This document describes a set of experimental extensions to LLVM. Use with caution. Because the intrinsics have experimental status, compatibility across LLVM releases is not guaranteed.
LLVM currently supports an alternate mechanism for conservative garbage collection support using the gcroot intrinsic. The mechanism described here shares little in common with the alternate gcroot implementation and it is hoped that this mechanism will eventually replace the gc_root mechanism.
To collect dead objects, garbage collectors must be able to identify any references to objects contained within executing code, and, depending on the collector, potentially update them. The collector does not need this information at all points in code - that would make the problem much harder - but only at well-defined points in the execution known as ‘safepoints’ For most collectors, it is sufficient to track at least one copy of each unique pointer value. However, for a collector which wishes to relocate objects directly reachable from running code, a higher standard is required.
One additional challenge is that the compiler may compute intermediate results (“derived pointers”) which point outside of the allocation or even into the middle of another allocation. The eventual use of this intermediate value must yield an address within the bounds of the allocation, but such “exterior derived pointers” may be visible to the collector. Given this, a garbage collector can not safely rely on the runtime value of an address to indicate the object it is associated with. If the garbage collector wishes to move any object, the compiler must provide a mapping, for each pointer, to an indication of its allocation.
To simplify the interaction between a collector and the compiled code, most garbage collectors are organized in terms of three abstractions: load barriers, store barriers, and safepoints.
Note that the term ‘safepoint’ is somewhat overloaded. It refers to both the location at which the machine state is parsable and the coordination protocol involved in bring application threads to a point at which the collector can safely use that information. The term “statepoint” as used in this document refers exclusively to the former.
This document focuses on the last item - compiler support for safepoints in generated code. We will assume that an outside mechanism has decided where to place safepoints. From our perspective, all safepoints will be function calls. To support relocation of objects directly reachable from values in compiled code, the collector must be able to:
This document describes the mechanism by which an LLVM based compiler can provide this information to a language runtime/collector, and ensure that all pointers can be read and updated if desired. The heart of the approach is to construct (or rewrite) the IR in a manner where the possible updates performed by the garbage collector are explicitly visible in the IR. Doing so requires that we:
At the most abstract level, inserting a safepoint can be thought of as replacing a call instruction with a call to a multiple return value function which both calls the original target of the call, returns it’s result, and returns updated values for any live pointers to garbage collected objects.
Note that the task of identifying all live pointers to garbage collected values, transforming the IR to expose a pointer giving the base object for every such live pointer, and inserting all the intrinsics correctly is explicitly out of scope for this document. The recommended approach is to use the utility passes described below.
This abstract function call is concretely represented by a sequence of intrinsic calls known collectively as a “statepoint relocation sequence”.
Let’s consider a simple call in LLVM IR:
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
gc "statepoint-example" {
call void ()* @foo()
ret i8 addrspace(1)* %obj
}
Depending on our language we may need to allow a safepoint during the execution of foo. If so, we need to let the collector update local values in the current frame. If we don’t, we’ll be accessing a potential invalid reference once we eventually return from the call.
In this example, we need to relocate the SSA value %obj. Since we can’t actually change the value in the SSA value %obj, we need to introduce a new SSA value %obj.relocated which represents the potentially changed value of %obj after the safepoint and update any following uses appropriately. The resulting relocation sequence is:
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
gc "statepoint-example" {
%0 = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj)
%obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %0, i32 7, i32 7)
ret i8 addrspace(1)* %obj.relocated
}
Ideally, this sequence would have been represented as a M argument, N return value function (where M is the number of values being relocated + the original call arguments and N is the original return value + each relocated value), but LLVM does not easily support such a representation.
Instead, the statepoint intrinsic marks the actual site of the safepoint or statepoint. The statepoint returns a token value (which exists only at compile time). To get back the original return value of the call, we use the gc.result intrinsic. To get the relocation of each pointer in turn, we use the gc.relocate intrinsic with the appropriate index. Note that both the gc.relocate and gc.result are tied to the statepoint. The combination forms a “statepoint relocation sequence” and represents the entirety of a parseable call or ‘statepoint’.
When lowered, this example would generate the following x86 assembly:
.globl test1
.align 16, 0x90
pushq %rax
callq foo
.Ltmp1:
movq (%rsp), %rax # This load is redundant (oops!)
popq %rdx
retq
Each of the potentially relocated values has been spilled to the stack, and a record of that location has been recorded to the Stack Map section. If the garbage collector needs to update any of these pointers during the call, it knows exactly what to change.
