The DW_OP_deref* therefore can be used on any storage kind, including memory location descriptions of different address spaces. Therefore, the DW_OP_xderef* operations are unnecessary, except to become a more compact way to encode a non-default address space address followed by dereferencing it. See A.2.5.4.3.4 Special Value Operations.

2.9 Support for Vector Base Types ¶

The vector registers of the AMDGPU are represented as their full wavefront size, meaning the wavefront size times the dword size. This reflects the actual hardware and allows the compiler to generate DWARF for languages that map a thread to the complete wavefront. It also allows more efficient DWARF to be generated to describe the CFI as only a single expression is required for the whole vector register, rather than a separate expression for each lane’s dword of the vector register. It also allows the compiler to produce DWARF that indexes the vector register if it spills scalar registers into portions of a vector register.

Since DWARF stack value entries have a base type and AMDGPU registers are a vector of dwords, the ability to specify that a base type is a vector is required.

See DW_AT_LLVM_vector_size in A.5.1 Base Type Entries.

2.10 DWARF Operations to Create Vector Composite Location Descriptions ¶

AMDGPU optimized code may spill vector registers to non-global address space memory, and this spilling may be done only for SIMT lanes that are active on entry to the subprogram. To support this the CFI rule for the partially spilled register needs to use an expression that uses the EXEC register as a bit mask to select between the register (for inactive lanes) and the stack spill location (for active lanes that are spilled). This needs to evaluate to a location description, and not a value, as a debugger needs to change the value if the user assigns to the variable.

Another usage is to create an expression that evaluates to provide a vector of logical PCs for active and inactive lanes in a SIMT execution model. Again the EXEC register is used to select between active and inactive PC values. In order to represent a vector of PC values, a way to create a composite location description that is a vector of a single location is used.

It may be possible to use existing DWARF to incrementally build the composite location description, possibly using the DWARF operations for control flow to create a loop. However, for the AMDGPU that would require loop iteration of 64. A concern is that the resulting DWARF would have a significant size and would be reasonably common as it is needed for every vector register that is spilled in a function. AMDGPU can have up to 512 vector registers. Another concern is the time taken to evaluate such non-trivial expressions repeatedly.

To avoid these issues, a composite location description that can be created as a masked select is proposed. In addition, an operation that creates a composite location description that is a vector on another location description is needed. These operations generate the composite location description using a single DWARF operation that combines all lanes of the vector in one step. The DWARF expression is more compact, and can be evaluated by a consumer far more efficiently.

An example that uses these operations is referenced in the C. Further Examples appendix.

See DW_OP_LLVM_select_bit_piece and DW_OP_LLVM_extend in A.2.5.4.4.6 Composite Location Description Operations.

2.11 DWARF Operation to Access Call Frame Entry Registers ¶

As described in 2.10 DWARF Operations to Create Vector Composite Location Descriptions, a DWARF expression involving the set of SIMT lanes active on entry to a subprogram is required. The SIMT active lane mask may be held in a register that is modified as the subprogram executes. However, its value may be saved on entry to the subprogram.

The Call Frame Information (CFI) already encodes such register saving, so it is more efficient to provide an operation to return the location of a saved register than have to generate a loclist to describe the same information. This is now possible since 2.1 Allow Location Description on the DWARF Expression Stack allows location descriptions on the stack.

See DW_OP_LLVM_call_frame_entry_reg in A.2.5.4.4.1 General Location Description Operations and A.6.4 Call Frame Information.

2.12 Support for Source Languages Mapped to SIMT Hardware ¶

If the source language is mapped onto the AMDGPU wavefronts in a SIMT manner, then the variable DWARF location expressions must compute the location for a single lane of the wavefront. Therefore, a DWARF operation is required to denote the current lane, much like DW_OP_push_object_address denotes the current object. See DW_OP_LLVM_push_lane in A.2.5.4.3.1 Literal Operations.

In addition, a way is needed for the compiler to communicate how many source language threads of execution are mapped to a target architecture thread’s SIMT lanes. See DW_AT_LLVM_lanes in A.3.3.5 Low-Level Information.

2.13 Support for Divergent Control Flow of SIMT Hardware ¶

If the source language is mapped onto the AMDGPU wavefronts in a SIMT manner the compiler can use the AMDGPU execution mask register to control which lanes are active. To describe the conceptual location of non-active lanes requires an attribute that has an expression that computes the source location PC for each lane.

For efficiency, the expression calculates the source location the wavefront as a whole. This can be done using the DW_OP_LLVM_select_bit_piece (see 2.10 DWARF Operations to Create Vector Composite Location Descriptions) operation.

The AMDGPU may update the execution mask to perform whole wavefront operations. Therefore, there is a need for an attribute that computes the current active lane mask. This can have an expression that may evaluate to the SIMT active lane mask register or to a saved mask when in whole wavefront execution mode.

An example that uses these attributes is referenced in the C. Further Examples appendix.

See DW_AT_LLVM_lane_pc and DW_AT_LLVM_active_lane in A.2.5.4.4.6 Composite Location Description Operations.

2.14 Define Source Language Memory Classes ¶

AMDGPU supports languages, such as OpenCL [OpenCL], that define source language memory classes. Support is added to define language specific memory spaces so they can be used in a consistent way by consumers.

Support for using memory spaces in defining source language types and data object allocation is also added.

See A.2.14 Memory Spaces.

2.15 Define Augmentation Strings to Support Multiple Extensions ¶

A DW_AT_LLVM_augmentation attribute is added to a compilation unit debugger information entry to indicate that there is additional target architecture specific information in the debugging information entries of that compilation unit. This allows a consumer to know what extensions are present in the debugger information entries as is possible with the augmentation string of other sections. See .

The format that should be used for an augmentation string is also recommended. This allows a consumer to parse the string when it contains information from multiple vendors. Augmentation strings occur in the DW_AT_LLVM_augmentation attribute, in the lookup by name table, and in the CFI Common Information Entry (CIE).

See A.3.1.1 Full and Partial Compilation Unit Entries, A.6.1.1.4.1 Section Header, and A.6.4.1 Structure of Call Frame Information.

2.16 Support Embedding Source Text for Online Compilation ¶

AMDGPU supports programming languages that include online compilation where the source text may be created at runtime. For example, the OpenCL and HIP language runtimes support online compilation. To support is, a way to embed the source text in the debug information is provided.

See A.6.2 Line Number Information.

2.17 Allow MD5 Checksums to be Optionally Present ¶

In DWARF Version 5 the file timestamp and file size can be optional, but if the MD5 checksum is present it must be valid for all files. This is a problem if using link time optimization to combine compilation units where some have MD5 checksums and some do not. Therefore, sSupport to allow MD5 checksums to be optionally present in the line table is added.

See A.6.2 Line Number Information.

2.18 Add the HIP Programing Language ¶

The HIP programming language [HIP], which is supported by the AMDGPU, is added.

See Language Names.

2.19 Support for Source Language Optimizations that Result in Concurrent Iteration Execution ¶

A compiler can perform loop optimizations that result in the generated code executing multiple iterations concurrently. For example, software pipelining schedules multiple iterations in an interleaved fashion to allow the instructions of one iteration to hide the latencies of the instructions of another iteration. Another example is vectorization that can exploit SIMD hardware to allow a single instruction to execute multiple iterations using vector registers.

Note that although this is similar to SIMT execution, the way a client debugger uses the information is fundamentally different. In SIMT execution the debugger needs to present the concurrent execution as distinct source language threads that the user can list and switch focus between. With iteration concurrency optimizations, such as software pipelining and vectorized SIMD, the debugger must not present the concurrency as distinct source language threads. Instead, it must inform the user that multiple loop iterations are executing in parallel and allow the user to select between them.

In general, SIMT execution fixes the number of concurrent executions per target architecture thread. However, both software pipelining and SIMD vectorization may vary the number of concurrent iterations for different loops executed by a single source language thread.

It is possible for the compiler to use both SIMT concurrency and iteration concurrency techniques in the code of a single source language thread.

Therefore, a DWARF operation is required to denote the current concurrent iteration instance, much like DW_OP_push_object_address denotes the current object. See DW_OP_LLVM_push_iteration in A.2.5.4.3.1 Literal Operations.

In addition, a way is needed for the compiler to communicate how many source language loop iterations are executing concurrently. See DW_AT_LLVM_iterations in A.3.3.5 Low-Level Information.

2.20 DWARF Operation to Create Runtime Overlay Composite Location Description ¶

It is common in SIMD vectorization for the compiler to generate code that promotes portions of an array into vector registers. For example, if the hardware has vector registers with 8 elements, and 8 wide SIMD instructions, the compiler may vectorize a loop so that is executes 8 iterations concurrently for each vectorized loop iteration.

On the first iteration of the generated vectorized loop, iterations 0 to 7 of the source language loop will be executed using SIMD instructions. Then on the next iteration of the generated vectorized loop, iteration 8 to 15 will be executed, and so on.

If the source language loop accesses an array element based on the loop iteration index, the compiler may read the element into a register for the duration of that iteration. Next iteration it will read the next element into the register, and so on. With SIMD, this generalizes to the compiler reading array elements 0 to 7 into a vector register on the first vectorized loop iteration, then array elements 8 to 15 on the next iteration, and so on.

The DWARF location description for the array needs to express that all elements are in memory, except the slice that has been promoted to the vector register. The starting position of the slice is a runtime value based on the iteration index modulo the vectorization size. This cannot be expressed by DW_OP_piece and DW_OP_bit_piece which only allow constant offsets to be expressed.

Therefore, a new operator is defined that takes two location descriptions, an offset and a size, and creates a composite that effectively uses the second location description as an overlay of the first, positioned according to the offset and size. See DW_OP_LLVM_overlay and DW_OP_LLVM_bit_overlay in A.2.5.4.4.6 Composite Location Description Operations.

Consider an array that has been partially registerized such that the currently processed elements are held in registers, whereas the remainder of the array remains in memory. Consider the loop in this C function, for example:

extern void foo(uint32_t dst[], uint32_t src[], int len) {
  for (int i = 0; i < len; ++i)
    dst[i] += src[i];
}

Inside the loop body, the machine code loads src[i] and dst[i] into registers, adds them, and stores the result back into dst[i].

Considering the location of dst and src in the loop body, the elements dst[i] and src[i] would be located in registers, all other elements are located in memory. Let register R0 contain the base address of dst, register R1 contain i, and register R2 contain the registerized dst[i] element. We can describe the location of dst as a memory location with a register location overlaid at a runtime offset involving i:

// 1. Memory location description of dst elements located in memory:
DW_OP_breg0 0

// 2. Register location description of element dst[i] is located in R2:
DW_OP_reg2

// 3. Offset of the register within the memory of dst:
DW_OP_breg1 0
DW_OP_lit4
DW_OP_mul

// 4. The size of the register element:
DW_OP_lit4

// 5. Make a composite location description for dst that is the memory #1 with
//    the register #2 positioned as an overlay at offset #3 of size #4:
DW_OP_LLVM_overlay

2.21 Support for Source Language Memory Spaces ¶

AMDGPU supports languages, such as OpenCL, that define source language memory spaces. Support is added to define language specific memory spaces so they can be used in a consistent way by consumers. See A.2.14 Memory Spaces.

A new attribute DW_AT_LLVM_memory_space is added to support using memory spaces in defining source language pointer and reference types (see A.5.3 Type Modifier Entries) and data object allocation (see A.4.1 Data Object Entries).

2.22 Expression Operation Vendor Extensibility Opcode ¶

The vendor extension encoding space for DWARF expression operations accommodates only 32 unique operations. In practice, the lack of a central registry and a desire for backwards compatibility means vendor extensions are never retired, even when standard versions are accepted into DWARF proper. This has produced a situation where the effective encoding space available for new vendor extensions is miniscule today.

To expand this encoding space a new DWARF operation DW_OP_LLVM_user is added which acts as a “prefix” for vendor extensions. It is followed by a ULEB128 encoded vendor extension opcode, which is then followed by the operands of the corresponding vendor extension operation.

This approach allows all remaining operations defined in these extensions to be encoded without conflicting with existing vendor extensions.

See DW_OP_LLVM_user in A.2.5.4.0 Vendor Extension Operations.

A. Changes Relative to DWARF Version 5 ¶

Note

This appendix provides changes relative to DWARF Version 5. It has been defined such that it is backwards compatible with DWARF Version 5. Non-normative text is shown in italics. The section numbers generally correspond to those in the DWARF Version 5 standard unless specified otherwise. Definitions are given for the additional operations, as well as clarifying how existing expression operations, CFI operations, and attributes behave with respect to generalized location descriptions that support address spaces and multiple places.

The names for the new operations, attributes, and constants include “LLVM“ and are encoded with vendor specific codes so these extensions can be implemented as an LLVM vendor extension to DWARF Version 5. New operations other than DW_OP_LLVM_user are “prefixed” by DW_OP_LLVM_user to make enough encoding space available for their implementation.

Note

Notes are included to describe how the changes are to be applied to the DWARF Version 5 standard. They also describe rational and issues that may need further consideration.

A.2 General Description ¶

A.2.2 Attribute Types ¶

Note

This augments DWARF Version 5 section 2.2 and Table 2.2.

The following table provides the additional attributes.

Attribute names¶
Attribute	Usage
`DW_AT_LLVM_active_lane`	SIMT active lanes (see A.3.3.5 Low-Level Information)
`DW_AT_LLVM_augmentation`	Compilation unit augmentation string (see A.3.1.1 Full and Partial Compilation Unit Entries)
`DW_AT_LLVM_lane_pc`	SIMT lane program location (see A.3.3.5 Low-Level Information)
`DW_AT_LLVM_lanes`	SIMT lane count (see A.3.3.5 Low-Level Information)
`DW_AT_LLVM_iterations`	Concurrent iteration count (see A.3.3.5 Low-Level Information)
`DW_AT_LLVM_vector_size`	Base type vector size (see A.5.1 Base Type Entries)
`DW_AT_LLVM_address_space`	Architecture specific address space (see A.2.13 Address Spaces)
`DW_AT_LLVM_memory_space`	Pointer or reference types (see 5.3 “Type Modifier Entries”) Data objects (see 4.1 “Data Object Entries”)

A.2.5 DWARF Expressions ¶

Note

This section, and its nested sections, replaces DWARF Version 5 section 2.5 and section 2.6. The new DWARF expression operation extensions are defined as well as clarifying the extensions to already existing DWARF Version 5 operations. It is based on the text of the existing DWARF Version 5 standard.

DWARF expressions describe how to compute a value or specify a location.

The evaluation of a DWARF expression can provide the location of an object, the value of an array bound, the length of a dynamic string, the desired value itself, and so on.

If the evaluation of a DWARF expression does not encounter an error, then it can either result in a value (see A.2.5.2 DWARF Expression Value) or a location description (see A.2.5.3 DWARF Location Description). When a DWARF expression is evaluated, it may be specified whether a value or location description is required as the result kind.

If a result kind is specified, and the result of the evaluation does not match the specified result kind, then the implicit conversions described in A.2.5.4.4.3 Memory Location Description Operations are performed if valid. Otherwise, the DWARF expression is ill-formed.

If the evaluation of a DWARF expression encounters an evaluation error, then the result is an evaluation error.

Note

Decided to define the concept of an evaluation error. An alternative is to introduce an undefined value base type in a similar way to location descriptions having an undefined location description. Then operations that encounter an evaluation error can return the undefined location description or value with an undefined base type.

All operations that act on values would return an undefined entity if given an undefined value. The expression would then always evaluate to completion, and can be tested to determine if it is an undefined entity.

However, this would add considerable additional complexity and does not match that GDB throws an exception when these evaluation errors occur.

If a DWARF expression is ill-formed, then the result is undefined.

The following sections detail the rules for when a DWARF expression is ill-formed or results in an evaluation error.

A DWARF expression can either be encoded as an operation expression (see A.2.5.4 DWARF Operation Expressions), or as a location list expression (see A.2.5.5 DWARF Location List Expressions).

A.2.5.1 DWARF Expression Evaluation Context ¶

A DWARF expression is evaluated in a context that can include a number of context elements. If multiple context elements are specified then they must be self consistent or the result of the evaluation is undefined. The context elements that can be specified are:

A current result kind

The kind of result required by the DWARF expression evaluation. If specified it can be a location description or a value.

A current thread

The target architecture thread identifier. For source languages that are not implemented using a SIMT execution model, this corresponds to the source program thread of execution for which a user presented expression is currently being evaluated. For source languages that are implemented using a SIMT execution model, this together with the current lane corresponds to the source program thread of execution for which a user presented expression is currently being evaluated.

It is required for operations that are related to target architecture threads.

For example, the DW_OP_regval_type operation, or the DW_OP_form_tls_address and DW_OP_LLVM_form_aspace_address operations when given an address space that is target architecture thread specific.

A current lane

The 0 based SIMT lane identifier to be used in evaluating a user presented expression. This applies to source languages that are implemented for a target architecture using a SIMT execution model. These implementations map source language threads of execution to lanes of the target architecture threads.

It is required for operations that are related to SIMT lanes.

For example, the DW_OP_LLVM_push_lane operation and DW_OP_LLVM_form_aspace_address operation when given an address space that is SIMT lane specific.

If specified, it must be consistent with the value of the DW_AT_LLVM_lanes attribute of the subprogram corresponding to context’s frame and program location. It is consistent if the value is greater than or equal to 0 and less than the, possibly default, value of the DW_AT_LLVM_lanes attribute. Otherwise the result is undefined.

A current iteration

The 0 based source language iteration instance to be used in evaluating a user presented expression. This applies to target architectures that support optimizations that result in executing multiple source language loop iterations concurrently.

For example, software pipelining and SIMD vectorization.

It is required for operations that are related to source language loop iterations.

For example, the DW_OP_LLVM_push_iteration operation.

If specified, it must be consistent with the value of the DW_AT_LLVM_iterations attribute of the subprogram corresponding to context’s frame and program location. It is consistent if the value is greater than or equal to 0 and less than the, possibly default, value of the DW_AT_LLVM_iterations attribute. Otherwise the result is undefined.

A current call frame

The target architecture call frame identifier. It identifies a call frame that corresponds to an active invocation of a subprogram in the current thread. It is identified by its address on the call stack. The address is referred to as the Canonical Frame Address (CFA). The call frame information is used to determine the CFA for the call frames of the current thread’s call stack (see A.6.4 Call Frame Information).

It is required for operations that specify target architecture registers to support virtual unwinding of the call stack.

For example, the DW_OP_*reg* operations.

If specified, it must be an active call frame in the current thread. If the current lane is specified, then that lane must have been active on entry to the call frame (see the DW_AT_LLVM_lane_pc attribute). Otherwise the result is undefined.

If it is the currently executing call frame, then it is termed the top call frame.

A current program location

The target architecture program location corresponding to the current call frame of the current thread.

The program location of the top call frame is the target architecture program counter for the current thread. The call frame information is used to obtain the value of the return address register to determine the program location of the other call frames (see A.6.4 Call Frame Information).

It is required for the evaluation of location list expressions to select amongst multiple program location ranges. It is required for operations that specify target architecture registers to support virtual unwinding of the call stack (see A.6.4 Call Frame Information).

If specified:

If the current lane is not specified:

If the current call frame is the top call frame, it must be the current target architecture program location.

