mirror of
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955318d58d
Summary: This patch changes gc.statepoint intrinsic's return type to token type instead of i32 type. Using token types could prevent LLVM to merge different gc.statepoint nodes into PHI nodes and cause further problems with gc relocations. The patch also changes the way on how gc.relocate and gc.result look for their corresponding gc.statepoint on unwind path. The current implementation uses the selector value extracted from a { i8*, i32 } landingpad as a hook to find the gc.statepoint, while the patch directly uses a token type landingpad (http://reviews.llvm.org/D15405) to find the gc.statepoint.
Reviewers: sanjoy, JosephTremoulet, pgavlin, igor-laevsky, mjacob
Subscribers: reames, mjacob, sanjoy, llvm-commits
Differential Revision: http://reviews.llvm.org/D15662
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256443 91177308-0d34-0410-b5e6-96231b3b80d8
784 lines
34 KiB
ReStructuredText
784 lines
34 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 ``gcroot`` intrinsic. The mechanism
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described here shares little in common with the alternate ``gcroot``
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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 fragment 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 to use the :ref:`utility passes
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<statepoint-utilities>` described below.
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This abstract function call is concretely represented by a sequence of
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intrinsic calls known collectively as a "statepoint relocation sequence".
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Let's consider a simple call in LLVM IR:
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.. code-block:: llvm
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define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
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gc "statepoint-example" {
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call void ()* @foo()
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ret i8 addrspace(1)* %obj
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}
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Depending on our language we may need to allow a safepoint during the execution
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of ``foo``. If so, we need to let the collector update local values in the
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current frame. If we don't, we'll be accessing a potential invalid reference
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once we eventually return from the call.
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In this example, we need to relocate the SSA value ``%obj``. Since we can't
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actually change the value in the SSA value ``%obj``, we need to introduce a new
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SSA value ``%obj.relocated`` which represents the potentially changed value of
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``%obj`` after the safepoint and update any following uses appropriately. The
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resulting relocation sequence is:
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.. code-block:: llvm
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define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
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gc "statepoint-example" {
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%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)
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%obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %0, i32 7, i32 7)
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ret i8 addrspace(1)* %obj.relocated
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}
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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 relocation
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sequence" and represents the entirety of a parseable call or 'statepoint'.
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When lowered, this example would generate the following x86 assembly:
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.. code-block:: gas
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.globl test1
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.align 16, 0x90
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pushq %rax
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callq foo
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.Ltmp1:
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movq (%rsp), %rax # This load is redundant (oops!)
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popq %rdx
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retq
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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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The relevant parts of the StackMap section for our example are:
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.. code-block:: gas
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# This describes the call site
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# Stack Maps: callsite 2882400000
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.quad 2882400000
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.long .Ltmp1-test1
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.short 0
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# .. 8 entries skipped ..
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# This entry describes the spill slot which is directly addressable
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# off RSP with offset 0. Given the value was spilled with a pushq,
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# that makes sense.
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# Stack Maps: Loc 8: Direct RSP [encoding: .byte 2, .byte 8, .short 7, .int 0]
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.byte 2
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.byte 8
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.short 7
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.long 0
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This example was taken from the tests for the :ref:`RewriteStatepointsForGC` utility pass. As such, it's full StackMap can be easily examined with the following command.
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.. code-block:: bash
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opt -rewrite-statepoints-for-gc test/Transforms/RewriteStatepointsForGC/basics.ll -S | llc -debug-only=stackmaps
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Base & Derived Pointers
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^^^^^^^^^^^^^^^^^^^^^^^
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A "base pointer" is one which points to the starting address of an allocation
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(object). A "derived pointer" is one which is offset from a base pointer by
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some amount. When relocating objects, a garbage collector needs to be able
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to relocate each derived pointer associated with an allocation to the same
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offset from the new address.
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"Interior derived pointers" remain within the bounds of the allocation
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they're associated with. As a result, the base object can be found at
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runtime provided the bounds of allocations are known to the runtime system.
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"Exterior derived pointers" are outside the bounds of the associated object;
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they may even fall within *another* allocations address range. As a result,
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there is no way for a garbage collector to determine which allocation they
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are associated with at runtime and compiler support is needed.