The relevant parts of the StackMap section for our example are:
# This describes the call site
# Stack Maps: callsite 2882400000
.quad 2882400000
.long .Ltmp1-test1
.short 0
# .. 8 entries skipped ..
# This entry describes the spill slot which is directly addressable
# off RSP with offset 0. Given the value was spilled with a pushq,
# that makes sense.
# Stack Maps: Loc 8: Direct RSP [encoding: .byte 2, .byte 8, .short 7, .int 0]
.byte 2
.byte 8
.short 7
.long 0
This example was taken from the tests for the RewriteStatepointsForGC utility pass. As such, it’s full StackMap can be easily examined with the following command.
opt -rewrite-statepoints-for-gc test/Transforms/RewriteStatepointsForGC/basics.ll -S | llc -debug-only=stackmaps
A “base pointer” is one which points to the starting address of an allocation (object). A “derived pointer” is one which is offset from a base pointer by some amount. When relocating objects, a garbage collector needs to be able to relocate each derived pointer associated with an allocation to the same offset from the new address.
“Interior derived pointers” remain within the bounds of the allocation they’re associated with. As a result, the base object can be found at runtime provided the bounds of allocations are known to the runtime system.
“Exterior derived pointers” are outside the bounds of the associated object; they may even fall within another allocations address range. As a result, there is no way for a garbage collector to determine which allocation they are associated with at runtime and compiler support is needed.
The gc.relocate intrinsic supports an explicit operand for describing the allocation associated with a derived pointer. This operand is frequently referred to as the base operand, but does not strictly speaking have to be a base pointer, but it does need to lie within the bounds of the associated allocation. Some collectors may require that the operand be an actual base pointer rather than merely an internal derived pointer. Note that during lowering both the base and derived pointer operands are required to be live over the associated call safepoint even if the base is otherwise unused afterwards.
If we extend our previous example to include a pointless derived pointer, we get:
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
gc "statepoint-example" {
%gep = getelementptr i8, i8 addrspace(1)* %obj, i64 20000
%token = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj, i8 addrspace(1)* %gep)
%obj.relocated = call i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %token, i32 7, i32 7)
%gep.relocated = call i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %token, i32 7, i32 8)
%p = getelementptr i8, i8 addrspace(1)* %gep, i64 -20000
ret i8 addrspace(1)* %p
}
Note that in this example %p and %obj.relocate are the same address and we could replace one with the other, potentially removing the derived pointer from the live set at the safepoint entirely.
As a practical consideration, many garbage-collected systems allow code that is collector-aware (“managed code”) to call code that is not collector-aware (“unmanaged code”). It is common that such calls must also be safepoints, since it is desirable to allow the collector to run during the execution of unmanaged code. Futhermore, it is common that coordinating the transition from managed to unmanaged code requires extra code generation at the call site to inform the collector of the transition. In order to support these needs, a statepoint may be marked as a GC transition, and data that is necessary to perform the transition (if any) may be provided as additional arguments to the statepoint.
Note that although in many cases statepoints may be inferred to be GC transitions based on the function symbols involved (e.g. a call from a function with GC strategy “foo” to a function with GC strategy “bar”), indirect calls that are also GC transitions must also be supported. This requirement is the driving force behind the decision to require that GC transitions are explicitly marked.
Let’s revisit the sample given above, this time treating the call to @foo as a GC transition. Depending on our target, the transition code may need to access some extra state in order to inform the collector of the transition. Let’s assume a hypothetical GC–somewhat unimaginatively named “hypothetical-gc” –that requires that a TLS variable must be written to before and after a call to unmanaged code. The resulting relocation sequence is:
@flag = thread_local global i32 0, align 4
define i8 addrspace(1)* @test1(i8 addrspace(1) *%obj)
gc "hypothetical-gc" {
%0 = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 1, i32* @Flag, i32 0, i8 addrspace(1)* %obj)
%obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %0, i32 7, i32 7)
ret i8 addrspace(1)* %obj.relocated
}
During lowering, this will result in a instruction selection DAG that looks something like:
CALLSEQ_START
...
GC_TRANSITION_START (lowered i32 *@Flag), SRCVALUE i32* Flag
STATEPOINT
GC_TRANSITION_END (lowered i32 *@Flag), SRCVALUE i32 *Flag
...
