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Specifically, gc.result benefits from this greatly. Instead of: gc.result.int.* gc.result.float.* gc.result.ptr.* ... We now have a gc.result.* that can specialize to literally any type. Differential Revision: http://reviews.llvm.org/D7020 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226857 91177308-0d34-0410-b5e6-96231b3b80d8
406 lines
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ReStructuredText
406 lines
16 KiB
ReStructuredText
=====================================
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Garbage Collection Safepoints in LLVM
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=====================================
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.. contents::
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:local:
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:depth: 2
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Status
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=======
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This document describes a set of experimental extensions to LLVM. Use
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with caution. Because the intrinsics have experimental status,
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compatibility across LLVM releases is not guaranteed.
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LLVM currently supports an alternate mechanism for conservative
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garbage collection support using the gc_root intrinsic. The mechanism
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described here shares little in common with the alternate
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implementation and it is hoped that this mechanism will eventually
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replace the gc_root mechanism.
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Overview
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========
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To collect dead objects, garbage collectors must be able to identify
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any references to objects contained within executing code, and,
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depending on the collector, potentially update them. The collector
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does not need this information at all points in code - that would make
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the problem much harder - but only at well-defined points in the
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execution known as 'safepoints' For most collectors, it is sufficient
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to track at least one copy of each unique pointer value. However, for
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a collector which wishes to relocate objects directly reachable from
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running code, a higher standard is required.
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One additional challenge is that the compiler may compute intermediate
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results ("derived pointers") which point outside of the allocation or
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even into the middle of another allocation. The eventual use of this
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intermediate value must yield an address within the bounds of the
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allocation, but such "exterior derived pointers" may be visible to the
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collector. Given this, a garbage collector can not safely rely on the
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runtime value of an address to indicate the object it is associated
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with. If the garbage collector wishes to move any object, the
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compiler must provide a mapping, for each pointer, to an indication of
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its allocation.
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To simplify the interaction between a collector and the compiled code,
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most garbage collectors are organized in terms of three abstractions:
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load barriers, store barriers, and safepoints.
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#. A load barrier is a bit of code executed immediately after the
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machine load instruction, but before any use of the value loaded.
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Depending on the collector, such a barrier may be needed for all
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loads, merely loads of a particular type (in the original source
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language), or none at all.
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#. Analogously, a store barrier is a code fragement that runs
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immediately before the machine store instruction, but after the
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computation of the value stored. The most common use of a store
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barrier is to update a 'card table' in a generational garbage
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collector.
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#. A safepoint is a location at which pointers visible to the compiled
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code (i.e. currently in registers or on the stack) are allowed to
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change. After the safepoint completes, the actual pointer value
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may differ, but the 'object' (as seen by the source language)
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pointed to will not.
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Note that the term 'safepoint' is somewhat overloaded. It refers to
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both the location at which the machine state is parsable and the
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coordination protocol involved in bring application threads to a
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point at which the collector can safely use that information. The
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term "statepoint" as used in this document refers exclusively to the
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former.
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This document focuses on the last item - compiler support for
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safepoints in generated code. We will assume that an outside
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mechanism has decided where to place safepoints. From our
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perspective, all safepoints will be function calls. To support
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relocation of objects directly reachable from values in compiled code,
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the collector must be able to:
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#. identify every copy of a pointer (including copies introduced by
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the compiler itself) at the safepoint,
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#. identify which object each pointer relates to, and
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#. potentially update each of those copies.
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This document describes the mechanism by which an LLVM based compiler
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can provide this information to a language runtime/collector, and
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ensure that all pointers can be read and updated if desired. The
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heart of the approach is to construct (or rewrite) the IR in a manner
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where the possible updates performed by the garbage collector are
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explicitly visible in the IR. Doing so requires that we:
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#. create a new SSA value for each potentially relocated pointer, and
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ensure that no uses of the original (non relocated) value is
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reachable after the safepoint,
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#. specify the relocation in a way which is opaque to the compiler to
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ensure that the optimizer can not introduce new uses of an
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unrelocated value after a statepoint. This prevents the optimizer
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from performing unsound optimizations.
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#. recording a mapping of live pointers (and the allocation they're
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associated with) for each statepoint.
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At the most abstract level, inserting a safepoint can be thought of as
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replacing a call instruction with a call to a multiple return value
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function which both calls the original target of the call, returns
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it's result, and returns updated values for any live pointers to
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garbage collected objects.
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Note that the task of identifying all live pointers to garbage
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collected values, transforming the IR to expose a pointer giving the
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base object for every such live pointer, and inserting all the
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intrinsics correctly is explicitly out of scope for this document.
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The recommended approach is described in the section of Late
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Safepoint Placement below.
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This abstract function call is concretely represented by a sequence of
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intrinsic calls known as a 'statepoint sequence'.
