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Garbage Collection Safepoints in LLVM

Status

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 gc_root intrinsic. The mechanism described here shares little in common with the alternate implementation and it is hoped that this mechanism will eventually replace the gc_root mechanism.

Overview

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.

  1. A load barrier is a bit of code executed immediately after the machine load instruction, but before any use of the value loaded. Depending on the collector, such a barrier may be needed for all loads, merely loads of a particular type (in the original source language), or none at all.
  2. Analogously, a store barrier is a code fragement that runs immediately before the machine store instruction, but after the computation of the value stored. The most common use of a store barrier is to update a 'card table' in a generational garbage collector.
  3. A safepoint is a location at which pointers visible to the compiled code (i.e. currently in registers or on the stack) are allowed to change. After the safepoint completes, the actual pointer value may differ, but the 'object' (as seen by the source language) pointed to will not.
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:

  1. identify every copy of a pointer (including copies introduced by the compiler itself) at the safepoint,
  2. identify which object each pointer relates to, and
  3. potentially update each of those copies.

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:

  1. create a new SSA value for each potentially relocated pointer, and ensure that no uses of the original (non relocated) value is reachable after the safepoint,
  2. specify the relocation in a way which is opaque to the compiler to ensure that the optimizer can not introduce new uses of an unrelocated value after a statepoint. This prevents the optimizer from performing unsound optimizations.
  3. recording a mapping of live pointers (and the allocation they're associated with) for each statepoint.

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 described in the section of Late Safepoint Placement below.

This abstract function call is concretely represented by a sequence of intrinsic calls known as a 'statepoint sequence'.

Let's consider a simple call in LLVM IR:
todo

Depending on our language we may need to allow a safepoint during the execution of the function called from this site. If so, we need to let the collector update local values in the current frame.

Let's say we need to relocate SSA values 'a', 'b', and 'c' at this safepoint. To represent this, we would generate the statepoint sequence:

todo

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 sequence" and represents the entitety of a parseable call or 'statepoint'.

When lowered, this example would generate the following x86 assembly::
put assembly here

Each of the potentially relocated values has been spilled to the stack, and a record of that location has been recorded to the :ref:`Stack Map section <stackmap-section>`. If the garbage collector needs to update any of these pointers during the call, it knows exactly what to change.

Intrinsics

'''gc.statepoint''' Intrinsic

Syntax:

declare i32
  @gc.statepoint(func_type <target>, i64 <#call args>.
                 i64 <unused>, ... (call parameters),
                 i64 <# deopt args>, ... (deopt parameters),
                 ... (gc parameters))

Overview:

The statepoint intrinsic represents a call which is parse-able by the runtime.

Operands:

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 'unused' operand is unused and likely to be removed. Please do not use.

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 '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.

Semantics:

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.

'''gc.result''' Intrinsic

Syntax:

declare type*
  @gc.result_ptr(i32 %statepoint_token)

declare fX
  @gc.result_float(i32 %statepoint_token)

declare iX
  @gc.result_int(i32 %statepoint_token)

Overview:

'''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.

Operands:

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 i32, only the value defined by a '''gc.statepoint''' is legal here.

Semantics:

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''.

'''gc.relocate''' Intrinsic

Syntax:

declare <type> addrspace(1)*
  @gc.relocate(i32 %statepoint_token, i32 %base_offset, i32 %pointer_offset)

Overview:

A ''gc.relocate'' returns the potentially relocated value of a pointer at the safepoint.

Operands:

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 i32, 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 base pointer for the pointer being relocated. This index must land within the 'gc parameter' section of the statepoint's argument list.

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.

Semantics:

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''.

Stack Map Format

Locations for each pointer value which may need read and/or updated by the runtime or collector are provided via the :ref:`Stack Map format <stackmap-format>` specified in the PatchPoint documentation.

Each statepoint generates the following Locations:

  • Constant which describes number of following deopt Locations (not operands)
  • Variable number of Locations, one for each deopt parameter listed in the IR statepoint (same number as described by previous Constant)
  • Variable number of Locations pairs, one pair for each unique pointer which needs relocated. The first Location in each pair describes the base pointer for the object. The second is the derived pointer actually being relocated. It is guaranteed that the base pointer must also appear explicitly as a relocation pair if used after the statepoint. There may be fewer pairs then gc parameters in the IR statepoint. Each unique pair will occur at least once; duplicates are possible.

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 ID field of the 'StkMapRecord' for a statepoint is meaningless and it's value is explicitly unspecified.

The LiveOut section of the StkMapRecord will be empty for a statepoint record.

Safepoint Semantics & Verification

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 llvmdev if you're interested in experimenting with the current version.

Bugs and Enhancements

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 llvmdev, and patches should be sent to llvm-commits for review.