CALLSEQ_END
In order to generate the necessary transition code, the backend for each target supported by “hypothetical-gc” must be modified to lower GC_TRANSITION_START and GC_TRANSITION_END nodes appropriately when the “hypothetical-gc” strategy is in use for a particular function. Assuming that such lowering has been added for X86, the generated assembly would be:
.globl test1
.align 16, 0x90
pushq %rax
movl $1, %fs:Flag@TPOFF
callq foo
movl $0, %fs:Flag@TPOFF
.Ltmp1:
movq (%rsp), %rax # This load is redundant (oops!)
popq %rdx
retq
Note that the design as presented above is not fully implemented: in particular, strategy-specific lowering is not present, and all GC transitions are emitted as as single no-op before and after the call instruction. These no-ops are often removed by the backend during dead machine instruction elimination.
declare token
@llvm.experimental.gc.statepoint(i64 <id>, i32 <num patch bytes>,
func_type <target>,
i64 <#call args>, i64 <flags>,
... (call parameters),
i64 <# transition args>, ... (transition parameters),
i64 <# deopt args>, ... (deopt parameters),
... (gc parameters))
The statepoint intrinsic represents a call which is parse-able by the runtime.
The ‘id’ operand is a constant integer that is reported as the ID field in the generated stackmap. LLVM does not interpret this parameter in any way and its meaning is up to the statepoint user to decide. Note that LLVM is free to duplicate code containing statepoint calls, and this may transform IR that had a unique ‘id’ per lexical call to statepoint to IR that does not.
If ‘num patch bytes’ is non-zero then the call instruction corresponding to the statepoint is not emitted and LLVM emits ‘num patch bytes’ bytes of nops in its place. LLVM will emit code to prepare the function arguments and retrieve the function return value in accordance to the calling convention; the former before the nop sequence and the latter after the nop sequence. It is expected that the user will patch over the ‘num patch bytes’ bytes of nops with a calling sequence specific to their runtime before executing the generated machine code. There are no guarantees with respect to the alignment of the nop sequence. Unlike Stack maps and patch points in LLVM statepoints do not have a concept of shadow bytes. Note that semantically the statepoint still represents a call or invoke to ‘target’, and the nop sequence after patching is expected to represent an operation equivalent to a call or invoke to ‘target’.
The ‘target’ operand is the function actually being called. The target can be specified as either a symbolic LLVM function, or as an arbitrary Value of appropriate function type. Note that the function type must match the signature of the callee and the types of the ‘call parameters’ arguments.
The ‘#call args’ operand is the number of arguments to the actual call. It must exactly match the number of arguments passed in the ‘call parameters’ variable length section.
The ‘flags’ operand is used to specify extra information about the statepoint. This is currently only used to mark certain statepoints as GC transitions. This operand is a 64-bit integer with the following layout, where bit 0 is the least significant bit:
Bit # Usage 0 Set if the statepoint is a GC transition, cleared otherwise. 1-63 Reserved for future use; must be cleared.
The ‘call parameters’ arguments are simply the arguments which need to be passed to the call target. They will be lowered according to the specified calling convention and otherwise handled like a normal call instruction. The number of arguments must exactly match what is specified in ‘# call args’. The types must match the signature of ‘target’.
The ‘transition parameters’ arguments contain an arbitrary list of Values which need to be passed to GC transition code. They will be lowered and passed as operands to the appropriate GC_TRANSITION nodes in the selection DAG. It is assumed that these arguments must be available before and after (but not necessarily during) the execution of the callee. The ‘# transition args’ field indicates how many operands are to be interpreted as ‘transition parameters’.
The ‘deopt parameters’ arguments contain an arbitrary list of Values which is meaningful to the runtime. The runtime may read any of these values, but is assumed not to modify them. If the garbage collector might need to modify one of these values, it must also be listed in the ‘gc pointer’ argument list. The ‘# deopt args’ field indicates how many operands are to be interpreted as ‘deopt parameters’.
The ‘gc parameters’ arguments contain every pointer to a garbage collector object which potentially needs to be updated by the garbage collector. Note that the argument list must explicitly contain a base pointer for every derived pointer listed. The order of arguments is unimportant. Unlike the other variable length parameter sets, this list is not length prefixed.
A statepoint is assumed to read and write all memory. As a result, memory operations can not be reordered past a statepoint. It is illegal to mark a statepoint as being either ‘readonly’ or ‘readnone’.
Note that legal IR can not perform any memory operation on a ‘gc pointer’ argument of the statepoint in a location statically reachable from the statepoint. Instead, the explicitly relocated value (from a gc.relocate) must be used.
declare type*
@llvm.experimental.gc.result(token %statepoint_token)
gc.result extracts the result of the original call instruction which was replaced by the gc.statepoint. The gc.result intrinsic is actually a family of three intrinsics due to an implementation limitation. Other than the type of the return value, the semantics are the same.