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Let's consider a simple call in LLVM IR:
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todo
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Depending on our language we may need to allow a safepoint during the
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execution of the function called from this site. If so, we need to
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let the collector update local values in the current frame.
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Let's say we need to relocate SSA values 'a', 'b', and 'c' at this
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safepoint. To represent this, we would generate the statepoint
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sequence:
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todo
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Ideally, this sequence would have been represented as a M argument, N
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return value function (where M is the number of values being
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relocated + the original call arguments and N is the original return
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value + each relocated value), but LLVM does not easily support such a
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representation.
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Instead, the statepoint intrinsic marks the actual site of the
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safepoint or statepoint. The statepoint returns a token value (which
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exists only at compile time). To get back the original return value
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of the call, we use the 'gc.result' intrinsic. To get the relocation
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of each pointer in turn, we use the 'gc.relocate' intrinsic with the
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appropriate index. Note that both the gc.relocate and gc.result are
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tied to the statepoint. The combination forms a "statepoint sequence"
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and represents the entitety of a parseable call or 'statepoint'.
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When lowered, this example would generate the following x86 assembly::
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put assembly here
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Each of the potentially relocated values has been spilled to the
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stack, and a record of that location has been recorded to the
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:ref:`Stack Map section <stackmap-section>`. If the garbage collector
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needs to update any of these pointers during the call, it knows
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exactly what to change.
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Intrinsics
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===========
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'''gc.statepoint''' Intrinsic
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Syntax:
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"""""""
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::
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declare i32
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@gc.statepoint(func_type <target>, i64 <#call args>.
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i64 <unused>, ... (call parameters),
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i64 <# deopt args>, ... (deopt parameters),
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... (gc parameters))
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Overview:
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"""""""""
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The statepoint intrinsic represents a call which is parse-able by the
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runtime.
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Operands:
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"""""""""
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The 'target' operand is the function actually being called. The
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target can be specified as either a symbolic LLVM function, or as an
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arbitrary Value of appropriate function type. Note that the function
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type must match the signature of the callee and the types of the 'call
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parameters' arguments.
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The '#call args' operand is the number of arguments to the actual
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call. It must exactly match the number of arguments passed in the
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'call parameters' variable length section.
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The 'unused' operand is unused and likely to be removed. Please do
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not use.
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The 'call parameters' arguments are simply the arguments which need to
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be passed to the call target. They will be lowered according to the
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specified calling convention and otherwise handled like a normal call
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instruction. The number of arguments must exactly match what is
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specified in '# call args'. The types must match the signature of
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'target'.
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The 'deopt parameters' arguments contain an arbitrary list of Values
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which is meaningful to the runtime. The runtime may read any of these
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values, but is assumed not to modify them. If the garbage collector
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might need to modify one of these values, it must also be listed in
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the 'gc pointer' argument list. The '# deopt args' field indicates
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how many operands are to be interpreted as 'deopt parameters'.
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The 'gc parameters' arguments contain every pointer to a garbage
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collector object which potentially needs to be updated by the garbage
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collector. Note that the argument list must explicitly contain a base
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pointer for every derived pointer listed. The order of arguments is
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unimportant. Unlike the other variable length parameter sets, this
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list is not length prefixed.
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Semantics:
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""""""""""
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A statepoint is assumed to read and write all memory. As a result,
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memory operations can not be reordered past a statepoint. It is
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illegal to mark a statepoint as being either 'readonly' or 'readnone'.
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Note that legal IR can not perform any memory operation on a 'gc
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pointer' argument of the statepoint in a location statically reachable
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from the statepoint. Instead, the explicitly relocated value (from a
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''gc.relocate'') must be used.
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'''gc.result''' Intrinsic
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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Syntax:
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"""""""
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::
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declare type*
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@gc.result(i32 %statepoint_token)
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Overview:
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"""""""""
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'''gc.result''' extracts the result of the original call instruction
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which was replaced by the '''gc.statepoint'''. The '''gc.result'''
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intrinsic is actually a family of three intrinsics due to an
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implementation limitation. Other than the type of the return value,
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the semantics are the same.
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Operands:
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"""""""""
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The first and only argument is the '''gc.statepoint''' which starts
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the safepoint sequence of which this '''gc.result'' is a part.
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Despite the typing of this as a generic i32, *only* the value defined
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by a '''gc.statepoint''' is legal here.
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Semantics:
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""""""""""
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The ''gc.result'' represents the return value of the call target of
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the ''statepoint''. The type of the ''gc.result'' must exactly match
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the type of the target. If the call target returns void, there will
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be no ''gc.result''.
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A ''gc.result'' is modeled as a 'readnone' pure function. It has no
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side effects since it is just a projection of the return value of the
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previous call represented by the ''gc.statepoint''.