If the current call frame F is not the top call frame, it must be the program location associated with the call site in the current caller frame F that invoked the callee frame.

If the current lane is specified and the architecture program location LPC computed by the DW_AT_LLVM_lane_pc attribute for the current lane is not the undefined location description (indicating the lane was not active on entry to the call frame), it must be LPC.

Otherwise the result is undefined.

A current compilation unit

The compilation unit debug information entry that contains the DWARF expression being evaluated.

It is required for operations that reference debug information associated with the same compilation unit, including indicating if such references use the 32-bit or 64-bit DWARF format. It can also provide the default address space address size if no current target architecture is specified.

For example, the DW_OP_constx and DW_OP_addrx operations.

Note that this compilation unit may not be the same as the compilation unit determined from the loaded code object corresponding to the current program location. For example, the evaluation of the expression E associated with a DW_AT_location attribute of the debug information entry operand of the DW_OP_call* operations is evaluated with the compilation unit that contains E and not the one that contains the DW_OP_call* operation expression.

A current target architecture

The target architecture.

It is required for operations that specify target architecture specific entities.

For example, target architecture specific entities include DWARF register identifiers, DWARF lane identifiers, DWARF address space identifiers, the default address space, and the address space address sizes.

If specified:

If the current frame is specified, then the current target architecture must be the same as the target architecture of the current frame.

If the current frame is specified and is the top frame, and if the current thread is specified, then the current target architecture must be the same as the target architecture of the current thread.

If the current compilation unit is specified, then the current target architecture default address space address size must be the same as the address_size field in the header of the current compilation unit and any associated entry in the .debug_aranges section.

If the current program location is specified, then the current target architecture must be the same as the target architecture of any line number information entry (see A.6.2 Line Number Information) corresponding to the current program location.

If the current program location is specified, then the current target architecture default address space address size must be the same as the address_size field in the header of any entry corresponding to the current program location in the .debug_addr, .debug_line, .debug_rnglists, .debug_rnglists.dwo, .debug_loclists, and .debug_loclists.dwo sections.

Otherwise the result is undefined.

A current object

The location description of a program object.

It is required for the DW_OP_push_object_address operation.

For example, the DW_AT_data_location attribute on type debug information entries specifies the program object corresponding to a runtime descriptor as the current object when it evaluates its associated expression.

The result is undefined if the location description is invalid (see A.2.5.3 DWARF Location Description).

An initial stack

This is a list of values or location descriptions that will be pushed on the operation expression evaluation stack in the order provided before evaluation of an operation expression starts.

Some debugger information entries have attributes that evaluate their DWARF expression value with initial stack entries. In all other cases the initial stack is empty.

The result is undefined if any location descriptions are invalid (see A.2.5.3 DWARF Location Description).

If the evaluation requires a context element that is not specified, then the result of the evaluation is an error.

A DWARF expression for a location description may be able to be evaluated without a thread, lane, call frame, program location, or architecture context. For example, the location of a global variable may be able to be evaluated without such context. If the expression evaluates with an error then it may indicate the variable has been optimized and so requires more context.

The DWARF expression for call frame information (see A.6.4 Call Frame Information) operations are restricted to those that do not require the compilation unit context to be specified.

The DWARF is ill-formed if all the address_size fields in the headers of all the entries in the .debug_info, .debug_addr, .debug_line, .debug_rnglists, .debug_rnglists.dwo, .debug_loclists, and .debug_loclists.dwo sections corresponding to any given program location do not match.

A.2.5.2 DWARF Expression Value ¶

A value has a type and a literal value. It can represent a literal value of any supported base type of the target architecture. The base type specifies the size, encoding, and endianity of the literal value.

Note

It may be desirable to add an implicit pointer base type encoding. It would be used for the type of the value that is produced when the DW_OP_deref* operation retrieves the full contents of an implicit pointer location storage created by the DW_OP_implicit_pointer or DW_OP_LLVM_aspace_implicit_pointer operations. The literal value would record the debugging information entry and byte displacement specified by the associated DW_OP_implicit_pointer or DW_OP_LLVM_aspace_implicit_pointer operations.

There is a distinguished base type termed the generic type, which is an integral type that has the size of an address in the target architecture default address space, a target architecture defined endianity, and unspecified signedness.

The generic type is the same as the unspecified type used for stack operations defined in DWARF Version 4 and before.

An integral type is a base type that has an encoding of DW_ATE_signed, DW_ATE_signed_char, DW_ATE_unsigned, DW_ATE_unsigned_char, DW_ATE_boolean, or any target architecture defined integral encoding in the inclusive range DW_ATE_lo_user to DW_ATE_hi_user.

Note

It is unclear if DW_ATE_address is an integral type. GDB does not seem to consider it as integral.

A.2.5.3 DWARF Location Description ¶

Debugging information must provide consumers a way to find the location of program variables, determine the bounds of dynamic arrays and strings, and possibly to find the base address of a subprogram’s call frame or the return address of a subprogram. Furthermore, to meet the needs of recent computer architectures and optimization techniques, debugging information must be able to describe the location of an object whose location changes over the object’s lifetime, and may reside at multiple locations simultaneously during parts of an object’s lifetime.

Information about the location of program objects is provided by location descriptions.

Location descriptions can consist of one or more single location descriptions.

A single location description specifies the location storage that holds a program object and a position within the location storage where the program object starts. The position within the location storage is expressed as a bit offset relative to the start of the location storage.

A location storage is a linear stream of bits that can hold values. Each location storage has a size in bits and can be accessed using a zero-based bit offset. The ordering of bits within a location storage uses the bit numbering and direction conventions that are appropriate to the current language on the target architecture.

There are five kinds of location storage:

memory location storage: Corresponds to the target architecture memory address spaces.
register location storage: Corresponds to the target architecture registers.
implicit location storage: Corresponds to fixed values that can only be read.
undefined location storage: Indicates no value is available and therefore cannot be read or written.
composite location storage: Allows a mixture of these where some bits come from one location storage and some from another location storage, or from disjoint parts of the same location storage.

Note

It may be better to add an implicit pointer location storage kind used by the DW_OP_implicit_pointer and DW_OP_LLVM_aspace_implicit_pointer operations. It would specify the debugger information entry and byte offset provided by the operations.

Location descriptions are a language independent representation of addressing rules.

They can be the result of evaluating a debugger information entry attribute that specifies an operation expression of arbitrary complexity. In this usage they can describe the location of an object as long as its lifetime is either static or the same as the lexical block (see :ref:`amdgpu-dwarf-lexical-block-entries`) that owns it, and it does not move during its lifetime.
They can be the result of evaluating a debugger information entry attribute that specifies a location list expression. In this usage they can describe the location of an object that has a limited lifetime, changes its location during its lifetime, or has multiple locations over part or all of its lifetime.

If a location description has more than one single location description, the DWARF expression is ill-formed if the object value held in each single location description’s position within the associated location storage is not the same value, except for the parts of the value that are uninitialized.

A location description that has more than one single location description can only be created by a location list expression that has overlapping program location ranges, or certain expression operations that act on a location description that has more than one single location description. There are no operation expression operations that can directly create a location description with more than one single location description.

A location description with more than one single location description can be used to describe objects that reside in more than one piece of storage at the same time. An object may have more than one location as a result of optimization. For example, a value that is only read may be promoted from memory to a register for some region of code, but later code may revert to reading the value from memory as the register may be used for other purposes. For the code region where the value is in a register, any change to the object value must be made in both the register and the memory so both regions of code will read the updated value.

A consumer of a location description with more than one single location description can read the object’s value from any of the single location descriptions (since they all refer to location storage that has the same value), but must write any changed value to all the single location descriptions.

The evaluation of an expression may require context elements to create a location description. If such a location description is accessed, the storage it denotes is that associated with the context element values specified when the location description was created, which may differ from the context at the time it is accessed.

For example, creating a register location description requires the thread context: the location storage is for the specified register of that thread. Creating a memory location description for an address space may required a thread and a lane context: the location storage is the memory associated with that thread and lane.

If any of the context elements required to create a location description change, the location description becomes invalid and accessing it is undefined.

Examples of context that can invalidate a location description are:

The thread context is required and execution causes the thread to terminate.
The call frame context is required and further execution causes the call frame to return to the calling frame.
The program location is required and further execution of the thread occurs. That could change the location list entry or call frame information entry that applies.
An operation uses call frame information:
- Any of the frames used in the virtual call frame unwinding return.
- The top call frame is used, the program location is used to select the call frame information entry, and further execution of the thread occurs.

A DWARF expression can be used to compute a location description for an object. A subsequent DWARF expression evaluation can be given the object location description as the object context or initial stack context to compute a component of the object. The final result is undefined if the object location description becomes invalid between the two expression evaluations.

A change of a thread’s program location may not make a location description invalid, yet may still render it as no longer meaningful. Accessing such a location description, or using it as the object context or initial stack context of an expression evaluation, may produce an undefined result.

For example, a location description may specify a register that no longer holds the intended program object after a program location change. One way to avoid such problems is to recompute location descriptions associated with threads when their program locations change.

A.2.5.4 DWARF Operation Expressions ¶

An operation expression is comprised of a stream of operations, each consisting of an opcode followed by zero or more operands. The number of operands is implied by the opcode.

Operations represent a postfix operation on a simple stack machine. Each stack entry can hold either a value or a location description. Operations can act on entries on the stack, including adding entries and removing entries. If the kind of a stack entry does not match the kind required by the operation and is not implicitly convertible to the required kind (see A.2.5.4.4.3 Memory Location Description Operations), then the DWARF operation expression is ill-formed.

Evaluation of an operation expression starts with an empty stack on which the entries from the initial stack provided by the context are pushed in the order provided. Then the operations are evaluated, starting with the first operation of the stream. Evaluation continues until either an operation has an evaluation error, or until one past the last operation of the stream is reached.

The result of the evaluation is:

If an operation has an evaluation error, or an operation evaluates an expression that has an evaluation error, then the result is an evaluation error.
If the current result kind specifies a location description, then:
- If the stack is empty, the result is a location description with one undefined location description.
  
  This rule is for backwards compatibility with DWARF Version 5 which has no explicit operation to create an undefined location description, and uses an empty operation expression for this purpose.
- If the top stack entry is a location description, or can be converted to one (see A.2.5.4.4.3 Memory Location Description Operations), then the result is that, possibly converted, location description. Any other entries on the stack are discarded.
- Otherwise the DWARF expression is ill-formed.
  
  Note
  
  Could define this case as returning an implicit location description as if the DW_OP_implicit operation is performed.
If the current result kind specifies a value, then:
- If the top stack entry is a value, or can be converted to one (see A.2.5.4.4.3 Memory Location Description Operations), then the result is that, possibly converted, value. Any other entries on the stack are discarded.
- Otherwise the DWARF expression is ill-formed.
If the current result kind is not specified, then:
- If the stack is empty, the result is a location description with one undefined location description.
  
  This rule is for backwards compatibility with DWARF Version 5 which has no explicit operation to create an undefined location description, and uses an empty operation expression for this purpose.
  
  Note
  
  This rule is consistent with the rule above for when a location description is requested. However, GDB appears to report this as an error and no GDB tests appear to cause an empty stack for this case.
- Otherwise, the top stack entry is returned. Any other entries on the stack are discarded.

An operation expression is encoded as a byte block with some form of prefix that specifies the byte count. It can be used:

as the value of a debugging information entry attribute that is encoded using class exprloc (see A.7.5.5 Classes and Forms),
as the operand to certain operation expression operations,
as the operand to certain call frame information operations (see A.6.4 Call Frame Information),
and in location list entries (see A.2.5.5 DWARF Location List Expressions).

A.2.5.4.0 Vendor Extension Operations ¶

DW_OP_LLVM_user

DW_OP_LLVM_user encodes a vendor extension operation. It has at least one operand: a ULEB128 constant identifying a vendor extension operation. The remaining operands are defined by the vendor extension. The vendor extension opcode 0 is reserved and cannot be used by any vendor extension.

The DW_OP_user encoding space can be understood to supplement the space defined by DW_OP_lo_user and DW_OP_hi_user that is allocated by the standard for the same purpose.

A.2.5.4.1 Stack Operations ¶

Note

This section replaces DWARF Version 5 section 2.5.1.3.

The following operations manipulate the DWARF stack. Operations that index the stack assume that the top of the stack (most recently added entry) has index 0. They allow the stack entries to be either a value or location description.

If any stack entry accessed by a stack operation is an incomplete composite location description (see A.2.5.4.4.6 Composite Location Description Operations), then the DWARF expression is ill-formed.

Note

These operations now support stack entries that are values and location descriptions.

Note

If it is desired to also make them work with incomplete composite location descriptions, then would need to define that the composite location storage specified by the incomplete composite location description is also replicated when a copy is pushed. This ensures that each copy of the incomplete composite location description can update the composite location storage they specify independently.

DW_OP_dup

DW_OP_dup duplicates the stack entry at the top of the stack.
DW_OP_drop

DW_OP_drop pops the stack entry at the top of the stack and discards it.
DW_OP_pick

DW_OP_pick has a single unsigned 1-byte operand that represents an index I. A copy of the stack entry with index I is pushed onto the stack.
DW_OP_over

DW_OP_over pushes a copy of the entry with index 1.

This is equivalent to a DW_OP_pick 1 operation.
DW_OP_swap

DW_OP_swap swaps the top two stack entries. The entry at the top of the stack becomes the second stack entry, and the second stack entry becomes the top of the stack.
DW_OP_rot

DW_OP_rot rotates the first three stack entries. The entry at the top of the stack becomes the third stack entry, the second entry becomes the top of the stack, and the third entry becomes the second entry.

Examples illustrating many of these stack operations are found in Appendix D.1.2 on page 289.

A.2.5.4.2 Control Flow Operations ¶

Note

This section replaces DWARF Version 5 section 2.5.1.5.

The following operations provide simple control of the flow of a DWARF operation expression.

DW_OP_nop

DW_OP_nop is a place holder. It has no effect on the DWARF stack entries.
DW_OP_le, DW_OP_ge, DW_OP_eq, DW_OP_lt, DW_OP_gt, DW_OP_ne

Note

The same as in DWARF Version 5 section 2.5.1.5.
DW_OP_skip

DW_OP_skip is an unconditional branch. Its single operand is a 2-byte signed integer constant. The 2-byte constant is the number of bytes of the DWARF expression to skip forward or backward from the current operation, beginning after the 2-byte constant.

If the updated position is at one past the end of the last operation, then the operation expression evaluation is complete.

Otherwise, the DWARF expression is ill-formed if the updated operation position is not in the range of the first to last operation inclusive, or not at the start of an operation.
DW_OP_bra

DW_OP_bra is a conditional branch. Its single operand is a 2-byte signed integer constant. This operation pops the top of stack. If the value popped is not the constant 0, the 2-byte constant operand is the number of bytes of the DWARF operation expression to skip forward or backward from the current operation, beginning after the 2-byte constant.

If the updated position is at one past the end of the last operation, then the operation expression evaluation is complete.

Otherwise, the DWARF expression is ill-formed if the updated operation position is not in the range of the first to last operation inclusive, or not at the start of an operation.
DW_OP_call2, DW_OP_call4, DW_OP_call_ref

DW_OP_call2, DW_OP_call4, and DW_OP_call_ref perform DWARF procedure calls during evaluation of a DWARF operation expression.

DW_OP_call2 and DW_OP_call4, have one operand that is, respectively, a 2-byte or 4-byte unsigned offset DR that represents the byte offset of a debugging information entry D relative to the beginning of the current compilation unit.

DW_OP_call_ref has one operand that is a 4-byte unsigned value in the 32-bit DWARF format, or an 8-byte unsigned value in the 64-bit DWARF format, that represents the byte offset DR of a debugging information entry D relative to the beginning of the .debug_info section that contains the current compilation unit. D may not be in the current compilation unit.

Note

DWARF Version 5 states that DR can be an offset in a .debug_info section other than the one that contains the current compilation unit. It states that relocation of references from one executable or shared object file to another must be performed by the consumer. But given that DR is defined as an offset in a .debug_info section this seems impossible. If DR was defined as an implementation defined value, then the consumer could choose to interpret the value in an implementation defined manner to reference a debug information in another executable or shared object.

In ELF the .debug_info section is in a non-PT_LOAD segment so standard dynamic relocations cannot be used. But even if they were loaded segments and dynamic relocations were used, DR would need to be the address of D, not an offset in a .debug_info section. That would also need DR to be the size of a global address. So it would not be possible to use the 32-bit DWARF format in a 64-bit global address space. In addition, the consumer would need to determine what executable or shared object the relocated address was in so it could determine the containing compilation unit.

GDB only interprets DR as an offset in the .debug_info section that contains the current compilation unit.

This comment also applies to DW_OP_implicit_pointer and DW_OP_LLVM_aspace_implicit_pointer.

Operand interpretation of DW_OP_call2, DW_OP_call4, and DW_OP_call_ref is exactly like that for DW_FORM_ref2, ``DW_FORM_ref4``*, and DW_FORM_ref_addr, respectively.

The call operation is evaluated by:
- If D has a DW_AT_location attribute that is encoded as a exprloc that specifies an operation expression E, then execution of the current operation expression continues from the first operation of E. Execution continues until one past the last operation of E is reached, at which point execution continues with the operation following the call operation. The operations of E are evaluated with the same current context, except current compilation unit is the one that contains D and the stack is the same as that being used by the call operation. After the call operation has been evaluated, the stack is therefore as it is left by the evaluation of the operations of E. Since E is evaluated on the same stack as the call operation, E can use, and/or remove entries already on the stack, and can add new entries to the stack.
  
  Values on the stack at the time of the call may be used as parameters by the called expression and values left on the stack by the called expression may be used as return values by prior agreement between the calling and called expressions.
- If D has a DW_AT_location attribute that is encoded as a loclist or loclistsptr, then the specified location list expression E is evaluated. The evaluation of E uses the current context, except the result kind is a location description, the compilation unit is the one that contains D, and the initial stack is empty. The location description result is pushed on the stack.
  
  Note
  
  This rule avoids having to define how to execute a matched location list entry operation expression on the same stack as the call when there are multiple matches. But it allows the call to obtain the location description for a variable or formal parameter which may use a location list expression.
  
  An alternative is to treat the case when D has a DW_AT_location attribute that is encoded as a loclist or loclistsptr, and the specified location list expression E’ matches a single location list entry with operation expression E, the same as the exprloc case and evaluate on the same stack.
  
  But this is not attractive as if the attribute is for a variable that happens to end with a non-singleton stack, it will not simply put a location description on the stack. Presumably the intent of using DW_OP_call* on a variable or formal parameter debugger information entry is to push just one location description on the stack. That location description may have more than one single location description.
  
  The previous rule for exprloc also has the same problem, as normally a variable or formal parameter location expression may leave multiple entries on the stack and only return the top entry.
  
  GDB implements DW_OP_call* by always executing E on the same stack. If the location list has multiple matching entries, it simply picks the first one and ignores the rest. This seems fundamentally at odds with the desire to support multiple places for variables.
  
  So, it feels like DW_OP_call* should both support pushing a location description on the stack for a variable or formal parameter, and also support being able to execute an operation expression on the same stack. Being able to specify a different operation expression for different program locations seems a desirable feature to retain.
  