The first and only argument is the gc.statepoint which starts the safepoint sequence of which this gc.result is a part. Despite the typing of this as a generic token, only the value defined by a gc.statepoint is legal here.
The gc.result represents the return value of the call target of the statepoint. The type of the gc.result must exactly match the type of the target. If the call target returns void, there will be no gc.result.
A gc.result is modeled as a ‘readnone’ pure function. It has no side effects since it is just a projection of the return value of the previous call represented by the gc.statepoint.
declare <pointer type>
@llvm.experimental.gc.relocate(token %statepoint_token,
i32 %base_offset,
i32 %pointer_offset)
A gc.relocate returns the potentially relocated value of a pointer at the safepoint.
The first argument is the gc.statepoint which starts the safepoint sequence of which this gc.relocation is a part. Despite the typing of this as a generic token, only the value defined by a gc.statepoint is legal here.
The second argument is an index into the statepoints list of arguments which specifies the allocation for the pointer being relocated. This index must land within the ‘gc parameter’ section of the statepoint’s argument list. The associated value must be within the object with which the pointer being relocated is associated. The optimizer is free to change which interior derived pointer is reported, provided that it does not replace an actual base pointer with another interior derived pointer. Collectors are allowed to rely on the base pointer operand remaining an actual base pointer if so constructed.
The third argument is an index into the statepoint’s list of arguments which specify the (potentially) derived pointer being relocated. It is legal for this index to be the same as the second argument if-and-only-if a base pointer is being relocated. This index must land within the ‘gc parameter’ section of the statepoint’s argument list.
The return value of gc.relocate is the potentially relocated value of the pointer specified by it’s arguments. It is unspecified how the value of the returned pointer relates to the argument to the gc.statepoint other than that a) it points to the same source language object with the same offset, and b) the ‘based-on’ relationship of the newly relocated pointers is a projection of the unrelocated pointers. In particular, the integer value of the pointer returned is unspecified.
A gc.relocate is modeled as a readnone pure function. It has no side effects since it is just a way to extract information about work done during the actual call modeled by the gc.statepoint.
Locations for each pointer value which may need read and/or updated by the runtime or collector are provided via the Stack Map format specified in the PatchPoint documentation.
Each statepoint generates the following Locations:
Note that the Locations used in each section may describe the same physical location. e.g. A stack slot may appear as a deopt location, a gc base pointer, and a gc derived pointer.
The LiveOut section of the StkMapRecord will be empty for a statepoint record.
The fundamental correctness property for the compiled code’s correctness w.r.t. the garbage collector is a dynamic one. It must be the case that there is no dynamic trace such that a operation involving a potentially relocated pointer is observably-after a safepoint which could relocate it. ‘observably-after’ is this usage means that an outside observer could observe this sequence of events in a way which precludes the operation being performed before the safepoint.
To understand why this ‘observable-after’ property is required, consider a null comparison performed on the original copy of a relocated pointer. Assuming that control flow follows the safepoint, there is no way to observe externally whether the null comparison is performed before or after the safepoint. (Remember, the original Value is unmodified by the safepoint.) The compiler is free to make either scheduling choice.
The actual correctness property implemented is slightly stronger than this. We require that there be no static path on which a potentially relocated pointer is ‘observably-after’ it may have been relocated. This is slightly stronger than is strictly necessary (and thus may disallow some otherwise valid programs), but greatly simplifies reasoning about correctness of the compiled code.
By construction, this property will be upheld by the optimizer if correctly established in the source IR. This is a key invariant of the design.
The existing IR Verifier pass has been extended to check most of the local restrictions on the intrinsics mentioned in their respective documentation. The current implementation in LLVM does not check the key relocation invariant, but this is ongoing work on developing such a verifier. Please ask on llvm-dev if you’re interested in experimenting with the current version.
The pass RewriteStatepointsForGC transforms a functions IR by replacing a gc.statepoint (with an optional gc.result) with a full relocation sequence, including all required gc.relocates. To function, the pass requires that the GC strategy specified for the function be able to reliably distinguish between GC references and non-GC references in IR it is given.