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The ``gc.relocate`` intrinsic supports an explicit operand for describing the
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allocation associated with a derived pointer. This operand is frequently
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referred to as the base operand, but does not strictly speaking have to be
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a base pointer, but it does need to lie within the bounds of the associated
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allocation. Some collectors may require that the operand be an actual base
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pointer rather than merely an internal derived pointer. Note that during
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lowering both the base and derived pointer operands are required to be live
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over the associated call safepoint even if the base is otherwise unused
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afterwards.
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If we extend our previous example to include a pointless derived pointer,
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we get:
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.. code-block:: llvm
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define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
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gc "statepoint-example" {
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%gep = getelementptr i8, i8 addrspace(1)* %obj, i64 20000
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%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)
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%obj.relocated = call i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %token, i32 7, i32 7)
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%gep.relocated = call i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %token, i32 7, i32 8)
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%p = getelementptr i8, i8 addrspace(1)* %gep, i64 -20000
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ret i8 addrspace(1)* %p
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}
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Note that in this example %p and %obj.relocate are the same address and we
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could replace one with the other, potentially removing the derived pointer
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from the live set at the safepoint entirely.
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GC Transitions
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^^^^^^^^^^^^^^^^^^
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As a practical consideration, many garbage-collected systems allow code that is
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collector-aware ("managed code") to call code that is not collector-aware
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("unmanaged code"). It is common that such calls must also be safepoints, since
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it is desirable to allow the collector to run during the execution of
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unmanaged code. Futhermore, it is common that coordinating the transition from
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managed to unmanaged code requires extra code generation at the call site to
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inform the collector of the transition. In order to support these needs, a
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statepoint may be marked as a GC transition, and data that is necessary to
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perform the transition (if any) may be provided as additional arguments to the
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statepoint.
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Note that although in many cases statepoints may be inferred to be GC
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transitions based on the function symbols involved (e.g. a call from a
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function with GC strategy "foo" to a function with GC strategy "bar"),
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indirect calls that are also GC transitions must also be supported. This
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requirement is the driving force behind the decision to require that GC
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transitions are explicitly marked.
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Let's revisit the sample given above, this time treating the call to ``@foo``
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as a GC transition. Depending on our target, the transition code may need to
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access some extra state in order to inform the collector of the transition.
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Let's assume a hypothetical GC--somewhat unimaginatively named "hypothetical-gc"
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--that requires that a TLS variable must be written to before and after a call
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to unmanaged code. The resulting relocation sequence is:
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.. code-block:: llvm
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@flag = thread_local global i32 0, align 4
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define i8 addrspace(1)* @test1(i8 addrspace(1) *%obj)
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gc "hypothetical-gc" {
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%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)
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%obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %0, i32 7, i32 7)
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ret i8 addrspace(1)* %obj.relocated
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}
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During lowering, this will result in a instruction selection DAG that looks
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something like:
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::
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CALLSEQ_START
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...
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GC_TRANSITION_START (lowered i32 *@Flag), SRCVALUE i32* Flag
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STATEPOINT
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GC_TRANSITION_END (lowered i32 *@Flag), SRCVALUE i32 *Flag
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...
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CALLSEQ_END
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In order to generate the necessary transition code, the backend for each target
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supported by "hypothetical-gc" must be modified to lower ``GC_TRANSITION_START``
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and ``GC_TRANSITION_END`` nodes appropriately when the "hypothetical-gc"
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strategy is in use for a particular function. Assuming that such lowering has
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been added for X86, the generated assembly would be:
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.. code-block:: gas
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.globl test1
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.align 16, 0x90
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pushq %rax
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movl $1, %fs:Flag@TPOFF
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callq foo
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movl $0, %fs:Flag@TPOFF
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.Ltmp1:
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movq (%rsp), %rax # This load is redundant (oops!)
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popq %rdx
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retq
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Note that the design as presented above is not fully implemented: in particular,
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strategy-specific lowering is not present, and all GC transitions are emitted as
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as single no-op before and after the call instruction. These no-ops are often
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removed by the backend during dead machine instruction elimination.
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Intrinsics
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===========
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'llvm.experimental.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 token
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@llvm.experimental.gc.statepoint(i64 <id>, i32 <num patch bytes>,
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func_type <target>,
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i64 <#call args>, i64 <flags>,
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... (call parameters),
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i64 <# transition args>, ... (transition 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 'id' operand is a constant integer that is reported as the ID
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field in the generated stackmap. LLVM does not interpret this
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parameter in any way and its meaning is up to the statepoint user to
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decide. Note that LLVM is free to duplicate code containing
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statepoint calls, and this may transform IR that had a unique 'id' per
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lexical call to statepoint to IR that does not.