  A solution to that is to have a distinct DW_AT_LLVM_proc attribute for the DW_TAG_dwarf_procedure debugging information entry. Then the DW_AT_location attribute expression is always executed separately and pushes a location description (that may have multiple single location descriptions), and the DW_AT_LLVM_proc attribute expression is always executed on the same stack and can leave anything on the stack.
  
  The DW_AT_LLVM_proc attribute could have the new classes exprproc, loclistproc, and loclistsptrproc to indicate that the expression is executed on the same stack. exprproc is the same encoding as exprloc. loclistproc and loclistsptrproc are the same encoding as their non-proc counterparts, except the DWARF is ill-formed if the location list does not match exactly one location list entry and a default entry is required. These forms indicate explicitly that the matched single operation expression must be executed on the same stack. This is better than ad hoc special rules for loclistproc and loclistsptrproc which are currently clearly defined to always return a location description. The producer then explicitly indicates the intent through the attribute classes.
  
  Such a change would be a breaking change for how GDB implements DW_OP_call*. However, are the breaking cases actually occurring in practice? GDB could implement the current approach for DWARF Version 5, and the new semantics for DWARF Version 6 which has been done for some other features.
  
  Another option is to limit the execution to be on the same stack only to the evaluation of an expression E that is the value of a DW_AT_location attribute of a DW_TAG_dwarf_procedure debugging information entry. The DWARF would be ill-formed if E is a location list expression that does not match exactly one location list entry. In all other cases the evaluation of an expression E that is the value of a DW_AT_location attribute would evaluate E with the current context, except the result kind is a location description, the compilation unit is the one that contains D, and the initial stack is empty. The location description result is pushed on the stack.
- If D has a DW_AT_const_value attribute with a value V, then it is as if a DW_OP_implicit_value V operation was executed.
  
  This allows a call operation to be used to compute the location description for any variable or formal parameter regardless of whether the producer has optimized it to a constant. This is consistent with the DW_OP_implicit_pointer operation.
  
  Note
  
  Alternatively, could deprecate using DW_AT_const_value for DW_TAG_variable and DW_TAG_formal_parameter debugger information entries that are constants and instead use DW_AT_location with an operation expression that results in a location description with one implicit location description. Then this rule would not be required.
- Otherwise, there is no effect and no changes are made to the stack.
  
  Note
  
  In DWARF Version 5, if D does not have a DW_AT_location then DW_OP_call* is defined to have no effect. It is unclear that this is the right definition as a producer should be able to rely on using DW_OP_call* to get a location description for any non-DW_TAG_dwarf_procedure debugging information entries. Also, the producer should not be creating DWARF with DW_OP_call* to a DW_TAG_dwarf_procedure that does not have a DW_AT_location attribute. So, should this case be defined as an ill-formed DWARF expression?
The DW_TAG_dwarf_procedure debugging information entry can be used to define DWARF procedures that can be called.

A.2.5.4.3 Value Operations ¶

This section describes the operations that push values on the stack.

Each value stack entry has a type and a literal value. It can represent a literal value of any supported base type of the target architecture. The base type specifies the size, encoding, and endianity of the literal value.

The base type of value stack entries can be the distinguished generic type.

A.2.5.4.3.1 Literal Operations ¶

Note

This section replaces DWARF Version 5 section 2.5.1.1.

The following operations all push a literal value onto the DWARF stack.

Operations other than DW_OP_const_type push a value V with the generic type. If V is larger than the generic type, then V is truncated to the generic type size and the low-order bits used.

DW_OP_lit0, DW_OP_lit1, …, DW_OP_lit31

DW_OP_lit<N> operations encode an unsigned literal value N from 0 through 31, inclusive. They push the value N with the generic type.
DW_OP_const1u, DW_OP_const2u, DW_OP_const4u, DW_OP_const8u

DW_OP_const<N>u operations have a single operand that is a 1, 2, 4, or 8-byte unsigned integer constant U, respectively. They push the value U with the generic type.
DW_OP_const1s, DW_OP_const2s, DW_OP_const4s, DW_OP_const8s

DW_OP_const<N>s operations have a single operand that is a 1, 2, 4, or 8-byte signed integer constant S, respectively. They push the value S with the generic type.
DW_OP_constu

DW_OP_constu has a single unsigned LEB128 integer operand N. It pushes the value N with the generic type.
DW_OP_consts

DW_OP_consts has a single signed LEB128 integer operand N. It pushes the value N with the generic type.
DW_OP_constx

DW_OP_constx has a single unsigned LEB128 integer operand that represents a zero-based index into the .debug_addr section relative to the value of the DW_AT_addr_base attribute of the associated compilation unit. The value N in the .debug_addr section has the size of the generic type. It pushes the value N with the generic type.

The DW_OP_constx operation is provided for constants that require link-time relocation but should not be interpreted by the consumer as a relocatable address (for example, offsets to thread-local storage).
DW_OP_const_type

DW_OP_const_type has three operands. The first is an unsigned LEB128 integer DR that represents the byte offset of a debugging information entry D relative to the beginning of the current compilation unit, that provides the type T of the constant value. The second is a 1-byte unsigned integral constant S. The third is a block of bytes B, with a length equal to S.

TS is the bit size of the type T. The least significant TS bits of B are interpreted as a value V of the type D. It pushes the value V with the type D.

The DWARF is ill-formed if D is not a DW_TAG_base_type debugging information entry in the current compilation unit, or if TS divided by 8 (the byte size) and rounded up to a whole number is not equal to S.

While the size of the byte block B can be inferred from the type D definition, it is encoded explicitly into the operation so that the operation can be parsed easily without reference to the .debug_info section.
DW_OP_LLVM_push_lane New

DW_OP_LLVM_push_lane pushes the current lane as a value with the generic type.

For source languages that are implemented using a SIMT execution model, this is the zero-based lane number that corresponds to the source language thread of execution upon which the user is focused.

The value must be greater than or equal to 0 and less than the value of the DW_AT_LLVM_lanes attribute, otherwise the DWARF expression is ill-formed. See A.3.3.5 Low-Level Information.
DW_OP_LLVM_push_iteration New

DW_OP_LLVM_push_iteration pushes the current iteration as a value with the generic type.

For source language implementations with optimizations that cause multiple loop iterations to execute concurrently, this is the zero-based iteration number that corresponds to the source language concurrent loop iteration upon which the user is focused.

The value must be greater than or equal to 0 and less than the value of the DW_AT_LLVM_iterations attribute, otherwise the DWARF expression is ill-formed. See A.3.3.5 Low-Level Information.

A.2.5.4.3.2 Arithmetic and Logical Operations ¶

Note

This section is the same as DWARF Version 5 section 2.5.1.4.

A.2.5.4.3.3 Type Conversion Operations ¶

Note

This section is the same as DWARF Version 5 section 2.5.1.6.

A.2.5.4.3.4 Special Value Operations ¶

Note

This section replaces parts of DWARF Version 5 sections 2.5.1.2, 2.5.1.3, and 2.5.1.7.

There are these special value operations currently defined:

DW_OP_regval_type

DW_OP_regval_type has two operands. The first is an unsigned LEB128 integer that represents a register number R. The second is an unsigned LEB128 integer DR that represents the byte offset of a debugging information entry D relative to the beginning of the current compilation unit, that provides the type T of the register value.

The operation is equivalent to performing DW_OP_regx R; DW_OP_deref_type DR.

Note

Should DWARF allow the type T to be a larger size than the size of the register R? Restricting a larger bit size avoids any issue of conversion as the, possibly truncated, bit contents of the register is simply interpreted as a value of T. If a conversion is wanted it can be done explicitly using a DW_OP_convert operation.

GDB has a per register hook that allows a target specific conversion on a register by register basis. It defaults to truncation of bigger registers. Removing use of the target hook does not cause any test failures in common architectures. If the compiler for a target architecture did want some form of conversion, including a larger result type, it could always explicitly use the DW_OP_convert operation.

If T is a larger type than the register size, then the default GDB register hook reads bytes from the next register (or reads out of bounds for the last register!). Removing use of the target hook does not cause any test failures in common architectures (except an illegal hand written assembly test). If a target architecture requires this behavior, these extensions allow a composite location description to be used to combine multiple registers.
DW_OP_deref

S is the bit size of the generic type divided by 8 (the byte size) and rounded up to a whole number. DR is the offset of a hypothetical debug information entry D in the current compilation unit for a base type of the generic type.

The operation is equivalent to performing DW_OP_deref_type S, DR.
DW_OP_deref_size

DW_OP_deref_size has a single 1-byte unsigned integral constant that represents a byte result size S.

TS is the smaller of the generic type bit size and S scaled by 8 (the byte size). If TS is smaller than the generic type bit size then T is an unsigned integral type of bit size TS, otherwise T is the generic type. DR is the offset of a hypothetical debug information entry D in the current compilation unit for a base type T.

Note

Truncating the value when S is larger than the generic type matches what GDB does. This allows the generic type size to not be an integral byte size. It does allow S to be arbitrarily large. Should S be restricted to the size of the generic type rounded up to a multiple of 8?

The operation is equivalent to performing DW_OP_deref_type S, DR, except if T is not the generic type, the value V pushed is zero-extended to the generic type bit size and its type changed to the generic type.
DW_OP_deref_type

DW_OP_deref_type has two operands. The first is a 1-byte unsigned integral constant S. The second is an unsigned LEB128 integer DR that represents the byte offset of a debugging information entry D relative to the beginning of the current compilation unit, that provides the type T of the result value.

TS is the bit size of the type T.

While the size of the pushed value V can be inferred from the type T, it is encoded explicitly as the operand S so that the operation can be parsed easily without reference to the .debug_info section.

Note

It is unclear why the operand S is needed. Unlike DW_OP_const_type, the size is not needed for parsing. Any evaluation needs to get the base type T to push with the value to know its encoding and bit size.

It pops one stack entry that must be a location description L.

A value V of TS bits is retrieved from the location storage LS specified by one of the single location descriptions SL of L.

If L, or the location description of any composite location description part that is a subcomponent of L, has more than one single location description, then any one of them can be selected as they are required to all have the same value. For any single location description SL, bits are retrieved from the associated storage location starting at the bit offset specified by SL. For a composite location description, the retrieved bits are the concatenation of the N bits from each composite location part PL, where N is limited to the size of PL.

V is pushed on the stack with the type T.

Note

This definition makes it an evaluation error if L is a register location description that has less than TS bits remaining in the register storage. Particularly since these extensions extend location descriptions to have a bit offset, it would be odd to define this as performing sign extension based on the type, or be target architecture dependent, as the number of remaining bits could be any number. This matches the GDB implementation for DW_OP_deref_type.

These extensions define DW_OP_*breg* in terms of DW_OP_regval_type. DW_OP_regval_type is defined in terms of DW_OP_regx, which uses a 0 bit offset, and DW_OP_deref_type. Therefore, it requires the register size to be greater or equal to the address size of the address space. This matches the GDB implementation for DW_OP_*breg*.

The DWARF is ill-formed if D is not in the current compilation unit, D is not a DW_TAG_base_type debugging information entry, or if TS divided by 8 (the byte size) and rounded up to a whole number is not equal to S.

Note

This definition allows the base type to be a bit size since there seems no reason to restrict it.

It is an evaluation error if any bit of the value is retrieved from the undefined location storage or the offset of any bit exceeds the size of the location storage LS specified by any single location description SL of L.

See A.2.5.4.4.5 Implicit Location Description Operations for special rules concerning implicit location descriptions created by the DW_OP_implicit_pointer and DW_OP_LLVM_aspace_implicit_pointer operations.
DW_OP_xderef Deprecated

DW_OP_xderef pops two stack entries. The first must be an integral type value that represents an address A. The second must be an integral type value that represents a target architecture specific address space identifier AS.

The operation is equivalent to performing DW_OP_swap; DW_OP_LLVM_form_aspace_address; DW_OP_deref. The value V retrieved is left on the stack with the generic type.

This operation is deprecated as the DW_OP_LLVM_form_aspace_address operation can be used and provides greater expressiveness.
DW_OP_xderef_size Deprecated

DW_OP_xderef_size has a single 1-byte unsigned integral constant that represents a byte result size S.

It pops two stack entries. The first must be an integral type value that represents an address A. The second must be an integral type value that represents a target architecture specific address space identifier AS.

The operation is equivalent to performing DW_OP_swap; DW_OP_LLVM_form_aspace_address; DW_OP_deref_size S. The zero-extended value V retrieved is left on the stack with the generic type.

This operation is deprecated as the DW_OP_LLVM_form_aspace_address operation can be used and provides greater expressiveness.
DW_OP_xderef_type Deprecated

DW_OP_xderef_type has two operands. The first is a 1-byte unsigned integral constant S. The second operand is an unsigned LEB128 integer DR that represents the byte offset of a debugging information entry D relative to the beginning of the current compilation unit, that provides the type T of the result value.

It pops two stack entries. The first must be an integral type value that represents an address A. The second must be an integral type value that represents a target architecture specific address space identifier AS.

The operation is equivalent to performing DW_OP_swap; DW_OP_LLVM_form_aspace_address; DW_OP_deref_type S DR. The value V retrieved is left on the stack with the type T.

This operation is deprecated as the DW_OP_LLVM_form_aspace_address operation can be used and provides greater expressiveness.
DW_OP_entry_value Deprecated

DW_OP_entry_value pushes the value of an expression that is evaluated in the context of the calling frame.

It may be used to determine the value of arguments on entry to the current call frame provided they are not clobbered.

It has two operands. The first is an unsigned LEB128 integer S. The second is a block of bytes, with a length equal S, interpreted as a DWARF operation expression E.

E is evaluated with the current context, except the result kind is unspecified, the call frame is the one that called the current frame, the program location is the call site in the calling frame, the object is unspecified, and the initial stack is empty. The calling frame information is obtained by virtually unwinding the current call frame using the call frame information (see A.6.4 Call Frame Information).

If the result of E is a location description L (see A.2.5.4.4.4 Register Location Description Operations), and the last operation executed by E is a DW_OP_reg* for register R with a target architecture specific base type of T, then the contents of the register are retrieved as if a DW_OP_deref_type DR operation was performed where DR is the offset of a hypothetical debug information entry in the current compilation unit for T. The resulting value V s pushed on the stack.

Using DW_OP_reg* provides a more compact form for the case where the value was in a register on entry to the subprogram.

Note

It is unclear how this provides a more compact expression, as DW_OP_regval_type could be used which is marginally larger.

If the result of E is a value V, then V is pushed on the stack.

Otherwise, the DWARF expression is ill-formed.

The DW_OP_entry_value operation is deprecated as its main usage is provided by other means. DWARF Version 5 added the DW_TAG_call_site_parameter debugger information entry for call sites that has DW_AT_call_value, DW_AT_call_data_location, and DW_AT_call_data_value attributes that provide DWARF expressions to compute actual parameter values at the time of the call, and requires the producer to ensure the expressions are valid to evaluate even when virtually unwound. The DW_OP_LLVM_call_frame_entry_reg operation provides access to registers in the virtually unwound calling frame.

Note

GDB only implements DW_OP_entry_value when E is exactly DW_OP_reg* or DW_OP_breg*; DW_OP_deref*.

A.2.5.4.4 Location Description Operations ¶

This section describes the operations that push location descriptions on the stack.

A.2.5.4.4.1 General Location Description Operations ¶

Note

This section replaces part of DWARF Version 5 section 2.5.1.3.

DW_OP_LLVM_offset New

DW_OP_LLVM_offset pops two stack entries. The first must be an integral type value that represents a byte displacement B. The second must be a location description L.

It adds the value of B scaled by 8 (the byte size) to the bit offset of each single location description SL of L, and pushes the updated L.

It is an evaluation error if the updated bit offset of any SL is less than 0 or greater than or equal to the size of the location storage specified by SL.
DW_OP_LLVM_offset_uconst New

DW_OP_LLVM_offset_uconst has a single unsigned LEB128 integer operand that represents a byte displacement B.

The operation is equivalent to performing DW_OP_constu B; DW_OP_LLVM_offset.

This operation is supplied specifically to be able to encode more field displacements in two bytes than can be done with DW_OP_lit*; DW_OP_LLVM_offset.

Note

Should this be named DW_OP_LLVM_offset_uconst to match DW_OP_plus_uconst, or DW_OP_LLVM_offset_constu to match DW_OP_constu?
DW_OP_LLVM_bit_offset New

DW_OP_LLVM_bit_offset pops two stack entries. The first must be an integral type value that represents a bit displacement B. The second must be a location description L.

It adds the value of B to the bit offset of each single location description SL of L, and pushes the updated L.

It is an evaluation error if the updated bit offset of any SL is less than 0 or greater than or equal to the size of the location storage specified by SL.
DW_OP_push_object_address

DW_OP_push_object_address pushes the location description L of the current object.

This object may correspond to an independent variable that is part of a user presented expression that is being evaluated. The object location description may be determined from the variable’s own debugging information entry or it may be a component of an array, structure, or class whose address has been dynamically determined by an earlier step during user expression evaluation.

This operation provides explicit functionality (especially for arrays involving descriptors) that is analogous to the implicit push of the base location description of a structure prior to evaluation of a DW_AT_data_member_location to access a data member of a structure.

Note

This operation could be removed and the object location description specified as the initial stack as for DW_AT_data_member_location.

Or this operation could be used instead of needing to specify an initial stack. The latter approach is more composable as access to the object may be needed at any point of the expression, and passing it as the initial stack requires the entire expression to be aware where on the stack it is. If this were done, DW_AT_use_location would require a DW_OP_push_object2_address operation for the second object.

Or a more general way to pass an arbitrary number of arguments in and an operation to get the Nth one such as DW_OP_arg N. A vector of arguments would then be passed in the expression context rather than an initial stack. This could also resolve the issues with DW_OP_call* by allowing a specific number of arguments passed in and returned to be specified. The DW_OP_call* operation could then always execute on a separate stack: the number of arguments would be specified in a new call operation and taken from the callers stack, and similarly the number of return results specified and copied from the called stack back to the callee stack when the called expression was complete.

The only attribute that specifies a current object is DW_AT_data_location so the non-normative text seems to overstate how this is being used. Or are there other attributes that need to state they pass an object?
DW_OP_LLVM_call_frame_entry_reg New

DW_OP_LLVM_call_frame_entry_reg has a single unsigned LEB128 integer operand that represents a target architecture register number R.

It pushes a location description L that holds the value of register R on entry to the current subprogram as defined by the call frame information (see A.6.4 Call Frame Information).

If there is no call frame information defined, then the default rules for the target architecture are used. If the register rule is undefined, then the undefined location description is pushed. If the register rule is same value, then a register location description for R is pushed.

A.2.5.4.4.2 Undefined Location Description Operations ¶

Note

This section replaces DWARF Version 5 section 2.6.1.1.1.

The undefined location storage represents a piece or all of an object that is present in the source but not in the object code (perhaps due to optimization). Neither reading nor writing to the undefined location storage is meaningful.

An undefined location description specifies the undefined location storage. There is no concept of the size of the undefined location storage, nor of a bit offset for an undefined location description. The DW_OP_LLVM_*offset operations leave an undefined location description unchanged. The DW_OP_*piece operations can explicitly or implicitly specify an undefined location description, allowing any size and offset to be specified, and results in a part with all undefined bits.

DW_OP_LLVM_undefined New

DW_OP_LLVM_undefined pushes a location description L that comprises one undefined location description SL.