As an example, given this code:
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
gc "statepoint-example" {
call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0)
ret i8 addrspace(1)* %obj
}
The pass would produce this IR:
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
gc "statepoint-example" {
%0 = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj)
%obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %0, i32 12, i32 12)
ret i8 addrspace(1)* %obj.relocated
}
In the above examples, the addrspace(1) marker on the pointers is the mechanism that the statepoint-example GC strategy uses to distinguish references from non references. Address space 1 is not globally reserved for this purpose.
This pass can be used an utility function by a language frontend that doesn’t want to manually reason about liveness, base pointers, or relocation when constructing IR. As currently implemented, RewriteStatepointsForGC must be run after SSA construction (i.e. mem2ref).
RewriteStatepointsForGC will ensure that appropriate base pointers are listed for every relocation created. It will do so by duplicating code as needed to propagate the base pointer associated with each pointer being relocated to the appropriate safepoints. The implementation assumes that the following IR constructs produce base pointers: loads from the heap, addresses of global variables, function arguments, function return values. Constant pointers (such as null) are also assumed to be base pointers. In practice, this constraint can be relaxed to producing interior derived pointers provided the target collector can find the associated allocation from an arbitrary interior derived pointer.
In practice, RewriteStatepointsForGC can be run much later in the pass pipeline, after most optimization is already done. This helps to improve the quality of the generated code when compiled with garbage collection support. In the long run, this is the intended usage model. At this time, a few details have yet to be worked out about the semantic model required to guarantee this is always correct. As such, please use with caution and report bugs.
The pass PlaceSafepoints transforms a function’s IR by replacing any call or invoke instructions with appropriate gc.statepoint and gc.result pairs, and inserting safepoint polls sufficient to ensure running code checks for a safepoint request on a timely manner. This pass is expected to be run before RewriteStatepointsForGC and thus does not produce full relocation sequences.
As an example, given input IR of the following:
define void @test() gc "statepoint-example" {
call void @foo()
ret void
}
declare void @do_safepoint()
define void @gc.safepoint_poll() {
call void @do_safepoint()
ret void
}
This pass would produce the following IR:
define void @test() gc "statepoint-example" {
%safepoint_token = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @do_safepoint, i32 0, i32 0, i32 0, i32 0)
%safepoint_token1 = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0)
ret void
}
In this case, we’ve added an (unconditional) entry safepoint poll and converted the call into a gc.statepoint. Note that despite appearances, the entry poll is not necessarily redundant. We’d have to know that foo and test were not mutually recursive for the poll to be redundant. In practice, you’d probably want to your poll definition to contain a conditional branch of some form.
At the moment, PlaceSafepoints can insert safepoint polls at method entry and loop backedges locations. Extending this to work with return polls would be straight forward if desired.
PlaceSafepoints includes a number of optimizations to avoid placing safepoint polls at particular sites unless needed to ensure timely execution of a poll under normal conditions. PlaceSafepoints does not attempt to ensure timely execution of a poll under worst case conditions such as heavy system paging.
The implementation of a safepoint poll action is specified by looking up a function of the name gc.safepoint_poll in the containing Module. The body of this function is inserted at each poll site desired. While calls or invokes inside this method are transformed to a gc.statepoints, recursive poll insertion is not performed.
By default PlaceSafepoints passes in 0xABCDEF00 as the statepoint ID and 0 as the number of patchable bytes to the newly constructed gc.statepoint. These values can be configured on a per-callsite basis using the attributes "statepoint-id" and "statepoint-num-patch-bytes". If a call site is marked with a "statepoint-id" function attribute and its value is a positive integer (represented as a string), then that value is used as the ID of the newly constructed gc.statepoint. If a call site is marked with a "statepoint-num-patch-bytes" function attribute and its value is a positive integer, then that value is used as the ‘num patch bytes’ parameter of the newly constructed gc.statepoint. The "statepoint-id" and "statepoint-num-patch-bytes" attributes are not propagated to the gc.statepoint call or invoke if they could be successfully parsed.
If you are scheduling the RewriteStatepointsForGC pass late in the pass order, you should probably schedule this pass immediately before it. The exception would be if you need to preserve abstract frame information (e.g. for deoptimization or introspection) at safepoints. In that case, ask on the llvm-dev mailing list for suggestions.
Support for statepoint generation requires some code for each backend. Today, only X86_64 is supported.
Currently known bugs and enhancements under consideration can be tracked by performing a bugzilla search for [Statepoint] in the summary field. When filing new bugs, please use this tag so that interested parties see the newly filed bug. As with most LLVM features, design discussions take place on llvm-dev, and patches should be sent to llvm-commits for review.