LLVM Extensions¶

Introduction
General Assembly Syntax
- C99-style Hexadecimal Floating-point Constants
Machine-specific Assembly Syntax
Target Specific Behaviour

Introduction ¶

This document describes extensions to tools and formats LLVM seeks compatibility with.

General Assembly Syntax ¶

C99-style Hexadecimal Floating-point Constants ¶

LLVM’s assemblers allow floating-point constants to be written in C99’s hexadecimal format instead of decimal if desired.

.section .data
.float 0x1c2.2ap3

Machine-specific Assembly Syntax ¶

X86/COFF-Dependent ¶

Relocations ¶

The following additional relocation types are supported:

@IMGREL (AT&T syntax only) generates an image-relative relocation that corresponds to the COFF relocation types IMAGE_REL_I386_DIR32NB (32-bit) or IMAGE_REL_AMD64_ADDR32NB (64-bit).

.text
fun:
  mov foo@IMGREL(%ebx, %ecx, 4), %eax

.section .pdata
  .long fun@IMGREL
  .long (fun@imgrel + 0x3F)
  .long $unwind$fun@imgrel

.secrel32 generates a relocation that corresponds to the COFF relocation types IMAGE_REL_I386_SECREL (32-bit) or IMAGE_REL_AMD64_SECREL (64-bit).

.secidx relocation generates an index of the section that contains the target. It corresponds to the COFF relocation types IMAGE_REL_I386_SECTION (32-bit) or IMAGE_REL_AMD64_SECTION (64-bit).

.section .debug$S,"rn"
  .long 4
  .long 242
  .long 40
  .secrel32 _function_name + 0
  .secidx   _function_name
  ...

`.linkonce` Directive ¶

Syntax:

.linkonce [ comdat type ]

Supported COMDAT types:

discard: Discards duplicate sections with the same COMDAT symbol. This is the default if no type is specified.
one_only: If the symbol is defined multiple times, the linker issues an error.
same_size: Duplicates are discarded, but the linker issues an error if any have different sizes.
same_contents: Duplicates are discarded, but the linker issues an error if any duplicates do not have exactly the same content.
largest: Links the largest section from among the duplicates.
newest: Links the newest section from among the duplicates.

.section .text$foo
.linkonce
  ...

`.section` Directive ¶

MC supports passing the information in .linkonce at the end of .section. For example, these two codes are equivalent

.section secName, "dr", discard, "Symbol1"
.globl Symbol1
Symbol1:
.long 1

.section secName, "dr"
.linkonce discard
.globl Symbol1
Symbol1:
.long 1

Note that in the combined form the COMDAT symbol is explicit. This extension exists to support multiple sections with the same name in different COMDATs:

.section secName, "dr", discard, "Symbol1"
.globl Symbol1
Symbol1:
.long 1

.section secName, "dr", discard, "Symbol2"
.globl Symbol2
Symbol2:
.long 1

In addition to the types allowed with .linkonce, .section also accepts associative. The meaning is that the section is linked if a certain other COMDAT section is linked. This other section is indicated by the comdat symbol in this directive. It can be any symbol defined in the associated section, but is usually the associated section’s comdat.

The following restrictions apply to the associated section:

It must be a COMDAT section.

It cannot be another associative COMDAT section.

In the following example the symbol sym is the comdat symbol of .foo and .bar is associated to .foo.

.section        .foo,"bw",discard, "sym"
.section        .bar,"rd",associative, "sym"

MC supports these flags in the COFF .section directive:

b: BSS section (IMAGE_SCN_CNT_INITIALIZED_DATA)

d: Data section (IMAGE_SCN_CNT_UNINITIALIZED_DATA)

n: Section is not loaded (IMAGE_SCN_LNK_REMOVE)

r: Read-only

s: Shared section

w: Writable

x: Executable section

y: Not readable

D: Discardable (IMAGE_SCN_MEM_DISCARDABLE)

These flags are all compatible with gas, with the exception of the D flag, which gnu as does not support. For gas compatibility, sections with a name starting with “.debug” are implicitly discardable.

ARM64/COFF-Dependent ¶

Relocations ¶

The following additional symbol variants are supported:

:secrel_lo12: generates a relocation that corresponds to the COFF relocation types IMAGE_REL_ARM64_SECREL_LOW12A or IMAGE_REL_ARM64_SECREL_LOW12L.

:secrel_hi12: generates a relocation that corresponds to the COFF relocation type IMAGE_REL_ARM64_SECREL_HIGH12A.

add x0, x0, :secrel_hi12:symbol
ldr x0, [x0, :secrel_lo12:symbol]

add x1, x1, :secrel_hi12:symbol
add x1, x1, :secrel_lo12:symbol
...