A.2.5.4.4.3 Memory Location Description Operations ¶

Note

This section replaces parts of DWARF Version 5 section 2.5.1.1, 2.5.1.2, 2.5.1.3, and 2.6.1.1.2.

Each of the target architecture specific address spaces has a corresponding memory location storage that denotes the linear addressable memory of that address space. The size of each memory location storage corresponds to the range of the addresses in the corresponding address space.

It is target architecture defined how address space location storage maps to target architecture physical memory. For example, they may be independent memory, or more than one location storage may alias the same physical memory possibly at different offsets and with different interleaving. The mapping may also be dictated by the source language address classes.

A memory location description specifies a memory location storage. The bit offset corresponds to a bit position within a byte of the memory. Bits accessed using a memory location description, access the corresponding target architecture memory starting at the bit position within the byte specified by the bit offset.

A memory location description that has a bit offset that is a multiple of 8 (the byte size) is defined to be a byte address memory location description. It has a memory byte address A that is equal to the bit offset divided by 8.

A memory location description that does not have a bit offset that is a multiple of 8 (the byte size) is defined to be a bit field memory location description. It has a bit position B equal to the bit offset modulo 8, and a memory byte address A equal to the bit offset minus B that is then divided by 8.

The address space AS of a memory location description is defined to be the address space that corresponds to the memory location storage associated with the memory location description.

A location description that is comprised of one byte address memory location description SL is defined to be a memory byte address location description. It has a byte address equal to A and an address space equal to AS of the corresponding SL.

DW_ASPACE_LLVM_none is defined as the target architecture default address space. See A.2.13 Address Spaces.

If a stack entry is required to be a location description, but it is a value V with the generic type, then it is implicitly converted to a location description L with one memory location description SL. SL specifies the memory location storage that corresponds to the target architecture default address space with a bit offset equal to V scaled by 8 (the byte size).

Note

If it is wanted to allow any integral type value to be implicitly converted to a memory location description in the target architecture default address space:

If a stack entry is required to be a location description, but is a value V with an integral type, then it is implicitly converted to a location description L with a one memory location description SL. If the type size of V is less than the generic type size, then the value V is zero extended to the size of the generic type. The least significant generic type size bits are treated as an unsigned value to be used as an address A. SL specifies memory location storage corresponding to the target architecture default address space with a bit offset equal to A scaled by 8 (the byte size).

The implicit conversion could also be defined as target architecture specific. For example, GDB checks if V is an integral type. If it is not it gives an error. Otherwise, GDB zero-extends V to 64 bits. If the GDB target defines a hook function, then it is called. The target specific hook function can modify the 64-bit value, possibly sign extending based on the original value type. Finally, GDB treats the 64-bit value V as a memory location address.

If a stack entry is required to be a location description, but it is an implicit pointer value IPV with the target architecture default address space, then it is implicitly converted to a location description with one single location description specified by IPV. See A.2.5.4.4.5 Implicit Location Description Operations.

Note

Is this rule required for DWARF Version 5 backwards compatibility? If not, it can be eliminated, and the producer can use DW_OP_LLVM_form_aspace_address.

If a stack entry is required to be a value, but it is a location description L with one memory location description SL in the target architecture default address space with a bit offset B that is a multiple of 8, then it is implicitly converted to a value equal to B divided by 8 (the byte size) with the generic type.

DW_OP_addr

DW_OP_addr has a single byte constant value operand, which has the size of the generic type, that represents an address A.

It pushes a location description L with one memory location description SL on the stack. SL specifies the memory location storage corresponding to the target architecture default address space with a bit offset equal to A scaled by 8 (the byte size).

If the DWARF is part of a code object, then A may need to be relocated. For example, in the ELF code object format, A must be adjusted by the difference between the ELF segment virtual address and the virtual address at which the segment is loaded.
DW_OP_addrx

DW_OP_addrx has a single unsigned LEB128 integer operand that represents a zero-based index into the .debug_addr section relative to the value of the DW_AT_addr_base attribute of the associated compilation unit. The address value A in the .debug_addr section has the size of the generic type.

It pushes a location description L with one memory location description SL on the stack. SL specifies the memory location storage corresponding to the target architecture default address space with a bit offset equal to A scaled by 8 (the byte size).

If the DWARF is part of a code object, then A may need to be relocated. For example, in the ELF code object format, A must be adjusted by the difference between the ELF segment virtual address and the virtual address at which the segment is loaded.
DW_OP_LLVM_form_aspace_address New

DW_OP_LLVM_form_aspace_address pops top two stack entries. The first must be an integral type value that represents a target architecture specific address space identifier AS. The second must be an integral type value that represents an address A.

The address size S is defined as the address bit size of the target architecture specific address space that corresponds to AS.

A is adjusted to S bits by zero extending if necessary, and then treating the least significant S bits as an unsigned value A’.

It pushes a location description L with one memory location description SL on the stack. SL specifies the memory location storage LS that corresponds to AS with a bit offset equal to A’ scaled by 8 (the byte size).

If AS is an address space that is specific to context elements, then LS corresponds to the location storage associated with the current context.

For example, if AS is for per thread storage then LS is the location storage for the current thread. For languages that are implemented using a SIMT execution model, then if AS is for per lane storage then LS is the location storage for the current lane of the current thread. Therefore, if L is accessed by an operation, the location storage selected when the location description was created is accessed, and not the location storage associated with the current context of the access operation.

The DWARF expression is ill-formed if AS is not one of the values defined by the target architecture specific DW_ASPACE_LLVM_* values.

See A.2.5.4.4.5 Implicit Location Description Operations for special rules concerning implicit pointer values produced by dereferencing implicit location descriptions created by the DW_OP_implicit_pointer and DW_OP_LLVM_aspace_implicit_pointer operations.
DW_OP_form_tls_address

DW_OP_form_tls_address pops one stack entry that must be an integral type value and treats it as a thread-local storage address TA.

It pushes a location description L with one memory location description SL on the stack. SL is the target architecture specific memory location description that corresponds to the thread-local storage address TA.

The meaning of the thread-local storage address TA is defined by the run-time environment. If the run-time environment supports multiple thread-local storage blocks for a single thread, then the block corresponding to the executable or shared library containing this DWARF expression is used.

Some implementations of C, C++, Fortran, and other languages, support a thread-local storage class. Variables with this storage class have distinct values and addresses in distinct threads, much as automatic variables have distinct values and addresses in each subprogram invocation. Typically, there is a single block of storage containing all thread-local variables declared in the main executable, and a separate block for the variables declared in each shared library. Each thread-local variable can then be accessed in its block using an identifier. This identifier is typically a byte offset into the block and pushed onto the DWARF stack by one of the DW_OP_const* operations prior to the DW_OP_form_tls_address operation. Computing the address of the appropriate block can be complex (in some cases, the compiler emits a function call to do it), and difficult to describe using ordinary DWARF location descriptions. Instead of forcing complex thread-local storage calculations into the DWARF expressions, the DW_OP_form_tls_address allows the consumer to perform the computation based on the target architecture specific run-time environment.
DW_OP_call_frame_cfa

DW_OP_call_frame_cfa pushes the location description L of the Canonical Frame Address (CFA) of the current subprogram, obtained from the call frame information on the stack. See A.6.4 Call Frame Information.

Although the value of the DW_AT_frame_base attribute of the debugger information entry corresponding to the current subprogram can be computed using a location list expression, in some cases this would require an extensive location list because the values of the registers used in computing the CFA change during a subprogram execution. If the call frame information is present, then it already encodes such changes, and it is space efficient to reference that using the DW_OP_call_frame_cfa operation.
DW_OP_fbreg

DW_OP_fbreg has a single signed LEB128 integer operand that represents a byte displacement B.

The location description L for the frame base of the current subprogram is obtained from the DW_AT_frame_base attribute of the debugger information entry corresponding to the current subprogram as described in A.3.3.5 Low-Level Information.

The location description L is updated as if the DW_OP_LLVM_offset_uconst B operation was applied. The updated L is pushed on the stack.
DW_OP_breg0, DW_OP_breg1, …, DW_OP_breg31

The DW_OP_breg<N> operations encode the numbers of up to 32 registers, numbered from 0 through 31, inclusive. The register number R corresponds to the N in the operation name.

They have a single signed LEB128 integer operand that represents a byte displacement B.

The address space identifier AS is defined as the one corresponding to the target architecture specific default address space.

The address size S is defined as the address bit size of the target architecture specific address space corresponding to AS.

The contents of the register specified by R are retrieved as if a DW_OP_regval_type R, DR operation was performed where DR is the offset of a hypothetical debug information entry in the current compilation unit for an unsigned integral base type of size S bits. B is added and the least significant S bits are treated as an unsigned value to be used as an address A.

They push a location description L comprising one memory location description LS on the stack. LS specifies the memory location storage that corresponds to AS with a bit offset equal to A scaled by 8 (the byte size).
DW_OP_bregx

DW_OP_bregx has two operands. The first is an unsigned LEB128 integer that represents a register number R. The second is a signed LEB128 integer that represents a byte displacement B.

The action is the same as for DW_OP_breg<N>, except that R is used as the register number and B is used as the byte displacement.
DW_OP_LLVM_aspace_bregx New

DW_OP_LLVM_aspace_bregx has two operands. The first is an unsigned LEB128 integer that represents a register number R. The second is a signed LEB128 integer that represents a byte displacement B. It pops one stack entry that is required to be an integral type value that represents a target architecture specific address space identifier AS.

The action is the same as for DW_OP_breg<N>, except that R is used as the register number, B is used as the byte displacement, and AS is used as the address space identifier.

The DWARF expression is ill-formed if AS is not one of the values defined by the target architecture specific DW_ASPACE_LLVM_* values.

Note

Could also consider adding DW_OP_LLVM_aspace_breg0, DW_OP_LLVM_aspace_breg1, ..., DW_OP_LLVM_aspace_bref31 which would save encoding size.

A.2.5.4.4.4 Register Location Description Operations ¶

Note

This section replaces DWARF Version 5 section 2.6.1.1.3.

There is a register location storage that corresponds to each of the target architecture registers. The size of each register location storage corresponds to the size of the corresponding target architecture register.

A register location description specifies a register location storage. The bit offset corresponds to a bit position within the register. Bits accessed using a register location description access the corresponding target architecture register starting at the specified bit offset.

DW_OP_reg0, DW_OP_reg1, …, DW_OP_reg31

DW_OP_reg<N> operations encode the numbers of up to 32 registers, numbered from 0 through 31, inclusive. The target architecture register number R corresponds to the N in the operation name.

The operation is equivalent to performing DW_OP_regx R.
DW_OP_regx

DW_OP_regx has a single unsigned LEB128 integer operand that represents a target architecture register number R.

If the current call frame is the top call frame, it pushes a location description L that specifies one register location description SL on the stack. SL specifies the register location storage that corresponds to R with a bit offset of 0 for the current thread.

If the current call frame is not the top call frame, call frame information (see A.6.4 Call Frame Information) is used to determine the location description that holds the register for the current call frame and current program location of the current thread. The resulting location description L is pushed.

Note that if call frame information is used, the resulting location description may be register, memory, or undefined.

An implementation may evaluate the call frame information immediately, or may defer evaluation until L is accessed by an operation. If evaluation is deferred, R and the current context can be recorded in L. When accessed, the recorded context is used to evaluate the call frame information, not the current context of the access operation.

These operations obtain a register location. To fetch the contents of a register, it is necessary to use DW_OP_regval_type, use one of the DW_OP_breg* register-based addressing operations, or use DW_OP_deref* on a register location description.

A.2.5.4.4.5 Implicit Location Description Operations ¶

Note

This section replaces DWARF Version 5 section 2.6.1.1.4.

Implicit location storage represents a piece or all of an object which has no actual location in the program but whose contents are nonetheless known, either as a constant or can be computed from other locations and values in the program.

An implicit location description specifies an implicit location storage. The bit offset corresponds to a bit position within the implicit location storage. Bits accessed using an implicit location description, access the corresponding implicit storage value starting at the bit offset.

DW_OP_implicit_value

DW_OP_implicit_value has two operands. The first is an unsigned LEB128 integer that represents a byte size S. The second is a block of bytes with a length equal to S treated as a literal value V.

An implicit location storage LS is created with the literal value V and a size of S.

It pushes location description L with one implicit location description SL on the stack. SL specifies LS with a bit offset of 0.
DW_OP_stack_value

DW_OP_stack_value pops one stack entry that must be a value V.

An implicit location storage LS is created with the literal value V using the size, encoding, and endianity specified by V’s base type.

It pushes a location description L with one implicit location description SL on the stack. SL specifies LS with a bit offset of 0.

The DW_OP_stack_value operation specifies that the object does not exist in memory, but its value is nonetheless known. In this form, the location description specifies the actual value of the object, rather than specifying the memory or register storage that holds the value.

See DW_OP_implicit_pointer (following) for special rules concerning implicit pointer values produced by dereferencing implicit location descriptions created by the DW_OP_implicit_pointer and DW_OP_LLVM_aspace_implicit_pointer operations.

Note: Since location descriptions are allowed on the stack, the DW_OP_stack_value operation no longer terminates the DWARF operation expression execution as in DWARF Version 5.
DW_OP_implicit_pointer

An optimizing compiler may eliminate a pointer, while still retaining the value that the pointer addressed. DW_OP_implicit_pointer allows a producer to describe this value.

DW_OP_implicit_pointer specifies an object is a pointer to the target architecture default address space that cannot be represented as a real pointer, even though the value it would point to can be described. In this form, the location description specifies a debugging information entry that represents the actual location description of the object to which the pointer would point. Thus, a consumer of the debug information would be able to access the dereferenced pointer, even when it cannot access the pointer itself.

DW_OP_implicit_pointer has two operands. The first operand is a 4-byte unsigned value in the 32-bit DWARF format, or an 8-byte unsigned value in the 64-bit DWARF format, that represents the byte offset DR of a debugging information entry D relative to the beginning of the .debug_info section that contains the current compilation unit. The second operand is a signed LEB128 integer that represents a byte displacement B.

Note that D might not be in the current compilation unit.

The first operand interpretation is exactly like that for DW_FORM_ref_addr.

The address space identifier AS is defined as the one corresponding to the target architecture specific default address space.

The address size S is defined as the address bit size of the target architecture specific address space corresponding to AS.

An implicit location storage LS is created with the debugging information entry D, address space AS, and size of S.

It pushes a location description L that comprises one implicit location description SL on the stack. SL specifies LS with a bit offset of 0.

It is an evaluation error if a DW_OP_deref* operation pops a location description L’, and retrieves S bits, such that any retrieved bits come from an implicit location storage that is the same as LS, unless both the following conditions are met:
1. All retrieved bits come from an implicit location description that refers to an implicit location storage that is the same as LS.
  
  Note that all bits do not have to come from the same implicit location description, as L’ may involve composite location descriptions.
2. The bits come from consecutive ascending offsets within their respective implicit location storage.
These rules are equivalent to retrieving the complete contents of LS.

If both the above conditions are met, then the value V pushed by the DW_OP_deref* operation is an implicit pointer value IPV with a target architecture specific address space of AS, a debugging information entry of D, and a base type of T. If AS is the target architecture default address space, then T is the generic type. Otherwise, T is a target architecture specific integral type with a bit size equal to S.

If IPV is either implicitly converted to a location description (only done if AS is the target architecture default address space) or used by DW_OP_LLVM_form_aspace_address (only done if the address space popped by DW_OP_LLVM_form_aspace_address is AS), then the resulting location description RL is:
- If D has a DW_AT_location attribute, the DWARF expression E from the DW_AT_location attribute is evaluated with the current context, except that the result kind is a location description, the compilation unit is the one that contains D, the object is unspecified, and the initial stack is empty. RL is the expression result.
  
  Note that E is evaluated with the context of the expression accessing IPV, and not the context of the expression that contained the DW_OP_implicit_pointer or DW_OP_LLVM_aspace_implicit_pointer operation that created L.
- If D has a DW_AT_const_value attribute, then an implicit location storage RLS is created from the DW_AT_const_value attribute’s value with a size matching the size of the DW_AT_const_value attribute’s value. RL comprises one implicit location description SRL. SRL specifies RLS with a bit offset of 0.
  
  Note
  
  If using DW_AT_const_value for variables and formal parameters is deprecated and instead DW_AT_location is used with an implicit location description, then this rule would not be required.
- Otherwise, it is an evaluation error.
The bit offset of RL is updated as if the DW_OP_LLVM_offset_uconst B operation was applied.

If a DW_OP_stack_value operation pops a value that is the same as IPV, then it pushes a location description that is the same as L.

It is an evaluation error if LS or IPV is accessed in any other manner.

The restrictions on how an implicit pointer location description created by DW_OP_implicit_pointer and DW_OP_LLVM_aspace_implicit_pointer can be used are to simplify the DWARF consumer. Similarly, for an implicit pointer value created by DW_OP_deref* and DW_OP_stack_value.
DW_OP_LLVM_aspace_implicit_pointer New

DW_OP_LLVM_aspace_implicit_pointer has two operands that are the same as for DW_OP_implicit_pointer.

It pops one stack entry that must be an integral type value that represents a target architecture specific address space identifier AS.

The location description L that is pushed on the stack is the same as for DW_OP_implicit_pointer, except that the address space identifier used is AS.

The DWARF expression is ill-formed if AS is not one of the values defined by the target architecture specific DW_ASPACE_LLVM_* values.

Note

This definition of DW_OP_LLVM_aspace_implicit_pointer may change when full support for address classes is added as required for languages such as OpenCL/SyCL.

Typically a DW_OP_implicit_pointer or DW_OP_LLVM_aspace_implicit_pointer operation is used in a DWARF expression E₁ of a DW_TAG_variable or DW_TAG_formal_parameter debugging information entry D₁‘s DW_AT_location attribute. The debugging information entry referenced by the DW_OP_implicit_pointer or DW_OP_LLVM_aspace_implicit_pointer operations is typically itself a DW_TAG_variable or DW_TAG_formal_parameter debugging information entry D₂ whose DW_AT_location attribute gives a second DWARF expression E₂.

D₁ and E₁ are describing the location of a pointer type object. D₂ and E₂ are describing the location of the object pointed to by that pointer object.

However, D₂ may be any debugging information entry that contains a DW_AT_location or DW_AT_const_value attribute (for example, DW_TAG_dwarf_procedure). By using E₂, a consumer can reconstruct the value of the object when asked to dereference the pointer described by E₁ which contains the DW_OP_implicit_pointer or DW_OP_LLVM_aspace_implicit_pointer operation.

A.2.5.4.4.6 Composite Location Description Operations ¶

Note

This section replaces DWARF Version 5 section 2.6.1.2.

A composite location storage represents an object or value which may be contained in part of another location storage or contained in parts of more than one location storage.

Each part has a part location description L and a part bit size S. L can have one or more single location descriptions SL. If there are more than one SL then that indicates that part is located in more than one place. The bits of each place of the part comprise S contiguous bits from the location storage LS specified by SL starting at the bit offset specified by SL. All the bits must be within the size of LS or the DWARF expression is ill-formed.