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If 'num patch bytes' is non-zero then the call instruction
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corresponding to the statepoint is not emitted and LLVM emits 'num
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patch bytes' bytes of nops in its place. LLVM will emit code to
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prepare the function arguments and retrieve the function return value
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in accordance to the calling convention; the former before the nop
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sequence and the latter after the nop sequence. It is expected that
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the user will patch over the 'num patch bytes' bytes of nops with a
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calling sequence specific to their runtime before executing the
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generated machine code. There are no guarantees with respect to the
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alignment of the nop sequence. Unlike :doc:`StackMaps` statepoints do
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not have a concept of shadow bytes. Note that semantically the
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statepoint still represents a call or invoke to 'target', and the nop
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sequence after patching is expected to represent an operation
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equivalent to a call or invoke to 'target'.
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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 'flags' operand is used to specify extra information about the
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statepoint. This is currently only used to mark certain statepoints
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as GC transitions. This operand is a 64-bit integer with the following
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layout, where bit 0 is the least significant bit:
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+-------+---------------------------------------------------+
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| Bit # | Usage |
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+=======+===================================================+
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| 0 | Set if the statepoint is a GC transition, cleared |
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| | otherwise. |
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+-------+---------------------------------------------------+
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| 1-63 | Reserved for future use; must be cleared. |
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+-------+---------------------------------------------------+
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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 'transition parameters' arguments contain an arbitrary list of
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Values which need to be passed to GC transition code. They will be
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lowered and passed as operands to the appropriate GC_TRANSITION nodes
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in the selection DAG. It is assumed that these arguments must be
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available before and after (but not necessarily during) the execution
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of the callee. The '# transition args' field indicates how many operands
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are to be interpreted as 'transition parameters'.
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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
|
|
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.
|
|
|
|
'llvm.experimental.gc.result' Intrinsic
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Syntax:
|
|
"""""""
|
|
|
|
::
|
|
|
|
declare type*
|
|
@llvm.experimental.gc.result(token %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 token, *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``.
|
|
|
|
'llvm.experimental.gc.relocate' Intrinsic
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Syntax:
|
|
"""""""
|
|
|
|
::
|
|
|
|
declare <pointer type>
|
|
@llvm.experimental.gc.relocate(token %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 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.
|
|
|
|
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``.
|
|
|
|
.. _statepoint-stackmap-format:
|
|
|
|
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 the calling convention of the call target. This
|
|
constant is a valid :ref:`calling convention identifier <callingconv>` for
|
|
the version of LLVM used to generate the stackmap. No additional compatibility
|
|
guarantees are made for this constant over what LLVM provides elsewhere w.r.t.
|
|
these identifiers.
|
|
* Constant which describes the flags passed to the statepoint intrinsic
|
|
* 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 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 llvm-dev if you're interested in
|
|
experimenting with the current version.
|
|
|
|
.. _statepoint-utilities:
|
|
|
|
Utility Passes for Safepoint Insertion
|
|
======================================
|
|
|
|
.. _RewriteStatepointsForGC:
|
|
|
|
RewriteStatepointsForGC
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
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:
|
|
|
|
.. code-block:: llvm
|
|
|
|
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:
|
|
|
|
.. code-block:: llvm
|
|
|
|
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.
|
|
|
|
.. _PlaceSafepoints:
|
|
|
|
PlaceSafepoints
|
|
^^^^^^^^^^^^^^^^
|
|
|
|
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:
|
|
|
|
.. code-block:: llvm
|
|
|
|
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:
|
|
|
|
.. code-block:: llvm
|
|
|
|
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.
|
|
|
|
|
|
Supported Architectures
|
|
=======================
|
|
|
|
Support for statepoint generation requires some code for each backend.
|
|
Today, only X86_64 is supported.
|
|
|
|
Bugs and Enhancements
|
|
=====================
|
|
|
|
Currently known bugs and enhancements under consideration can be
|
|
tracked by performing a `bugzilla search
|
|
<http://llvm.org/bugs/buglist.cgi?cmdtype=runnamed&namedcmd=Statepoint%20Bugs&list_id=64342>`_
|
|
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
|
|
<http://lists.llvm.org/mailman/listinfo/llvm-dev>`_, and patches
|
|
should be sent to `llvm-commits
|
|
<http://lists.llvm.org/mailman/listinfo/llvm-commits>`_ for review.
|
|
|