ELF-Dependent ¶

`.section` Directive ¶

In order to support creating multiple sections with the same name and comdat, it is possible to add an unique number at the end of the .section directive. For example, the following code creates two sections named .text.

.section        .text,"ax",@progbits,unique,1
nop

.section        .text,"ax",@progbits,unique,2
nop

The unique number is not present in the resulting object at all. It is just used in the assembler to differentiate the sections.

The ‘o’ flag is mapped to SHF_LINK_ORDER. If it is present, a symbol must be given that identifies the section to be placed is the .sh_link.

.section .foo,"a",@progbits
.Ltmp:
.section .bar,"ao",@progbits,.Ltmp

which is equivalent to just

.section .foo,"a",@progbits
.section .bar,"ao",@progbits,.foo

`.linker-options` Section (linker options)¶

In order to support passing linker options from the frontend to the linker, a special section of type SHT_LLVM_LINKER_OPTIONS (usually named .linker-options though the name is not significant as it is identified by the type). The contents of this section is a simple pair-wise encoding of directives for consideration by the linker. The strings are encoded as standard null-terminated UTF-8 strings. They are emitted inline to avoid having the linker traverse the object file for retrieving the value. The linker is permitted to not honour the option and instead provide a warning/error to the user that the requested option was not honoured.

The section has type SHT_LLVM_LINKER_OPTIONS and has the SHF_EXCLUDE flag to ensure that the section is treated as opaque by linkers which do not support the feature and will not be emitted into the final linked binary.

This would be equivalent to the follow raw assembly:

.section ".linker-options","e",@llvm_linker_options
.asciz "option 1"
.asciz "value 1"
.asciz "option 2"
.asciz "value 2"

The following directives are specified:

lib

The parameter identifies a library to be linked against. The library will be looked up in the default and any specified library search paths (specified to this point).

libpath

The parameter identifies an additional library search path to be considered when looking up libraries after the inclusion of this option.

`SHT_LLVM_DEPENDENT_LIBRARIES` Section (Dependent Libraries)¶

This section contains strings specifying libraries to be added to the link by the linker.

The section should be consumed by the linker and not written to the output.

The strings are encoded as standard null-terminated UTF-8 strings.

For example:

.section ".deplibs","MS",@llvm_dependent_libraries,1
.asciz "library specifier 1"
.asciz "library specifier 2"

The interpretation of the library specifiers is defined by the consuming linker.

`SHT_LLVM_CALL_GRAPH_PROFILE` Section (Call Graph Profile)¶

This section is used to pass a call graph profile to the linker which can be used to optimize the placement of sections. It contains a sequence of (from symbol, to symbol, weight) tuples.

It shall have a type of SHT_LLVM_CALL_GRAPH_PROFILE (0x6fff4c02), shall have the SHF_EXCLUDE flag set, the sh_link member shall hold the section header index of the associated symbol table, and shall have a sh_entsize of 16. It should be named .llvm.call-graph-profile.

The contents of the section shall be a sequence of Elf_CGProfile entries.

typedef struct {
  Elf_Word cgp_from;
  Elf_Word cgp_to;
  Elf_Xword cgp_weight;
} Elf_CGProfile;

cgp_from: The symbol index of the source of the edge.
cgp_to: The symbol index of the destination of the edge.
cgp_weight: The weight of the edge.

This is represented in assembly as:

.cg_profile from, to, 42

.cg_profile directives are processed at the end of the file. It is an error if either from or to are undefined temporary symbols. If either symbol is a temporary symbol, then the section symbol is used instead. If either symbol is undefined, then that symbol is defined as if .weak symbol has been written at the end of the file. This forces the symbol to show up in the symbol table.

`SHT_LLVM_ADDRSIG` Section (address-significance table)¶

This section is used to mark symbols as address-significant, i.e. the address of the symbol is used in a comparison or leaks outside the translation unit. It has the same meaning as the absence of the LLVM attributes unnamed_addr and local_unnamed_addr.

Any sections referred to by symbols that are not marked as address-significant in any object file may be safely merged by a linker without breaking the address uniqueness guarantee provided by the C and C++ language standards.

The contents of the section are a sequence of ULEB128-encoded integers referring to the symbol table indexes of the address-significant symbols.

There are two associated assembly directives:

.addrsig

This instructs the assembler to emit an address-significance table. Without this directive, all symbols are considered address-significant.