A composite location storage can have zero or more parts. The parts are contiguous such that the zero-based location storage bit index will range over each part with no gaps between them. Therefore, the size of a composite location storage is the sum of the size of its parts. The DWARF expression is ill-formed if the size of the contiguous location storage is larger than the size of the memory location storage corresponding to the largest target architecture specific address space.

A composite location description specifies a composite location storage. The bit offset corresponds to a bit position within the composite location storage.

There are operations that create a composite location storage.

There are other operations that allow a composite location storage to be incrementally created. Each part is created by a separate operation. There may be one or more operations to create the final composite location storage. A series of such operations describes the parts of the composite location storage that are in the order that the associated part operations are executed.

To support incremental creation, a composite location storage can be in an incomplete state. When an incremental operation operates on an incomplete composite location storage, it adds a new part, otherwise it creates a new composite location storage. The DW_OP_LLVM_piece_end operation explicitly makes an incomplete composite location storage complete.

A composite location description that specifies a composite location storage that is incomplete is termed an incomplete composite location description. A composite location description that specifies a composite location storage that is complete is termed a complete composite location description.

If the top stack entry is a location description that has one incomplete composite location description SL after the execution of an operation expression has completed, SL is converted to a complete composite location description.

Note that this conversion does not happen after the completion of an operation expression that is evaluated on the same stack by the DW_OP_call* operations. Such executions are not a separate evaluation of an operation expression, but rather the continued evaluation of the same operation expression that contains the DW_OP_call* operation.

If a stack entry is required to be a location description L, but L has an incomplete composite location description, then the DWARF expression is ill-formed. The exception is for the operations involved in incrementally creating a composite location description as described below.

Note that a DWARF operation expression may arbitrarily compose composite location descriptions from any other location description, including those that have multiple single location descriptions, and those that have composite location descriptions.

The incremental composite location description operations are defined to be compatible with the definitions in DWARF Version 5.

DW_OP_piece

DW_OP_piece has a single unsigned LEB128 integer that represents a byte size S.

The action is based on the context:
- If the stack is empty, then a location description L comprised of one incomplete composite location description SL is pushed on the stack.
  
  An incomplete composite location storage LS is created with a single part P. P specifies a location description PL and has a bit size of S scaled by 8 (the byte size). PL is comprised of one undefined location description PSL.
  
  SL specifies LS with a bit offset of 0.
- Otherwise, if the top stack entry is a location description L comprised of one incomplete composite location description SL, then the incomplete composite location storage LS that SL specifies is updated to append a new part P. P specifies a location description PL and has a bit size of S scaled by 8 (the byte size). PL is comprised of one undefined location description PSL. L is left on the stack.
- Otherwise, if the top stack entry is a location description or can be converted to one, then it is popped and treated as a part location description PL. Then:
  - If the top stack entry (after popping PL) is a location description L comprised of one incomplete composite location description SL, then the incomplete composite location storage LS that SL specifies is updated to append a new part P. P specifies the location description PL and has a bit size of S scaled by 8 (the byte size). L is left on the stack.
  - Otherwise, a location description L comprised of one incomplete composite location description SL is pushed on the stack.
    
    An incomplete composite location storage LS is created with a single part P. P specifies the location description PL and has a bit size of S scaled by 8 (the byte size).
    
    SL specifies LS with a bit offset of 0.
- Otherwise, the DWARF expression is ill-formed
Many compilers store a single variable in sets of registers or store a variable partially in memory and partially in registers. DW_OP_piece provides a way of describing where a part of a variable is located.

If a non-0 byte displacement is required, the DW_OP_LLVM_offset operation can be used to update the location description before using it as the part location description of a DW_OP_piece operation.

The evaluation rules for the DW_OP_piece operation allow it to be compatible with the DWARF Version 5 definition.

Note

Since these extensions allow location descriptions to be entries on the stack, a simpler operation to create composite location descriptions could be defined. For example, just one operation that specifies how many parts, and pops pairs of stack entries for the part size and location description. Not only would this be a simpler operation and avoid the complexities of incomplete composite location descriptions, but it may also have a smaller encoding in practice. However, the desire for compatibility with DWARF Version 5 is likely a stronger consideration.
DW_OP_bit_piece

DW_OP_bit_piece has two operands. The first is an unsigned LEB128 integer that represents the part bit size S. The second is an unsigned LEB128 integer that represents a bit displacement B.

The action is the same as for DW_OP_piece, except that any part created has the bit size S, and the location description PL of any created part is updated as if the DW_OP_constu B; DW_OP_LLVM_bit_offset operations were applied.

DW_OP_bit_piece is used instead of DW_OP_piece when the piece to be assembled is not byte-sized or is not at the start of the part location description.

If a computed bit displacement is required, the DW_OP_LLVM_bit_offset operation can be used to update the location description before using it as the part location description of a DW_OP_bit_piece operation.

Note

The bit offset operand is not needed as DW_OP_LLVM_bit_offset can be used on the part’s location description.
DW_OP_LLVM_piece_end New

If the top stack entry is not a location description L comprised of one incomplete composite location description SL, then the DWARF expression is ill-formed.

Otherwise, the incomplete composite location storage LS specified by SL is updated to be a complete composite location description with the same parts.
DW_OP_LLVM_extend New

DW_OP_LLVM_extend has two operands. The first is an unsigned LEB128 integer that represents the element bit size S. The second is an unsigned LEB128 integer that represents a count C.

It pops one stack entry that must be a location description and is treated as the part location description PL.

A location description L comprised of one complete composite location description SL is pushed on the stack.

A complete composite location storage LS is created with C identical parts P. Each P specifies PL and has a bit size of S.

SL specifies LS with a bit offset of 0.

The DWARF expression is ill-formed if the element bit size or count are 0.
DW_OP_LLVM_select_bit_piece New

DW_OP_LLVM_select_bit_piece has two operands. The first is an unsigned LEB128 integer that represents the element bit size S. The second is an unsigned LEB128 integer that represents a count C.

It pops three stack entries. The first must be an integral type value that represents a bit mask value M. The second must be a location description that represents the one-location description L1. The third must be a location description that represents the zero-location description L0.

A complete composite location storage LS is created with C parts P_N ordered in ascending N from 0 to C-1 inclusive. Each P_N specifies location description PL_N and has a bit size of S.

PL_N is as if the DW_OP_LLVM_bit_offset N*S operation was applied to PLX_N.

PLX_N is the same as L0 if the N^th least significant bit of M is a zero, otherwise it is the same as L1.

A location description L comprised of one complete composite location description SL is pushed on the stack. SL specifies LS with a bit offset of 0.

The DWARF expression is ill-formed if S or C are 0, or if the bit size of M is less than C.

Note

Should the count operand for DW_OP_extend and DW_OP_select_bit_piece be changed to get the count value off the stack? This would allow support for architectures that have variable length vector instructions such as ARM and RISC-V.
DW_OP_LLVM_overlay New

DW_OP_LLVM_overlay pops four stack entries. The first must be an integral type value that represents the overlay byte size value S. The second must be an integral type value that represents the overlay byte offset value O. The third must be a location description that represents the overlay location description OL. The fourth must be a location description that represents the base location description BL.

The action is the same as for DW_OP_LLVM_bit_overlay, except that the overlay bit size BS and overlay bit offset BO used are S and O respectively scaled by 8 (the byte size).
DW_OP_LLVM_bit_overlay New

DW_OP_LLVM_bit_overlay pops four stack entries. The first must be an integral type value that represents the overlay bit size value BS. The second must be an integral type value that represents the overlay bit offset value BO. The third must be a location description that represents the overlay location description OL. The fourth must be a location description that represents the base location description BL.

The DWARF expression is ill-formed if BS or BO are negative values.

rbss(L) is the minimum remaining bit storage size of L which is defined as follows. LS is the location storage and LO is the location bit offset specified by a single location description SL of L. The remaining bit storage size RBSS of SL is the bit size of LS minus LO. rbss(L) is the minimum RBSS of each single location description SL of L.

The DWARF expression is ill-formed if rbss(BL) is less than BO plus BS.

If BS is 0, then the operation pushes BL.

If BO is 0 and BS equals rbss(BL), then the operation pushes OL.

Otherwise, the operation is equivalent to performing the following steps to push a composite location description.

The composite location description is conceptually the base location description BL with the overlay location description OL positioned as an overlay starting at the overlay offset BO and covering overlay bit size BS.
1. If BO is not 0 then push BL followed by performing the DW_OP_bit_piece BO, 0 operation.
2. Push OL followed by performing the DW_OP_bit_piece BS, 0 operation.
3. If rbss(BL) is greater than BO plus BS, push BL followed by performing the DW_OP_bit_piece (rbss(BL) - BO - BS), (BO + BS) operation.
4. Perform the DW_OP_LLVM_piece_end operation.

A.2.5.5 DWARF Location List Expressions ¶

Note

This section replaces DWARF Version 5 section 2.6.2.

To meet the needs of recent computer architectures and optimization techniques, debugging information must be able to describe the location of an object whose location changes over the object’s lifetime, and may reside at multiple locations during parts of an object’s lifetime. Location list expressions are used in place of operation expressions whenever the object whose location is being described has these requirements.

A location list expression consists of a series of location list entries. Each location list entry is one of the following kinds:

Bounded location description

This kind of location list entry provides an operation expression that evaluates to the location description of an object that is valid over a lifetime bounded by a starting and ending address. The starting address is the lowest address of the address range over which the location is valid. The ending address is the address of the first location past the highest address of the address range.

The location list entry matches when the current program location is within the given range.

There are several kinds of bounded location description entries which differ in the way that they specify the starting and ending addresses.

Default location description

This kind of location list entry provides an operation expression that evaluates to the location description of an object that is valid when no bounded location description entry applies.

The location list entry matches when the current program location is not within the range of any bounded location description entry.

Base address

This kind of location list entry provides an address to be used as the base address for beginning and ending address offsets given in certain kinds of bounded location description entries. The applicable base address of a bounded location description entry is the address specified by the closest preceding base address entry in the same location list. If there is no preceding base address entry, then the applicable base address defaults to the base address of the compilation unit (see DWARF Version 5 section 3.1.1).

In the case of a compilation unit where all of the machine code is contained in a single contiguous section, no base address entry is needed.

End-of-list

This kind of location list entry marks the end of the location list expression.

The address ranges defined by the bounded location description entries of a location list expression may overlap. When they do, they describe a situation in which an object exists simultaneously in more than one place.

If all of the address ranges in a given location list expression do not collectively cover the entire range over which the object in question is defined, and there is no following default location description entry, it is assumed that the object is not available for the portion of the range that is not covered.

The result of the evaluation of a DWARF location list expression is:

If the current program location is not specified, then it is an evaluation error.

Note

If the location list only has a single default entry, should that be considered a match if there is no program location? If there are non-default entries then it seems it has to be an evaluation error when there is no program location as that indicates the location depends on the program location which is not known.
If there are no matching location list entries, then the result is a location description that comprises one undefined location description.
Otherwise, the operation expression E of each matching location list entry is evaluated with the current context, except that the result kind is a location description, the object is unspecified, and the initial stack is empty. The location list entry result is the location description returned by the evaluation of E.

The result is a location description that is comprised of the union of the single location descriptions of the location description result of each matching location list entry.

A location list expression can only be used as the value of a debugger information entry attribute that is encoded using class loclist or loclistsptr (see A.7.5.5 Classes and Forms). The value of the attribute provides an index into a separate object file section called .debug_loclists or .debug_loclists.dwo (for split DWARF object files) that contains the location list entries.

A DW_OP_call* and DW_OP_implicit_pointer operation can be used to specify a debugger information entry attribute that has a location list expression. Several debugger information entry attributes allow DWARF expressions that are evaluated with an initial stack that includes a location description that may originate from the evaluation of a location list expression.

This location list representation, the loclist and loclistsptr class, and the related DW_AT_loclists_base attribute are new in DWARF Version 5. Together they eliminate most, or all of the code object relocations previously needed for location list expressions.

Note

The rest of this section is the same as DWARF Version 5 section 2.6.2.

A.2.13 Address Spaces ¶

Note

This is a new section after DWARF Version 5 section 2.12 Segmented Addresses.

DWARF address spaces correspond to target architecture specific linear addressable memory areas. They are used in DWARF expression location descriptions to describe in which target architecture specific memory area data resides.

Target architecture specific DWARF address spaces may correspond to hardware supported facilities such as memory utilizing base address registers, scratchpad memory, and memory with special interleaving. The size of addresses in these address spaces may vary. Their access and allocation may be hardware managed with each thread or group of threads having access to independent storage. For these reasons they may have properties that do not allow them to be viewed as part of the unified global virtual address space accessible by all threads.

It is target architecture specific whether multiple DWARF address spaces are supported and how source language memory spaces map to target architecture specific DWARF address spaces. A target architecture may map multiple source language memory spaces to the same target architecture specific DWARF address class. Optimization may determine that variable lifetime and access pattern allows them to be allocated in faster scratchpad memory represented by a different DWARF address space than the default for the source language memory space.

Although DWARF address space identifiers are target architecture specific, DW_ASPACE_LLVM_none is a common address space supported by all target architectures, and defined as the target architecture default address space.

DWARF address space identifiers are used by:

The DW_AT_LLVM_address_space attribute.
The DWARF expression operations: DW_OP_aspace_bregx, DW_OP_form_aspace_address, DW_OP_aspace_implicit_pointer, and DW_OP_xderef*.
The CFI instructions: DW_CFA_def_aspace_cfa and DW_CFA_def_aspace_cfa_sf.

Note

Currently, DWARF defines address class values as being target architecture specific, and defines a DW_AT_address_class attribute. With the removal of DW_AT_segment in DWARF 6, it is unclear how the address class is intended to be used as the term is not used elsewhere. Should these be replaced by this proposal’s more complete address space? Or are they intended to represent source language memory spaces such as in OpenCL?

A.2.14 Memory Spaces ¶

Note

This is a new section after DWARF Version 5 section 2.12 Segmented Addresses.

DWARF memory spaces are used for source languages that have the concept of memory spaces. They are used in the DW_AT_LLVM_memory_space attribute for pointer type, reference type, variable, formal parameter, and constant debugger information entries.

Each DWARF memory space is conceptually a separate source language memory space with its own lifetime and aliasing rules. DWARF memory spaces are used to specify the source language memory spaces that pointer type and reference type values refer, and to specify the source language memory space in which variables are allocated.

Although DWARF memory space identifiers are source language specific, DW_MSPACE_LLVM_none is a common memory space supported by all source languages, and defined as the source language default memory space.

The set of currently defined DWARF memory spaces, together with source language mappings, is given in Source language memory spaces.

Vendor defined source language memory spaces may be defined using codes in the range DW_MSPACE_LLVM_lo_user to DW_MSPACE_LLVM_hi_user.

Source language memory spaces¶
Memory Space Name	Meaning	C/C++	OpenCL	CUDA/HIP
`DW_MSPACE_LLVM_none`	generic	default	generic	default
`DW_MSPACE_LLVM_global`	global		global
`DW_MSPACE_LLVM_constant`	constant		constant	constant
`DW_MSPACE_LLVM_group`	thread-group		local	shared
`DW_MSPACE_LLVM_private`	thread		private
`DW_MSPACE_LLVM_lo_user`
`DW_MSPACE_LLVM_hi_user`

Note

The approach presented in Source language memory spaces is to define the default DW_MSPACE_LLVM_none to be the generic address class and not the global address class. This matches how CLANG and LLVM have added support for CUDA-like languages on top of existing C++ language support. This allows all addresses to be generic by default which matches CUDA-like languages.

An alternative approach is to define DW_MSPACE_LLVM_none as being the global memory space and then change DW_MSPACE_LLVM_global to DW_MSPACE_LLVM_generic. This would match the reality that languages that do not support multiple memory spaces only have one default global memory space. Generally, in these languages if they expose that the target architecture supports multiple memory spaces, the default one is still the global memory space. Then a language that does support multiple memory spaces has to explicitly indicate which pointers have the added ability to reference more than the global memory space. However, compilers generating DWARF for CUDA-like languages would then have to define every CUDA-like language pointer type or reference type with a DW_AT_LLVM_memory_space attribute of DW_MSPACE_LLVM_generic to match the language semantics.

A.3 Program Scope Entries ¶

Note

This section provides changes to existing debugger information entry attributes. These would be incorporated into the corresponding DWARF Version 5 chapter 3 sections.

A.3.1 Unit Entries ¶

A.3.1.1 Full and Partial Compilation Unit Entries ¶

Note

This augments DWARF Version 5 section 3.1.1 and Table 3.1.

Additional language codes defined for use with the DW_AT_language attribute are defined in Language Names.

Language Names¶
Language Name	Meaning
`DW_LANG_LLVM_HIP`	HIP Language.

The HIP language [HIP] can be supported by extending the C++ language.

Note

The following new attribute is added.

A DW_TAG_compile_unit debugger information entry for a compilation unit may have a DW_AT_LLVM_augmentation attribute, whose value is an augmentation string.

The augmentation string allows producers to indicate that there is additional vendor or target specific information in the debugging information entries. For example, this might be information about the version of vendor specific extensions that are being used.

If not present, or if the string is empty, then the compilation unit has no augmentation string.

The format for the augmentation string is:

[vendor:vX.Y[:options]]*

Where vendor is the producer, vX.Y specifies the major X and minor Y version number of the extensions used, and options is an optional string providing additional information about the extensions. The version number must conform to semantic versioning [SEMVER]. The options string must not contain the “]“ character.

For example:
[abc:v0.0][def:v1.2:feature-a=on,feature-b=3]

A.3.3 Subroutine and Entry Point Entries ¶

A.3.3.5 Low-Level Information ¶

A DW_TAG_subprogram, DW_TAG_inlined_subroutine, or DW_TAG_entry_point debugger information entry may have a DW_AT_return_addr attribute, whose value is a DWARF expression E.

The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The result of the evaluation is the location description L of the place where the return address for the current call frame’s subprogram or entry point is stored.

The DWARF is ill-formed if L is not comprised of one memory location description for one of the target architecture specific address spaces.

Note

It is unclear why DW_TAG_inlined_subroutine has a DW_AT_return_addr attribute but not a DW_AT_frame_base or DW_AT_static_link attribute. Seems it would either have all of them or none. Since inlined subprograms do not have a call frame it seems they would have none of these attributes.
A DW_TAG_subprogram or DW_TAG_entry_point debugger information entry may have a DW_AT_frame_base attribute, whose value is a DWARF expression E.

The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any.

The DWARF is ill-formed if E contains a DW_OP_fbreg operation, or the resulting location description L is not comprised of one single location description SL.

If SL is a register location description for register R, then L is replaced with the result of evaluating a DW_OP_bregx R, 0 operation. This computes the frame base memory location description in the target architecture default address space.

This allows the more compact DW_OP_reg* to be used instead of DW_OP_breg* 0.

Note

This rule could be removed and require the producer to create the required location description directly using DW_OP_call_frame_cfa, DW_OP_breg*, or DW_OP_LLVM_aspace_bregx. This would also then allow a target to implement the call frames within a large register.

Otherwise, the DWARF is ill-formed if SL is not a memory location description in any of the target architecture specific address spaces.

The resulting L is the frame base for the subprogram or entry point.

Typically, E will use the DW_OP_call_frame_cfa operation or be a stack pointer register plus or minus some offset.