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'''gc.relocate''' Intrinsic
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Syntax:
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"""""""
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::
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declare <type> addrspace(1)*
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@gc.relocate(i32 %statepoint_token, i32 %base_offset, i32 %pointer_offset)
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Overview:
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"""""""""
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A ''gc.relocate'' returns the potentially relocated value of a pointer
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at the safepoint.
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Operands:
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"""""""""
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The first argument is the '''gc.statepoint''' which starts the
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safepoint sequence of which this '''gc.relocation'' is a part.
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Despite the typing of this as a generic i32, *only* the value defined
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by a '''gc.statepoint''' is legal here.
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The second argument is an index into the statepoints list of arguments
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which specifies the base pointer for the pointer being relocated.
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This index must land within the 'gc parameter' section of the
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statepoint's argument list.
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The third argument is an index into the statepoint's list of arguments
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which specify the (potentially) derived pointer being relocated. It
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is legal for this index to be the same as the second argument
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if-and-only-if a base pointer is being relocated. This index must land
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within the 'gc parameter' section of the statepoint's argument list.
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Semantics:
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""""""""""
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The return value of ''gc.relocate'' is the potentially relocated value
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of the pointer specified by it's arguments. It is unspecified how the
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value of the returned pointer relates to the argument to the
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''gc.statepoint'' other than that a) it points to the same source
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language object with the same offset, and b) the 'based-on'
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relationship of the newly relocated pointers is a projection of the
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unrelocated pointers. In particular, the integer value of the pointer
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returned is unspecified.
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A ''gc.relocate'' is modeled as a 'readnone' pure function. It has no
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side effects since it is just a way to extract information about work
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done during the actual call modeled by the ''gc.statepoint''.
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Stack Map Format
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================
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Locations for each pointer value which may need read and/or updated by
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the runtime or collector are provided via the :ref:`Stack Map format
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<stackmap-format>` specified in the PatchPoint documentation.
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Each statepoint generates the following Locations:
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* Constant which describes number of following deopt *Locations* (not
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operands)
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* Variable number of Locations, one for each deopt parameter listed in
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the IR statepoint (same number as described by previous Constant)
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* Variable number of Locations pairs, one pair for each unique pointer
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which needs relocated. The first Location in each pair describes
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the base pointer for the object. The second is the derived pointer
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actually being relocated. It is guaranteed that the base pointer
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must also appear explicitly as a relocation pair if used after the
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statepoint. There may be fewer pairs then gc parameters in the IR
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statepoint. Each *unique* pair will occur at least once; duplicates
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are possible.
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Note that the Locations used in each section may describe the same
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physical location. e.g. A stack slot may appear as a deopt location,
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a gc base pointer, and a gc derived pointer.
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The ID field of the 'StkMapRecord' for a statepoint is meaningless and
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it's value is explicitly unspecified.
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The LiveOut section of the StkMapRecord will be empty for a statepoint
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record.
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Safepoint Semantics & Verification
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==================================
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The fundamental correctness property for the compiled code's
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correctness w.r.t. the garbage collector is a dynamic one. It must be
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the case that there is no dynamic trace such that a operation
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involving a potentially relocated pointer is observably-after a
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safepoint which could relocate it. 'observably-after' is this usage
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means that an outside observer could observe this sequence of events
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in a way which precludes the operation being performed before the
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safepoint.
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To understand why this 'observable-after' property is required,
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consider a null comparison performed on the original copy of a
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relocated pointer. Assuming that control flow follows the safepoint,
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there is no way to observe externally whether the null comparison is
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performed before or after the safepoint. (Remember, the original
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Value is unmodified by the safepoint.) The compiler is free to make
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either scheduling choice.
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The actual correctness property implemented is slightly stronger than
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this. We require that there be no *static path* on which a
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potentially relocated pointer is 'observably-after' it may have been
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relocated. This is slightly stronger than is strictly necessary (and
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thus may disallow some otherwise valid programs), but greatly
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simplifies reasoning about correctness of the compiled code.
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By construction, this property will be upheld by the optimizer if
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correctly established in the source IR. This is a key invariant of
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the design.
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The existing IR Verifier pass has been extended to check most of the
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local restrictions on the intrinsics mentioned in their respective
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documentation. The current implementation in LLVM does not check the
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key relocation invariant, but this is ongoing work on developing such
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a verifier. Please ask on llvmdev if you're interested in
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experimenting with the current version.
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Bugs and Enhancements
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=====================
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Currently known bugs and enhancements under consideration can be
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tracked by performing a `bugzilla search
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<http://llvm.org/bugs/buglist.cgi?cmdtype=runnamed&namedcmd=Statepoint%20Bugs&list_id=64342>`_
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for [Statepoint] in the summary field. When filing new bugs, please
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use this tag so that interested parties see the newly filed bug. As
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with most LLVM features, design discussions take place on `llvmdev
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<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_, and patches
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should be sent to `llvm-commits
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<http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits>`_ for review.
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