.addrsig_sym sym

This marks sym as address-significant.

`SHT_LLVM_SYMPART` Section (symbol partition specification)¶

This section is used to mark symbols with the partition that they belong to. An .llvm_sympart section consists of a null-terminated string specifying the name of the partition followed by a relocation referring to the symbol that belongs to the partition. It may be constructed as follows:

.section ".llvm_sympart","",@llvm_sympart
.asciz "libpartition.so"
.word symbol_in_partition

CodeView-Dependent ¶

`.cv_file` Directive ¶

Syntax:: .cv_file FileNumber FileName [ checksum ] [ checksumkind ]

`.cv_func_id` Directive ¶

Introduces a function ID that can be used with .cv_loc.

Syntax:: .cv_func_id FunctionId

`.cv_inline_site_id` Directive ¶

Introduces a function ID that can be used with .cv_loc. Includes inlined at source location information for use in the line table of the caller, whether the caller is a real function or another inlined call site.

Syntax:: .cv_inline_site_id FunctionId within Function inlined_at FileNumber Line [ Column ]

`.cv_loc` Directive ¶

The first number is a file number, must have been previously assigned with a .file directive, the second number is the line number and optionally the third number is a column position (zero if not specified). The remaining optional items are .loc sub-directives.

Syntax:: .cv_loc FunctionId FileNumber [ Line ] [ Column ] [ prologue_end ] [ is_stmt value ]

`.cv_linetable` Directive ¶

Syntax:: .cv_linetable FunctionId , FunctionStart , FunctionEnd

`.cv_inline_linetable` Directive ¶

Syntax:: .cv_inline_linetable PrimaryFunctionId , FileNumber Line FunctionStart FunctionEnd

`.cv_def_range` Directive ¶

The GapStart and GapEnd options may be repeated as needed.

Syntax:: .cv_def_range RangeStart RangeEnd [ GapStart GapEnd ] , bytes

`.cv_stringtable` Directive ¶

`.cv_filechecksums` Directive ¶

`.cv_filechecksumoffset` Directive ¶

Syntax:: .cv_filechecksumoffset FileNumber

`.cv_fpo_data` Directive ¶

Syntax:: .cv_fpo_data procsym

Target Specific Behaviour ¶

X86 ¶

Relocations ¶

@ABS8 can be applied to symbols which appear as immediate operands to instructions that have an 8-bit immediate form for that operand. It causes the assembler to use the 8-bit form and an 8-bit relocation (e.g. R_386_8 or R_X86_64_8) for the symbol.

For example:

cmpq $foo@ABS8, %rdi

This causes the assembler to select the form of the 64-bit cmpq instruction that takes an 8-bit immediate operand that is sign extended to 64 bits, as opposed to cmpq $foo, %rdi which takes a 32-bit immediate operand. This is also not the same as cmpb $foo, %dil, which is an 8-bit comparison.

Windows on ARM ¶

Stack Probe Emission ¶

The reference implementation (Microsoft Visual Studio 2012) emits stack probes in the following fashion:

movw r4, #constant
bl __chkstk
sub.w sp, sp, r4

However, this has the limitation of 32 MiB (±16MiB). In order to accommodate larger binaries, LLVM supports the use of -mcmodel=large to allow a 4GiB range via a slight deviation. It will generate an indirect jump as follows:

movw r4, #constant
movw r12, :lower16:__chkstk
movt r12, :upper16:__chkstk
blx r12
sub.w sp, sp, r4

Variable Length Arrays ¶

The reference implementation (Microsoft Visual Studio 2012) does not permit the emission of Variable Length Arrays (VLAs).

The Windows ARM Itanium ABI extends the base ABI by adding support for emitting a dynamic stack allocation. When emitting a variable stack allocation, a call to __chkstk is emitted unconditionally to ensure that guard pages are setup properly. The emission of this stack probe emission is handled similar to the standard stack probe emission.

The MSVC environment does not emit code for VLAs currently.

Windows on ARM64 ¶

Stack Probe Emission ¶

The reference implementation (Microsoft Visual Studio 2017) emits stack probes in the following fashion:

mov x15, #constant
bl __chkstk
sub sp, sp, x15, lsl #4

However, this has the limitation of 256 MiB (±128MiB). In order to accommodate larger binaries, LLVM supports the use of -mcmodel=large to allow a 8GiB (±4GiB) range via a slight deviation. It will generate an indirect jump as follows:

mov x15, #constant
adrp x16, __chkstk
add x16, x16, :lo12:__chkstk
blr x16
sub sp, sp, x15, lsl #4