The frame base for a subprogram is typically an address relative to the first unit of storage allocated for the subprogram’s stack frame. The DW_AT_frame_base attribute can be used in several ways:
1. In subprograms that need location lists to locate local variables, the DW_AT_frame_base can hold the needed location list, while all variables’ location descriptions can be simpler ones involving the frame base.
2. It can be used in resolving “up-level” addressing within nested routines. (See also DW_AT_static_link, below)
Some languages support nested subroutines. In such languages, it is possible to reference the local variables of an outer subroutine from within an inner subroutine. The DW_AT_static_link and DW_AT_frame_base attributes allow debuggers to support this same kind of referencing.
If a DW_TAG_subprogram or DW_TAG_entry_point debugger information entry is lexically nested, it may have a DW_AT_static_link attribute, whose value is a DWARF expression E.

The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The result of the evaluation is the location description L of the canonical frame address (see A.6.4 Call Frame Information) of the relevant call frame of the subprogram instance that immediately lexically encloses the current call frame’s subprogram or entry point.

The DWARF is ill-formed if L is not comprised of one memory location description for one of the target architecture specific address spaces.

In the context of supporting nested subroutines, the DW_AT_frame_base attribute value obeys the following constraints:
1. It computes a value that does not change during the life of the subprogram, and
2. The computed value is unique among instances of the same subroutine.
For typical DW_AT_frame_base use, this means that a recursive subroutine’s stack frame must have non-zero size.

If a debugger is attempting to resolve an up-level reference to a variable, it uses the nesting structure of DWARF to determine which subroutine is the lexical parent and the DW_AT_static_link value to identify the appropriate active frame of the parent. It can then attempt to find the reference within the context of the parent.

Note

The following new attributes are added.
For languages that are implemented using a SIMT execution model, a DW_TAG_subprogram, DW_TAG_inlined_subroutine, or DW_TAG_entry_point debugger information entry may have a DW_AT_LLVM_lanes attribute whose value is an integer constant that is the number of source language threads of execution per target architecture thread.

For example, a compiler may map source language threads of execution onto lanes of a target architecture thread using a SIMT execution model.

It is the static number of source language threads of execution per target architecture thread. It is not the dynamic number of source language threads of execution with which the target architecture thread was initiated, for example, due to smaller or partial work-groups.

If not present, the default value of 1 is used.

The DWARF is ill-formed if the value is less than or equal to 0.
For source languages that are implemented using a SIMT execution model, a DW_TAG_subprogram, DW_TAG_inlined_subroutine, or DW_TAG_entry_point debugging information entry may have a DW_AT_LLVM_lane_pc attribute whose value is a DWARF expression E.

The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any.

The resulting location description L is for a lane count sized vector of generic type elements. The lane count is the value of the DW_AT_LLVM_lanes attribute. Each element holds the conceptual program location of the corresponding lane. If the lane was not active when the current subprogram was called, its element is an undefined location description.

The DWARF is ill-formed if L does not have exactly one single location description.

DW_AT_LLVM_lane_pc allows the compiler to indicate conceptually where each SIMT lane of a target architecture thread is positioned even when it is in divergent control flow that is not active.

Typically, the result is a location description with one composite location description with each part being a location description with either one undefined location description or one memory location description.

If not present, the target architecture thread is not being used in a SIMT manner, and the thread’s current program location is used.
For languages that are implemented using a SIMT execution model, a DW_TAG_subprogram, DW_TAG_inlined_subroutine, or DW_TAG_entry_point debugger information entry may have a DW_AT_LLVM_active_lane attribute whose value is a DWARF expression E.

E is evaluated with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any.

The DWARF is ill-formed if L does not have exactly one single location description SL.

The active lane bit mask V for the current program location is obtained by reading from SL using a target architecture specific integral base type T that has a bit size equal to the value of the DW_AT_LLVM_lanes attribute of the subprogram corresponding to context’s frame and program location. The N^th least significant bit of the mask corresponds to the N^th lane. If the bit is 1 the lane is active, otherwise it is inactive. The result of the attribute is the value V.

Some targets may update the target architecture execution mask for regions of code that must execute with different sets of lanes than the current active lanes. For example, some code must execute with all lanes made temporarily active. DW_AT_LLVM_active_lane allows the compiler to provide the means to determine the source language active lanes at any program location. Typically, this attribute will use a loclist to express different locations of the active lane mask at different program locations.

If not present and DW_AT_LLVM_lanes is greater than 1, then the target architecture execution mask is used.
A DW_TAG_subprogram, DW_TAG_inlined_subroutine, or DW_TAG_entry_point debugger information entry may have a DW_AT_LLVM_iterations attribute whose value is an integer constant or a DWARF expression E. Its value is the number of source language loop iterations executing concurrently by the target architecture for a single source language thread of execution.

A compiler may generate code that executes more than one iteration of a source language loop concurrently using optimization techniques such as software pipelining or SIMD vectorization. The number of concurrent iterations may vary for different loop nests in the same subprogram. Typically, this attribute will use a loclist to express different values at different program locations.

If the attribute is an integer constant, then the value is the constant. The DWARF is ill-formed if the constant is less than or equal to 0.

Otherwise, E is evaluated with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The DWARF is ill-formed if the result is not a location description comprised of one implicit location description, that when read as the generic type, results in a value V that is less than or equal to 0. The result of the attribute is the value V.

If not present, the default value of 1 is used.

A.3.4 Call Site Entries and Parameters ¶

A.3.4.2 Call Site Parameters ¶

The call site entry may own DW_TAG_call_site_parameter debugging information entries representing the parameters passed to the call. Call site parameter entries occur in the same order as the corresponding parameters in the source. Each such entry has a DW_AT_location attribute which is a location description. This location description describes where the parameter is passed (usually either some register, or a memory location expressible as the contents of the stack register plus some offset).
A DW_TAG_call_site_parameter debugger information entry may have a DW_AT_call_value attribute, whose value is a DWARF operation expression E₁.

The result of the DW_AT_call_value attribute is obtained by evaluating E₁ with a context that has a result kind of a value, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The resulting value V₁ is the value of the parameter at the time of the call made by the call site.

For parameters passed by reference, where the code passes a pointer to a location which contains the parameter, or for reference type parameters, the DW_TAG_call_site_parameter debugger information entry may also have a DW_AT_call_data_location attribute whose value is a DWARF operation expression E₂, and a DW_AT_call_data_value attribute whose value is a DWARF operation expression E₃.

The value of the DW_AT_call_data_location attribute is obtained by evaluating E₂ with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The resulting location description L₂ is the location where the referenced parameter lives during the call made by the call site. If E₂ would just be a DW_OP_push_object_address, then the DW_AT_call_data_location attribute may be omitted.

Note

The DWARF Version 5 implies that DW_OP_push_object_address may be used but does not state what object must be specified in the context. Either DW_OP_push_object_address cannot be used, or the object to be passed in the context must be defined.

The value of the DW_AT_call_data_value attribute is obtained by evaluating E₃ with a context that has a result kind of a value, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The resulting value V₃ is the value in L₂ at the time of the call made by the call site.

The result of these attributes is undefined if the current call frame is not for the subprogram containing the DW_TAG_call_site_parameter debugger information entry or the current program location is not for the call site containing the DW_TAG_call_site_parameter debugger information entry in the current call frame.

The consumer may have to virtually unwind to the call site (see A.6.4 Call Frame Information) in order to evaluate these attributes. This will ensure the source language thread of execution upon which the user is focused corresponds to the call site needed to evaluate the expression.

If it is not possible to avoid the expressions of these attributes from accessing registers or memory locations that might be clobbered by the subprogram being called by the call site, then the associated attribute should not be provided.

The reason for the restriction is that the parameter may need to be accessed during the execution of the callee. The consumer may virtually unwind from the called subprogram back to the caller and then evaluate the attribute expressions. The call frame information (see A.6.4 Call Frame Information) will not be able to restore registers that have been clobbered, and clobbered memory will no longer have the value at the time of the call.
Each call site parameter entry may also have a DW_AT_call_parameter attribute which contains a reference to a DW_TAG_formal_parameter entry, DW_AT_type attribute referencing the type of the parameter or DW_AT_name attribute describing the parameter’s name.

Examples using call site entries and related attributes are found in Appendix D.15.

A.3.5 Lexical Block Entries ¶

Note

This section is the same as DWARF Version 5 section 3.5.

A.4 Data Object and Object List Entries ¶

Note

This section provides changes to existing debugger information entry attributes. These would be incorporated into the corresponding DWARF Version 5 chapter 4 sections.

A.4.1 Data Object Entries ¶

Program variables, formal parameters and constants are represented by debugging information entries with the tags DW_TAG_variable, DW_TAG_formal_parameter and DW_TAG_constant, respectively.

The tag DW_TAG_constant is used for languages that have true named constants.

The debugging information entry for a program variable, formal parameter or constant may have the following attributes:

A DW_AT_location attribute, whose value is a DWARF expression E that describes the location of a variable or parameter at run-time.

The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The result of the evaluation is the location description of the base of the data object.

See A.2.5.4.2 Control Flow Operations for special evaluation rules used by the DW_OP_call* operations.

Note

Delete the description of how the DW_OP_call* operations evaluate a DW_AT_location attribute as that is now described in the operations.

Note

See the discussion about the DW_AT_location attribute in the DW_OP_call* operation. Having each attribute only have a single purpose and single execution semantics seems desirable. It makes it easier for the consumer that no longer have to track the context. It makes it easier for the producer as it can rely on a single semantics for each attribute.

For that reason, limiting the DW_AT_location attribute to only supporting evaluating the location description of an object, and using a different attribute and encoding class for the evaluation of DWARF expression procedures on the same operation expression stack seems desirable.
DW_AT_const_value

Note

Could deprecate using the DW_AT_const_value attribute for DW_TAG_variable or DW_TAG_formal_parameter debugger information entries that have been optimized to a constant. Instead, DW_AT_location could be used with a DWARF expression that produces an implicit location description now that any location description can be used within a DWARF expression. This allows the DW_OP_call* operations to be used to push the location description of any variable regardless of how it is optimized.
DW_AT_LLVM_memory_space

A DW_AT_memory_space attribute with a constant value representing a source language specific DWARF memory space (see 2.14 “Memory Spaces”). If omitted, defaults to DW_MSPACE_none.

A.4.2 Common Block Entries ¶

A common block entry also has a DW_AT_location attribute whose value is a DWARF expression E that describes the location of the common block at run-time. The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The result of the evaluation is the location description of the base of the common block. See A.2.5.4.2 Control Flow Operations for special evaluation rules used by the DW_OP_call* operations.

A.5 Type Entries ¶

Note

This section provides changes to existing debugger information entry attributes. These would be incorporated into the corresponding DWARF Version 5 chapter 5 sections.

A.5.1 Base Type Entries ¶

Note

The following new attribute is added.

A DW_TAG_base_type debugger information entry for a base type T may have a DW_AT_LLVM_vector_size attribute whose value is an integer constant that is the vector type size N.

The representation of a vector base type is as N contiguous elements, each one having the representation of a base type T’ that is the same as T without the DW_AT_LLVM_vector_size attribute.

If a DW_TAG_base_type debugger information entry does not have a DW_AT_LLVM_vector_size attribute, then the base type is not a vector type.

The DWARF is ill-formed if N is not greater than 0.

Note

LLVM has mention of a non-upstreamed debugger information entry that is intended to support vector types. However, that was not for a base type so would not be suitable as the type of a stack value entry. But perhaps that could be replaced by using this attribute.

Note

Compare this with the DW_AT_GNU_vector extension supported by GNU. Is it better to add an attribute to the existing DW_TAG_base_type debug entry, or allow some forms of DW_TAG_array_type (those that have the DW_AT_GNU_vector attribute) to be used as stack entry value types?

A.5.3 Type Modifier Entries ¶

Note

This section augments DWARF Version 5 section 5.3.

A modified type entry describing a pointer or reference type (using DW_TAG_pointer_type, DW_TAG_reference_type or DW_TAG_rvalue_reference_type) may have a DW_AT_LLVM_memory_space attribute with a constant value representing a source language specific DWARF memory space (see A.2.14 Memory Spaces). If omitted, defaults to DW_MSPACE_LLVM_none.

A modified type entry describing a pointer or reference type (using DW_TAG_pointer_type, DW_TAG_reference_type or DW_TAG_rvalue_reference_type) may have a DW_AT_LLVM_address_space attribute with a constant value AS representing an architecture specific DWARF address space (see A.2.13 Address Spaces). If omitted, defaults to DW_ASPACE_LLVM_none. DR is the offset of a hypothetical debug information entry D in the current compilation unit for an integral base type matching the address size of AS. An object P having the given pointer or reference type are dereferenced as if the DW_OP_push_object_address; DW_OP_deref_type DR; DW_OP_constu AS; DW_OP_form_aspace_address operation expression was evaluated with the current context except: the result kind is location description; the initial stack is empty; and the object is the location description of P.

Note

What if the current context does not have a current target architecture defined?

Note

With the expanded support for DWARF address spaces, it may be worth examining if they can be used for what was formerly supported by DWARF 5 segments. That would include specifying the address space of all code addresses (compilation units, subprograms, subprogram entries, labels, subprogram types, etc.). Either the code address attributes could be extended to allow a exprloc form (so that DW_OP_form_aspace_address can be used) or the DW_AT_LLVM_address_space attribute be allowed on all DIEs that allow DW_AT_segment.

A.5.7 Structure, Union, Class and Interface Type Entries ¶

A.5.7.3 Derived or Extended Structures, Classes and Interfaces ¶

For a DW_AT_data_member_location attribute there are two cases:
1. If the attribute is an integer constant B, it provides the offset in bytes from the beginning of the containing entity.
  
  The result of the attribute is obtained by evaluating a DW_OP_LLVM_offset B operation with an initial stack comprising the location description of the beginning of the containing entity. The result of the evaluation is the location description of the base of the member entry.
  
  If the beginning of the containing entity is not byte aligned, then the beginning of the member entry has the same bit displacement within a byte.
2. Otherwise, the attribute must be a DWARF expression E which is evaluated with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an initial stack comprising the location description of the beginning of the containing entity, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The result of the evaluation is the location description of the base of the member entry.
Note

The beginning of the containing entity can now be any location description, including those with more than one single location description, and those with single location descriptions that are of any kind and have any bit offset.

A.5.7.8 Member Function Entries ¶

An entry for a virtual function also has a DW_AT_vtable_elem_location attribute whose value is a DWARF expression E.

The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an initial stack comprising the location description of the object of the enclosing type, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The result of the evaluation is the location description of the slot for the function within the virtual function table for the enclosing class.

A.5.14 Pointer to Member Type Entries ¶

The DW_TAG_ptr_to_member_type debugging information entry has a DW_AT_use_location attribute whose value is a DWARF expression E. It is used to compute the location description of the member of the class to which the pointer to member entry points.

The method used to find the location description of a given member of a class, structure, or union is common to any instance of that class, structure, or union and to any instance of the pointer to member type. The method is thus associated with the pointer to member type, rather than with each object that has a pointer to member type.

The DW_AT_use_location DWARF expression is used in conjunction with the location description for a particular object of the given pointer to member type and for a particular structure or class instance.

The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an unspecified object, the compilation unit that contains E, an initial stack comprising two entries, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The first stack entry is the value of the pointer to member object itself. The second stack entry is the location description of the base of the entire class, structure, or union instance containing the member whose location is being calculated. The result of the evaluation is the location description of the member of the class to which the pointer to member entry points.

A.5.18 Dynamic Properties of Types ¶

A.5.18.1 Data Location ¶

Some languages may represent objects using descriptors to hold information, including a location and/or run-time parameters, about the data that represents the value for that object.

The DW_AT_data_location attribute may be used with any type that provides one or more levels of hidden indirection and/or run-time parameters in its representation. Its value is a DWARF operation expression E which computes the location description of the data for an object. When this attribute is omitted, the location description of the data is the same as the location description of the object.

The result of the attribute is obtained by evaluating E with a context that has a result kind of a location description, an object that is the location description of the data descriptor, the compilation unit that contains E, an empty initial stack, and other context elements corresponding to the source language thread of execution upon which the user is focused, if any. The result of the evaluation is the location description of the base of the member entry.

E will typically involve an operation expression that begins with a DW_OP_push_object_address operation which loads the location description of the object which can then serve as a descriptor in subsequent calculation.

Note

Since DW_AT_data_member_location, DW_AT_use_location, and DW_AT_vtable_elem_location allow both operation expressions and location list expressions, why does DW_AT_data_location not allow both? In all cases they apply to data objects so less likely that optimization would cause different operation expressions for different program location ranges. But if supporting for some then should be for all.

It seems odd this attribute is not the same as DW_AT_data_member_location in having an initial stack with the location description of the object since the expression has to need it.

A.6 Other Debugging Information ¶

Note

This section provides changes to existing debugger information entry attributes. These would be incorporated into the corresponding DWARF Version 5 chapter 6 sections.

A.6.1 Accelerated Access ¶

A.6.1.1 Lookup By Name ¶

A.6.1.1.1 Contents of the Name Index ¶

Note

The following provides changes to DWARF Version 5 section 6.1.1.1.

The rule for debugger information entries included in the name index in the optional .debug_names section is extended to also include named DW_TAG_variable debugging information entries with a DW_AT_location attribute that includes a DW_OP_LLVM_form_aspace_address operation.

The name index must contain an entry for each debugging information entry that defines a named subprogram, label, variable, type, or namespace, subject to the following rules:

DW_TAG_variable debugging information entries with a DW_AT_location attribute that includes a DW_OP_addr, DW_OP_LLVM_form_aspace_address, or DW_OP_form_tls_address operation are included; otherwise, they are excluded.

A.6.1.1.4 Data Representation of the Name Index ¶

A.6.1.1.4.1 Section Header ¶

Note

The following provides an addition to DWARF Version 5 section 6.1.1.4.1 item 14 augmentation_string.

A null-terminated UTF-8 vendor specific augmentation string, which provides additional information about the contents of this index. If provided, the recommended format for augmentation string is:

[vendor:vX.Y[:options]]*

Where vendor is the producer, vX.Y specifies the major X and minor Y version number of the extensions used in the DWARF of the compilation unit, and options is an optional string providing additional information about the extensions. The version number must conform to semantic versioning [SEMVER]. The options string must not contain the “]” character.

For example:

[abc:v0.0][def:v1.2:feature-a=on,feature-b=3]

Note

This is different to the definition in DWARF Version 5 but is consistent with the other augmentation strings and allows multiple vendor extensions to be supported.

A.6.2 Line Number Information ¶

A.6.2.4 The Line Number Program Header ¶

A.6.2.4.1 Standard Content Descriptions ¶

Note

This augments DWARF Version 5 section 6.2.4.1.

DW_LNCT_LLVM_source

The component is a null-terminated UTF-8 source text string with “\n“ line endings. This content code is paired with the same forms as DW_LNCT_path. It can be used for file name entries.

The value is an empty null-terminated string if no source is available. If the source is available but is an empty file then the value is a null-terminated single “\n“.

When the source field is present, consumers can use the embedded source instead of attempting to discover the source on disk using the file path provided by the DW_LNCT_path field. When the source field is absent, consumers can access the file to get the source text.

This is particularly useful for programming languages that support runtime compilation and runtime generation of source text. In these cases, the source text does not reside in any permanent file. For example, the OpenCL language [:ref:`OpenCL <amdgpu-dwarf-OpenCL>`] supports online compilation.
DW_LNCT_LLVM_is_MD5

DW_LNCT_LLVM_is_MD5 indicates if the DW_LNCT_MD5 content kind, if present, is valid: when 0 it is not valid and when 1 it is valid. If DW_LNCT_LLVM_is_MD5 content kind is not present, and DW_LNCT_MD5 content kind is present, then the MD5 checksum is valid.

DW_LNCT_LLVM_is_MD5 is always paired with the DW_FORM_udata form.

This allows a compilation unit to have a mixture of files with and without MD5 checksums. This can happen when multiple relocatable files are linked together.

A.6.4 Call Frame Information ¶

Note

This section provides changes to existing call frame information and defines instructions added by these extensions. Additional support is added for address spaces. Register unwind DWARF expressions are generalized to allow any location description, including those with composite and implicit location descriptions.

These changes would be incorporated into the DWARF Version 5 section 6.4.

A.6.4.1 Structure of Call Frame Information ¶

The register rules are:

undefined

A register that has this rule has no recoverable value in the previous frame. The previous value of this register is the undefined location description (see A.2.5.4.4.2 Undefined Location Description Operations).

By convention, the register is not preserved by a callee.

same value

This register has not been modified from the previous caller frame.

If the current frame is the top frame, then the previous value of this register is the location description L that specifies one register location description SL. SL specifies the register location storage that corresponds to the register with a bit offset of 0 for the current thread.

If the current frame is not the top frame, then the previous value of this register is the location description obtained using the call frame information for the callee frame and callee program location invoked by the current caller frame for the same register.

By convention, the register is preserved by the callee, but the callee has not modified it.

offset(N)

N is a signed byte offset. The previous value of this register is saved at the location description computed as if the DWARF operation expression DW_OP_LLVM_offset N is evaluated with the current context, except the result kind is a location description, the compilation unit is unspecified, the object is unspecified, and an initial stack comprising the location description of the current CFA (see A.2.5.4 DWARF Operation Expressions).

val_offset(N)

N is a signed byte offset. The previous value of this register is the memory byte address of the location description computed as if the DWARF operation expression DW_OP_LLVM_offset N is evaluated with the current context, except the result kind is a location description, the compilation unit is unspecified, the object is unspecified, and an initial stack comprising the location description of the current CFA (see A.2.5.4 DWARF Operation Expressions).

The DWARF is ill-formed if the CFA location description is not a memory byte address location description, or if the register size does not match the size of an address in the address space of the current CFA location description.

Since the CFA location description is required to be a memory byte address location description, the value of val_offset(N) will also be a memory byte address location description since it is offsetting the CFA location description by N bytes. Furthermore, the value of val_offset(N) will be a memory byte address in the same address space as the CFA location description.

Note

Should DWARF allow the address size to be a different size to the size of the register? Requiring them to be the same bit size avoids any issue of conversion as the bit contents of the register is simply interpreted as a value of the address.

GDB has a per register hook that allows a target specific conversion on a register by register basis. It defaults to truncation of bigger registers, and to actually reading bytes from the next register (or reads out of bounds for the last register) for smaller registers. There are no GDB tests that read a register out of bounds (except an illegal hand written assembly test).

register(R)

This register has been stored in another register numbered R.

The previous value of this register is the location description obtained using the call frame information for the current frame and current program location for register R.

The DWARF is ill-formed if the size of this register does not match the size of register R or if there is a cyclic dependency in the call frame information.

Note

Should this also allow R to be larger than this register? If so is the value stored in the low order bits and it is undefined what is stored in the extra upper bits?

expression(E)

The previous value of this register is located at the location description produced by evaluating the DWARF operation expression E (see A.2.5.4 DWARF Operation Expressions).

E is evaluated with the current context, except the result kind is a location description, the compilation unit is unspecified, the object is unspecified, and an initial stack comprising the location description of the current CFA (see A.2.5.4 DWARF Operation Expressions).

val_expression(E)

The previous value of this register is located at the implicit location description created from the value produced by evaluating the DWARF operation expression E (see A.2.5.4 DWARF Operation Expressions).

E is evaluated with the current context, except the result kind is a value, the compilation unit is unspecified, the object is unspecified, and an initial stack comprising the location description of the current CFA (see A.2.5.4 DWARF Operation Expressions).

The DWARF is ill-formed if the resulting value type size does not match the register size.

Note

This has limited usefulness as the DWARF expression E can only produce values up to the size of the generic type. This is due to not allowing any operations that specify a type in a CFI operation expression. This makes it unusable for registers that are larger than the generic type. However, expression(E) can be used to create an implicit location description of any size.

architectural

The rule is defined externally to this specification by the augmenter.

This table would be extremely large if actually constructed as described. Most of the entries at any point in the table are identical to the ones above them. The whole table can be represented quite compactly by recording just the differences starting at the beginning address of each subroutine in the program.

The virtual unwind information is encoded in a self-contained section called .debug_frame. Entries in a .debug_frame section are aligned on a multiple of the address size relative to the start of the section and come in two forms: a Common Information Entry (CIE) and a Frame Description Entry (FDE).

If the range of code addresses for a function is not contiguous, there may be multiple CIEs and FDEs corresponding to the parts of that function.

A Common Information Entry (CIE) holds information that is shared among many Frame Description Entries (FDE). There is at least one CIE in every non-empty .debug_frame section. A CIE contains the following fields, in order:

length (initial length)

A constant that gives the number of bytes of the CIE structure, not including the length field itself (see Section 7.2.2 Initial Length Values). The size of the length field plus the value of length must be an integral multiple of the address size specified in the address_size field.
CIE_id (4 or 8 bytes, see A.7.4 32-Bit and 64-Bit DWARF Formats)

A constant that is used to distinguish CIEs from FDEs.

In the 32-bit DWARF format, the value of the CIE id in the CIE header is 0xffffffff; in the 64-bit DWARF format, the value is 0xffffffffffffffff.
version (ubyte)

A version number (see Section 7.24 Call Frame Information). This number is specific to the call frame information and is independent of the DWARF version number.

The value of the CIE version number is 4.

Note

Would this be increased to 5 to reflect the changes in these extensions?
augmentation (sequence of UTF-8 characters)

A null-terminated UTF-8 string that identifies the augmentation to this CIE or to the FDEs that use it. If a reader encounters an augmentation string that is unexpected, then only the following fields can be read:
- CIE: length, CIE_id, version, augmentation
- FDE: length, CIE_pointer, initial_location, address_range
If there is no augmentation, this value is a zero byte.

The augmentation string allows users to indicate that there is additional vendor and target architecture specific information in the CIE or FDE which is needed to virtually unwind a stack frame. For example, this might be information about dynamically allocated data which needs to be freed on exit from the routine.

Because the .debug_frame section is useful independently of any .debug_info section, the augmentation string always uses UTF-8 encoding.

The recommended format for the augmentation string is:

[vendor:vX.Y[:options]]*

Where vendor is the producer, vX.Y specifies the major X and minor Y version number of the extensions used, and options is an optional string providing additional information about the extensions. The version number must conform to semantic versioning [SEMVER]. The options string must not contain the “]“ character.

For example:
[abc:v0.0][def:v1.2:feature-a=on,feature-b=3]
address_size (ubyte)

The size of a target address in this CIE and any FDEs that use it, in bytes. If a compilation unit exists for this frame, its address size must match the address size here.
segment_selector_size (ubyte)

The size of a segment selector in this CIE and any FDEs that use it, in bytes.
code_alignment_factor (unsigned LEB128)

A constant that is factored out of all advance location instructions (see A.6.4.2.1 Row Creation Instructions). The resulting value is (operand * code_alignment_factor).
data_alignment_factor (signed LEB128)

A constant that is factored out of certain offset instructions (see A.6.4.2.2 CFA Definition Instructions and A.6.4.2.3 Register Rule Instructions). The resulting value is (operand * data_alignment_factor).
return_address_register (unsigned LEB128)

An unsigned LEB128 constant that indicates which column in the rule table represents the return address of the subprogram. Note that this column might not correspond to an actual machine register.

The value of the return address register is used to determine the program location of the caller frame. The program location of the top frame is the target architecture program counter value of the current thread.
initial_instructions (array of ubyte)

A sequence of rules that are interpreted to create the initial setting of each column in the table.

The default rule for all columns before interpretation of the initial instructions is the undefined rule. However, an ABI authoring body or a compilation system authoring body may specify an alternate default value for any or all columns.
padding (array of ubyte)

Enough DW_CFA_nop instructions to make the size of this entry match the length value above.

An FDE contains the following fields, in order:

length (initial length)

A constant that gives the number of bytes of the header and instruction stream for this subprogram, not including the length field itself (see Section 7.2.2 Initial Length Values). The size of the length field plus the value of length must be an integral multiple of the address size.
CIE_pointer (4 or 8 bytes, see A.7.4 32-Bit and 64-Bit DWARF Formats)

A constant offset into the .debug_frame section that denotes the CIE that is associated with this FDE.
initial_location (segment selector and target address)

The address of the first location associated with this table entry. If the segment_selector_size field of this FDE’s CIE is non-zero, the initial location is preceded by a segment selector of the given length.
address_range (target address)

The number of bytes of program instructions described by this entry.
instructions (array of ubyte)

A sequence of table defining instructions that are described in A.6.4.2 Call Frame Instructions.
padding (array of ubyte)

Enough DW_CFA_nop instructions to make the size of this entry match the length value above.

A.6.4.2 Call Frame Instructions ¶

Each call frame instruction is defined to take 0 or more operands. Some of the operands may be encoded as part of the opcode (see A.7.24 Call Frame Information). The instructions are defined in the following sections.

Some call frame instructions have operands that are encoded as DWARF operation expressions E (see A.2.5.4 DWARF Operation Expressions). The DWARF operations that can be used in E have the following restrictions:

DW_OP_addrx, DW_OP_call2, DW_OP_call4, DW_OP_call_ref, DW_OP_const_type, DW_OP_constx, DW_OP_convert, DW_OP_deref_type, DW_OP_fbreg, DW_OP_implicit_pointer, DW_OP_regval_type, DW_OP_reinterpret, and DW_OP_xderef_type operations are not allowed because the call frame information must not depend on other debug sections.
DW_OP_push_object_address is not allowed because there is no object context to provide a value to push.
DW_OP_LLVM_push_lane and DW_OP_LLVM_push_iteration are not allowed because the call frame instructions describe the actions for the whole target architecture thread, not the lanes or iterations independently.
DW_OP_call_frame_cfa and DW_OP_entry_value are not allowed because their use would be circular.
DW_OP_LLVM_call_frame_entry_reg is not allowed if evaluating E causes a circular dependency between DW_OP_LLVM_call_frame_entry_reg operations.

For example, if a register R1 has a DW_CFA_def_cfa_expression instruction that evaluates a DW_OP_LLVM_call_frame_entry_reg operation that specifies register R2, and register R2 has a DW_CFA_def_cfa_expression instruction that that evaluates a DW_OP_LLVM_call_frame_entry_reg operation that specifies register R1.

Call frame instructions to which these restrictions apply include DW_CFA_def_cfa_expression, DW_CFA_expression, and DW_CFA_val_expression.

A.6.4.2.1 Row Creation Instructions ¶

Note

These instructions are the same as in DWARF Version 5 section 6.4.2.1.

A.6.4.2.2 CFA Definition Instructions ¶

DW_CFA_def_cfa

The DW_CFA_def_cfa instruction takes two unsigned LEB128 operands representing a register number R and a (non-factored) byte displacement B. AS is set to the target architecture default address space identifier. The required action is to define the current CFA rule to be equivalent to the result of evaluating the DWARF operation expression DW_OP_constu AS; DW_OP_LLVM_aspace_bregx R, B as a location description.
DW_CFA_def_cfa_sf

The DW_CFA_def_cfa_sf instruction takes two operands: an unsigned LEB128 value representing a register number R and a signed LEB128 factored byte displacement B. AS is set to the target architecture default address space identifier. The required action is to define the current CFA rule to be equivalent to the result of evaluating the DWARF operation expression DW_OP_constu AS; DW_OP_LLVM_aspace_bregx R, B * data_alignment_factor as a location description.

The action is the same as DW_CFA_def_cfa, except that the second operand is signed and factored.
DW_CFA_LLVM_def_aspace_cfa New

The DW_CFA_LLVM_def_aspace_cfa instruction takes three unsigned LEB128 operands representing a register number R, a (non-factored) byte displacement B, and a target architecture specific address space identifier AS. The required action is to define the current CFA rule to be equivalent to the result of evaluating the DWARF operation expression DW_OP_constu AS; DW_OP_LLVM_aspace_bregx R, B as a location description.

If AS is not one of the values defined by the target architecture specific DW_ASPACE_LLVM_* values then the DWARF expression is ill-formed.
DW_CFA_LLVM_def_aspace_cfa_sf New

The DW_CFA_LLVM_def_aspace_cfa_sf instruction takes three operands: an unsigned LEB128 value representing a register number R, a signed LEB128 factored byte displacement B, and an unsigned LEB128 value representing a target architecture specific address space identifier AS. The required action is to define the current CFA rule to be equivalent to the result of evaluating the DWARF operation expression DW_OP_constu AS; DW_OP_LLVM_aspace_bregx R, B * data_alignment_factor as a location description.

If AS is not one of the values defined by the target architecture specific DW_ASPACE_LLVM_* values, then the DWARF expression is ill-formed.

The action is the same as DW_CFA_aspace_def_cfa, except that the second operand is signed and factored.
DW_CFA_def_cfa_register

The DW_CFA_def_cfa_register instruction takes a single unsigned LEB128 operand representing a register number R. The required action is to define the current CFA rule to be equivalent to the result of evaluating the DWARF operation expression DW_OP_constu AS; DW_OP_LLVM_aspace_bregx R, B as a location description. B and AS are the old CFA byte displacement and address space respectively.

If the subprogram has no current CFA rule, or the rule was defined by a DW_CFA_def_cfa_expression instruction, then the DWARF is ill-formed.
DW_CFA_def_cfa_offset

The DW_CFA_def_cfa_offset instruction takes a single unsigned LEB128 operand representing a (non-factored) byte displacement B. The required action is to define the current CFA rule to be equivalent to the result of evaluating the DWARF operation expression DW_OP_constu AS; DW_OP_LLVM_aspace_bregx R, B as a location description. R and AS are the old CFA register number and address space respectively.

If the subprogram has no current CFA rule, or the rule was defined by a DW_CFA_def_cfa_expression instruction, then the DWARF is ill-formed.
DW_CFA_def_cfa_offset_sf

The DW_CFA_def_cfa_offset_sf instruction takes a signed LEB128 operand representing a factored byte displacement B. The required action is to define the current CFA rule to be equivalent to the result of evaluating the DWARF operation expression DW_OP_constu AS; DW_OP_LLVM_aspace_bregx R, B * data_alignment_factor as a location description. R and AS are the old CFA register number and address space respectively.

If the subprogram has no current CFA rule, or the rule was defined by a DW_CFA_def_cfa_expression instruction, then the DWARF is ill-formed.

The action is the same as DW_CFA_def_cfa_offset, except that the operand is signed and factored.
DW_CFA_def_cfa_expression

The DW_CFA_def_cfa_expression instruction takes a single operand encoded as a DW_FORM_exprloc value representing a DWARF operation expression E. The required action is to define the current CFA rule to be equivalent to the result of evaluating E with the current context, except the result kind is a location description, the compilation unit is unspecified, the object is unspecified, and an empty initial stack.

See A.6.4.2 Call Frame Instructions regarding restrictions on the DWARF expression operations that can be used in E.

The DWARF is ill-formed if the result of evaluating E is not a memory byte address location description.

A.6.4.2.3 Register Rule Instructions ¶

DW_CFA_undefined

The DW_CFA_undefined instruction takes a single unsigned LEB128 operand that represents a register number R. The required action is to set the rule for the register specified by R to undefined.
DW_CFA_same_value

The DW_CFA_same_value instruction takes a single unsigned LEB128 operand that represents a register number R. The required action is to set the rule for the register specified by R to same value.
DW_CFA_offset

The DW_CFA_offset instruction takes two operands: a register number R (encoded with the opcode) and an unsigned LEB128 constant representing a factored displacement B. The required action is to change the rule for the register specified by R to be an offset(B * data_alignment_factor) rule.

Note

Seems this should be named DW_CFA_offset_uf since the offset is unsigned factored.
DW_CFA_offset_extended

The DW_CFA_offset_extended instruction takes two unsigned LEB128 operands representing a register number R and a factored displacement B. This instruction is identical to DW_CFA_offset, except for the encoding and size of the register operand.

Note

Seems this should be named DW_CFA_offset_extended_uf since the displacement is unsigned factored.
DW_CFA_offset_extended_sf

The DW_CFA_offset_extended_sf instruction takes two operands: an unsigned LEB128 value representing a register number R and a signed LEB128 factored displacement B. This instruction is identical to DW_CFA_offset_extended, except that B is signed.
DW_CFA_val_offset

The DW_CFA_val_offset instruction takes two unsigned LEB128 operands representing a register number R and a factored displacement B. The required action is to change the rule for the register indicated by R to be a val_offset(B * data_alignment_factor) rule.

Note

Seems this should be named DW_CFA_val_offset_uf since the displacement is unsigned factored.

Note

An alternative is to define DW_CFA_val_offset to implicitly use the target architecture default address space, and add another operation that specifies the address space.
DW_CFA_val_offset_sf

The DW_CFA_val_offset_sf instruction takes two operands: an unsigned LEB128 value representing a register number R and a signed LEB128 factored displacement B. This instruction is identical to DW_CFA_val_offset, except that B is signed.
DW_CFA_register

The DW_CFA_register instruction takes two unsigned LEB128 operands representing register numbers R1 and R2 respectively. The required action is to set the rule for the register specified by R1 to be a register(R2) rule.
DW_CFA_expression

The DW_CFA_expression instruction takes two operands: an unsigned LEB128 value representing a register number R, and a DW_FORM_block value representing a DWARF operation expression E. The required action is to change the rule for the register specified by R to be an expression(E) rule.

That is, E computes the location description where the register value can be retrieved.

See A.6.4.2 Call Frame Instructions regarding restrictions on the DWARF expression operations that can be used in E.
DW_CFA_val_expression

The DW_CFA_val_expression instruction takes two operands: an unsigned LEB128 value representing a register number R, and a DW_FORM_block value representing a DWARF operation expression E. The required action is to change the rule for the register specified by R to be a val_expression(E) rule.

That is, E computes the value of register R.

See A.6.4.2 Call Frame Instructions regarding restrictions on the DWARF expression operations that can be used in E.

If the result of evaluating E is not a value with a base type size that matches the register size, then the DWARF is ill-formed.
DW_CFA_restore

The DW_CFA_restore instruction takes a single operand (encoded with the opcode) that represents a register number R. The required action is to change the rule for the register specified by R to the rule assigned it by the initial_instructions in the CIE.
DW_CFA_restore_extended

The DW_CFA_restore_extended instruction takes a single unsigned LEB128 operand that represents a register number R. This instruction is identical to DW_CFA_restore, except for the encoding and size of the register operand.

A.6.4.2.4 Row State Instructions ¶

Note

These instructions are the same as in DWARF Version 5 section 6.4.2.4.

A.6.4.2.5 Padding Instruction ¶

Note

These instructions are the same as in DWARF Version 5 section 6.4.2.5.

A.6.4.3 Call Frame Instruction Usage ¶

Note

The same as in DWARF Version 5 section 6.4.3.

A.6.4.4 Call Frame Calling Address ¶

Note

The same as in DWARF Version 5 section 6.4.4.

A.7 Data Representation ¶

Note

This section provides changes to existing debugger information entry attributes. These would be incorporated into the corresponding DWARF Version 5 chapter 7 sections.

A.7.4 32-Bit and 64-Bit DWARF Formats ¶

Note

This augments DWARF Version 5 section 7.4 list item 3’s table.

`.debug_info` section attribute form roles¶
Form	Role
DW_OP_LLVM_aspace_implicit_pointer	offset in `.debug_info`

A.7.5 Format of Debugging Information ¶

A.7.5.4 Attribute Encodings ¶

Note

This augments DWARF Version 5 section 7.5.4 and Table 7.5.

The following table gives the encoding of the additional debugging information entry attributes.

Attribute encodings¶
Attribute Name	Value	Classes
`DW_AT_LLVM_active_lane`	0x3e08	exprloc, loclist
`DW_AT_LLVM_augmentation`	0x3e09	string
`DW_AT_LLVM_lanes`	0x3e0a	constant
`DW_AT_LLVM_lane_pc`	0x3e0b	exprloc, loclist
`DW_AT_LLVM_vector_size`	0x3e0c	constant
`DW_AT_LLVM_iterations`	0x3e0a	constant, exprloc, loclist
`DW_AT_LLVM_address_space`	TBA	constant
`DW_AT_LLVM_memory_space`	TBA	constant

A.7.5.5 Classes and Forms ¶

Note

The following modifies the matching text in DWARF Version 5 section 7.5.5.

reference
There are four types of reference.
The first type of reference…

The second type of reference can identify any debugging information entry within a .debug_info section; in particular, it may refer to an entry in a different compilation unit from the unit containing the reference, and may refer to an entry in a different shared object file. This type of reference (DW_FORM_ref_addr) is an offset from the beginning of the .debug_info section of the target executable or shared object file, or, for references within a supplementary object file, an offset from the beginning of the local .debug_info section; it is relocatable in a relocatable object file and frequently relocated in an executable or shared object file. In the 32-bit DWARF format, this offset is a 4-byte unsigned value; in the 64-bit DWARF format, it is an 8-byte unsigned value (see A.7.4 32-Bit and 64-Bit DWARF Formats).

A debugging information entry that may be referenced by another compilation unit using DW_FORM_ref_addr must have a global symbolic name.

For a reference from one executable or shared object file to another, the reference is resolved by the debugger to identify the executable or shared object file and the offset into that file’s .debug_info section in the same fashion as the run time loader, either when the debug information is first read, or when the reference is used.

A.7.7 DWARF Expressions ¶

Note

Rename DWARF Version 5 section 7.7 to reflect the unification of location descriptions into DWARF expressions.

A.7.7.1 Operation Expressions ¶

Note

Rename DWARF Version 5 section 7.7.1 and delete section 7.7.2 to reflect the unification of location descriptions into DWARF expressions.

This augments DWARF Version 5 section 7.7.1 and Table 7.9, and adds a new table describing vendor extension operations for DW_OP_LLVM_user.

A DWARF operation expression is stored in a block of contiguous bytes. The bytes form a sequence of operations. Each operation is a 1-byte code that identifies that operation, followed by zero or more bytes of additional data. The encoding for the operation DW_OP_LLVM_user is described in DWARF Operation Encodings, and the encoding of all DW_OP_LLVM_user vendor extensions operations are described in DWARF DW_OP_LLVM_user Vendor Extension Operation Encodings.

DWARF Operation Encodings¶
Operation	Code	Number of Operands	Notes
`DW_OP_LLVM_user`	0xe9	1+	ULEB128 vendor extension opcode, followed by vendor extension operands defined in DWARF DW_OP_LLVM_user Vendor Extension Operation Encodings

DWARF DW_OP_LLVM_user Vendor Extension Operation Encodings¶
Operation	Vendor Extension Opcode	Number of Additional Operands	Notes
`DW_OP_LLVM_form_aspace_address`	0x02	0
`DW_OP_LLVM_push_lane`	0x03	0
`DW_OP_LLVM_offset`	0x04	0
`DW_OP_LLVM_offset_uconst`	0x05	1	ULEB128 byte displacement
`DW_OP_LLVM_bit_offset`	0x06	0
`DW_OP_LLVM_call_frame_entry_reg`	0x07	1	ULEB128 register number
`DW_OP_LLVM_undefined`	0x08	0
`DW_OP_LLVM_aspace_bregx`	0x09	2	ULEB128 register number, SLEB128 byte displacement
`DW_OP_LLVM_piece_end`	0x0a	0
`DW_OP_LLVM_extend`	0x0b	2	ULEB128 bit size, ULEB128 count
`DW_OP_LLVM_select_bit_piece`	0x0c	2	ULEB128 bit size, ULEB128 count
`DW_OP_LLVM_aspace_implicit_pointer`	TBA	2	4-byte or 8-byte offset of DIE, SLEB128 byte displacement
`DW_OP_LLVM_push_iteration`	TBA	0
`DW_OP_LLVM_overlay`	TBA	0
`DW_OP_LLVM_bit_overlay`	TBA	0

A.7.7.3 Location List Expressions ¶

Note

Rename DWARF Version 5 section 7.7.3 to reflect that location lists are a kind of DWARF expression.

A.7.12 Source Languages ¶

Note

This augments DWARF Version 5 section 7.12 and Table 7.17.

The following table gives the encoding of the additional DWARF languages.

Language encodings¶
Language Name	Value	Default Lower Bound
`DW_LANG_LLVM_HIP`	0x8100	0

A.7.14 Address Space Encodings ¶

Note

This is a new section after DWARF Version 5 section 7.13 “Address Class and Address Space Encodings”.

The value of the common address space encoding DW_ASPACE_LLVM_none is 0.

A.7.15 Memory Space Encodings ¶

Note

This is a new section after DWARF Version 5 section 7.13 “Address Class and Address Space Encodings”.

The encodings of the constants used for the currently defined memory spaces are given in Memory space encodings.

Memory space encodings¶
Memory Space Name	Value
`DW_MSPACE_LLVM_none`	0x0000
`DW_MSPACE_LLVM_global`	0x0001
`DW_MSPACE_LLVM_constant`	0x0002
`DW_MSPACE_LLVM_group`	0x0003
`DW_MSPACE_LLVM_private`	0x0004
`DW_MSPACE_LLVM_lo_user`	0x8000
`DW_MSPACE_LLVM_hi_user`	0xffff

A.7.22 Line Number Information ¶

Note

This augments DWARF Version 5 section 7.22 and Table 7.27.

The following table gives the encoding of the additional line number header entry formats.

Line number header entry format encodings¶
Line number header entry format name	Value
`DW_LNCT_LLVM_source`	0x2001
`DW_LNCT_LLVM_is_MD5`	0x2002

A.7.24 Call Frame Information ¶

Note

This augments DWARF Version 5 section 7.24 and Table 7.29.

The following table gives the encoding of the additional call frame information instructions.

Call frame instruction encodings¶
Instruction	High 2 Bits	Low 6 Bits	Operand 1	Operand 2	Operand 3
`DW_CFA_LLVM_def_aspace_cfa`	0	0x30	ULEB128 register	ULEB128 offset	ULEB128 address space
`DW_CFA_LLVM_def_aspace_cfa_sf`	0	0x31	ULEB128 register	SLEB128 offset	ULEB128 address space

A.7.32 Type Signature Computation ¶

Note

This augments (in alphabetical order) DWARF Version 5 section 7.32, Table 7.32.

Attributes used in type signature computation¶
`DW_AT_LLVM_address_space`
`DW_AT_LLVM_memory_space`
`DW_AT_LLVM_vector_size`

A. Attributes by Tag Value (Informative)¶

Note

This augments DWARF Version 5 Appendix A and Table A.1.

The following table provides the additional attributes that are applicable to debugger information entries.

Attributes by tag value¶
Tag Name	Applicable Attributes
`DW_TAG_base_type`	`DW_AT_LLVM_vector_size`
`DW_TAG_pointer_type`	`DW_AT_LLVM_address_space` `DW_AT_LLVM_memory_space`
`DW_TAG_reference_type`	`DW_AT_LLVM_address_space` `DW_AT_LLVM_memory_space`
`DW_TAG_rvalue_reference_type`	`DW_AT_LLVM_address_space` `DW_AT_LLVM_memory_space`
`DW_TAG_variable`	`DW_AT_LLVM_memory_space`
`DW_TAG_formal_parameter`	`DW_AT_LLVM_memory_space`
`DW_TAG_constant`	`DW_AT_LLVM_memory_space`
`DW_TAG_compile_unit`	`DW_AT_LLVM_augmentation`
`DW_TAG_entry_point`	`DW_AT_LLVM_active_lane` `DW_AT_LLVM_lane_pc` `DW_AT_LLVM_lanes` `DW_AT_LLVM_iterations`
`DW_TAG_inlined_subroutine`	`DW_AT_LLVM_active_lane` `DW_AT_LLVM_lane_pc` `DW_AT_LLVM_lanes` `DW_AT_LLVM_iterations`
`DW_TAG_subprogram`	`DW_AT_LLVM_active_lane` `DW_AT_LLVM_lane_pc` `DW_AT_LLVM_lanes` `DW_AT_LLVM_iterations`

D. Examples (Informative)¶

Note

This modifies the corresponding DWARF Version 5 Appendix D examples.

D.1 General Description Examples ¶

D.1.3 DWARF Location Description Examples ¶

DW_OP_offset_uconst 4: A structure member is four bytes from the start of the structure instance. The location description of the base of the structure instance is assumed to be already on the stack.
DW_OP_entry_value 1 DW_OP_reg5 DW_OP_offset_uconst 16: The address of the memory location is calculated by adding 16 to the value contained in register 5 upon entering the current subprogram.

D.2 Aggregate Examples ¶

D.2.1 Fortran Simple Array Example ¶

Figure D.4: Fortran array example: DWARF description

-------------------------------------------------------------------------------
! Description for type of 'ap'
!
1$: DW_TAG_array_type
        ! No name, default (Fortran) ordering, default stride
        DW_AT_type(reference to REAL)
        DW_AT_associated(expression=    ! Test 'ptr_assoc' flag
            DW_OP_push_object_address
            DW_OP_lit<n>                ! where n == offset(ptr_assoc)
            DW_OP_offset
            DW_OP_deref
            DW_OP_lit1                  ! mask for 'ptr_assoc' flag
            DW_OP_and)
        DW_AT_data_location(expression= ! Get raw data address
            DW_OP_push_object_address
            DW_OP_lit<n>                ! where n == offset(base)
            DW_OP_offset
            DW_OP_deref)                ! Type of index of array 'ap'
2$:     DW_TAG_subrange_type
            ! No name, default stride
            DW_AT_type(reference to INTEGER)
            DW_AT_lower_bound(expression=
                DW_OP_push_object_address
                DW_OP_lit<n>            ! where n ==
                                        !   offset(desc, dims) +
                                        !   offset(dims_str, lower_bound)
                DW_OP_offset
                DW_OP_deref)
            DW_AT_upper_bound(expression=
                DW_OP_push_object_address
                DW_OP_lit<n>            ! where n ==
                                        !   offset(desc, dims) +
                                        !   offset(dims_str, upper_bound)
                DW_OP_offset
                DW_OP_deref)
!  Note: for the m'th dimension, the second operator becomes
!  DW_OP_lit<n> where
!       n == offset(desc, dims)          +
!                (m-1)*sizeof(dims_str)  +
!                 offset(dims_str, [lower|upper]_bound)
!  That is, the expression does not get longer for each successive
!  dimension (other than to express the larger offsets involved).
3$: DW_TAG_structure_type
        DW_AT_name("array_ptr")
        DW_AT_byte_size(constant sizeof(REAL) + sizeof(desc<1>))
4$:     DW_TAG_member
            DW_AT_name("myvar")
            DW_AT_type(reference to REAL)
            DW_AT_data_member_location(constant 0)
5$:     DW_TAG_member
            DW_AT_name("ap");
            DW_AT_type(reference to 1$)
            DW_AT_data_member_location(constant sizeof(REAL))
6$: DW_TAG_array_type
        ! No name, default (Fortran) ordering, default stride
        DW_AT_type(reference to 3$)
        DW_AT_allocated(expression=       ! Test 'ptr_alloc' flag
            DW_OP_push_object_address
            DW_OP_lit<n>                  ! where n == offset(ptr_alloc)
            DW_OP_offset
            DW_OP_deref
            DW_OP_lit2                    ! Mask for 'ptr_alloc' flag
            DW_OP_and)
        DW_AT_data_location(expression=   ! Get raw data address
            DW_OP_push_object_address
            DW_OP_lit<n>                  ! where n == offset(base)
            DW_OP_offset
            DW_OP_deref)
7$:     DW_TAG_subrange_type
            ! No name, default stride
            DW_AT_type(reference to INTEGER)
            DW_AT_lower_bound(expression=
                DW_OP_push_object_address
                DW_OP_lit<n>              ! where n == ...
                DW_OP_offset
                DW_OP_deref)
            DW_AT_upper_bound(expression=
                DW_OP_push_object_address
                DW_OP_lit<n>              ! where n == ...
                DW_OP_offset
                DW_OP_deref)
8$: DW_TAG_variable
        DW_AT_name("arrayvar")
        DW_AT_type(reference to 6$)
        DW_AT_location(expression=
            ...as appropriate...)         ! Assume static allocation
-------------------------------------------------------------------------------

D.2.3 Fortran 2008 Assumed-rank Array Example ¶

Figure D.13: Sample DWARF for the array descriptor in Figure D.12

----------------------------------------------------------------------------
10$:  DW_TAG_array_type
        DW_AT_type(reference to real)
        DW_AT_rank(expression=
            DW_OP_push_object_address
            DW_OP_lit<n>
            DW_OP_offset
            DW_OP_deref)
        DW_AT_data_location(expression=
            DW_OP_push_object_address
            DW_OP_lit<n>
            DW_OP_offset
            DW_OP_deref)
11$:     DW_TAG_generic_subrange
            DW_AT_type(reference to integer)
            !   offset of rank in descriptor
            !   offset of data in descriptor
            DW_AT_lower_bound(expression=
            !   Looks up the lower bound of dimension i.
            !   Operation                       ! Stack effect
            !   (implicit)                      ! i
                DW_OP_lit<n>                    ! i sizeof(dim)
                DW_OP_mul                       ! dim[i]
                DW_OP_lit<n>                    ! dim[i] offsetof(dim)
                DW_OP_plus                      ! dim[i]+offset
                DW_OP_push_object_address       ! dim[i]+offsetof(dim) objptr
                DW_OP_swap                      ! objptr dim[i]+offsetof(dim)
                DW_OP_offset                    ! objptr.dim[i]
                DW_OP_lit<n>                    ! objptr.dim[i] offsetof(lb)
                DW_OP_offset                    ! objptr.dim[i].lowerbound
                DW_OP_deref)                    ! *objptr.dim[i].lowerbound
            DW_AT_upper_bound(expression=
            !   Looks up the upper bound of dimension i.
                DW_OP_lit<n>                    ! sizeof(dim)
                DW_OP_mul
                DW_OP_lit<n>                    ! offsetof(dim)
                DW_OP_plus
                DW_OP_push_object_address
                DW_OP_swap
                DW_OP_offset
                DW_OP_lit<n>                    ! offset of upperbound in dim
                DW_OP_offset
                DW_OP_deref)
            DW_AT_byte_stride(expression=
            !   Looks up the byte stride of dimension i.
                ...
            !   (analogous to DW_AT_upper_bound)
                )
----------------------------------------------------------------------------

Note

This example suggests that DW_AT_lower_bound and DW_AT_upper_bound evaluate an exprloc with an initial stack containing the rank value. The attribute definition should be updated to state this.

D.2.6 Ada Example ¶

Figure D.20: Ada example: DWARF description

----------------------------------------------------------------------------
11$:  DW_TAG_variable
          DW_AT_name("M")
          DW_AT_type(reference to INTEGER)
12$:  DW_TAG_array_type
          ! No name, default (Ada) order, default stride
          DW_AT_type(reference to INTEGER)
13$:      DW_TAG_subrange_type
              DW_AT_type(reference to INTEGER)
              DW_AT_lower_bound(constant 1)
              DW_AT_upper_bound(reference to variable M at 11$)
14$:  DW_TAG_variable
          DW_AT_name("VEC1")
          DW_AT_type(reference to array type at 12$)
      ...
21$:  DW_TAG_subrange_type
          DW_AT_name("TEENY")
          DW_AT_type(reference to INTEGER)
          DW_AT_lower_bound(constant 1)
          DW_AT_upper_bound(constant 100)
      ...
26$:  DW_TAG_structure_type
          DW_AT_name("REC2")
27$:      DW_TAG_member
              DW_AT_name("N")
              DW_AT_type(reference to subtype TEENY at 21$)
              DW_AT_data_member_location(constant 0)
28$:      DW_TAG_array_type
              ! No name, default (Ada) order, default stride
              ! Default data location
              DW_AT_type(reference to INTEGER)
29$:          DW_TAG_subrange_type
                  DW_AT_type(reference to subrange TEENY at 21$)
                  DW_AT_lower_bound(constant 1)
                  DW_AT_upper_bound(reference to member N at 27$)
30$:      DW_TAG_member
              DW_AT_name("VEC2")
              DW_AT_type(reference to array "subtype" at 28$)
              DW_AT_data_member_location(machine=
                  DW_OP_lit<n>                ! where n == offset(REC2, VEC2)
                  DW_OP_offset)
      ...
41$:  DW_TAG_variable
          DW_AT_name("OBJ2B")
          DW_AT_type(reference to REC2 at 26$)
          DW_AT_location(...as appropriate...)
----------------------------------------------------------------------------

C. Further Examples ¶

The AMD GPU specific usage of the features in these extensions, including examples, is available at User Guide for AMDGPU Backend section DWARF Debug Information.

Note

Change examples to use DW_OP_LLVM_offset instead of DW_OP_add when acting on a location description.

Need to provide examples of new features.

D. References ¶

[AMD] Advanced Micro Devices
[AMD-ROCgdb] AMD ROCm Debugger (ROCgdb)
[AMD-ROCm] AMD ROCm Platform
[AMDGPU-DWARF-LOC] Allow Location Descriptions on the DWARF Expression Stack
[AMDGPU-LLVM] User Guide for AMDGPU LLVM Backend
[CUDA] Nvidia CUDA Language
[DWARF] DWARF Debugging Information Format
[ELF] Executable and Linkable Format (ELF)
[GCC] GCC: The GNU Compiler Collection
[GDB] GDB: The GNU Project Debugger
[HIP] HIP Programming Guide
[HSA] Heterogeneous System Architecture (HSA) Foundation
[LLVM] The LLVM Compiler Infrastructure
[OpenCL] The OpenCL Specification Version 2.0
[Perforce-TotalView] Perforce TotalView HPC Debugging Software
[SEMVER] Semantic Versioning