mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-11-06 21:05:51 +00:00
fda8a1817d
When I originally committed the statepoint docs, I left placeholders for example IR fragments. I'm finally getting around to filling those in. I also added IR fragments to illustrate the usage of the PlaceSafepoints pass while I was at it. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230601 91177308-0d34-0410-b5e6-96231b3b80d8
581 lines
24 KiB
ReStructuredText
581 lines
24 KiB
ReStructuredText
=====================================
|
|
Garbage Collection Safepoints in LLVM
|
|
=====================================
|
|
|
|
.. contents::
|
|
:local:
|
|
:depth: 2
|
|
|
|
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 ``gcroot`` intrinsic. The mechanism
|
|
described here shares little in common with the alternate ``gcroot``
|
|
implementation and it is hoped that this mechanism will eventually
|
|
replace the gc_root mechanism.
|
|
|
|
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.
|
|
|
|
#. 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.
|
|
|
|
#. 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.
|
|
|
|
#. 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:
|
|
|
|
#. identify every copy of a pointer (including copies introduced by
|
|
the compiler itself) at the safepoint,
|
|
#. identify which object each pointer relates to, and
|
|
#. 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:
|
|
|
|
#. 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,
|
|
#. 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.
|
|
#. 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 to use the :ref:`utility passes
|
|
<statepoint-utilities>` described below.
|
|
|
|
This abstract function call is concretely represented by a sequence of
|
|
intrinsic calls known collectively as a "statepoint relocation sequence".
|
|
|
|
Let's consider a simple call in LLVM IR:
|
|
|
|
.. code-block:: llvm
|
|
|
|
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
|
|
gc "statepoint-example" {
|
|
call void ()* @foo()
|
|
ret i8 addrspace(1)* %obj
|
|
}
|
|
|
|
Depending on our language we may need to allow a safepoint during the execution
|
|
of ``foo``. If so, we need to let the collector update local values in the
|
|
current frame. If we don't, we'll be accessing a potential invalid reference
|
|
once we eventually return from the call.
|
|
|
|
In this example, we need to relocate the SSA value ``%obj``. Since we can't
|
|
actually change the value in the SSA value ``%obj``, we need to introduce a new
|
|
SSA value ``%obj.relocated`` which represents the potentially changed value of
|
|
``%obj`` after the safepoint and update any following uses appropriately. The
|
|
resulting relocation sequence is:
|
|
|
|
.. code-block:: llvm
|
|
|
|
define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj)
|
|
gc "statepoint-example" {
|
|
%0 = call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @foo, 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(i32 %0, i32 9, i32 9)
|
|
ret i8 addrspace(1)* %obj.relocated
|
|
}
|
|
|
|
Ideally, this sequence would have been represented as a M argument, N
|
|
return value function (where M is the number of values being
|
|
relocated + the original call arguments and N is the original return
|
|
value + each relocated value), but LLVM does not easily support such a
|
|
representation.
|
|
|
|
Instead, the statepoint intrinsic marks the actual site of the
|
|
safepoint or statepoint. The statepoint returns a token value (which
|
|
exists only at compile time). To get back the original return value
|
|
of the call, we use the ``gc.result`` intrinsic. To get the relocation
|
|
of each pointer in turn, we use the ``gc.relocate`` intrinsic with the
|
|
appropriate index. Note that both the ``gc.relocate`` and ``gc.result`` are
|
|
tied to the statepoint. The combination forms a "statepoint relocation
|
|
sequence" and represents the entitety of a parseable call or 'statepoint'.
|
|
|
|
When lowered, this example would generate the following x86 assembly:
|
|
|
|
.. code-block:: gas
|
|
|
|
.globl test1
|
|
.align 16, 0x90
|
|
pushq %rax
|
|
callq foo
|
|
.Ltmp1:
|
|
movq (%rsp), %rax # This load is redundant (oops!)
|
|
popq %rdx
|
|
retq
|
|
|
|
Each of the potentially relocated values has been spilled to the
|
|
stack, and a record of that location has been recorded to the
|
|
: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.
|
|
|
|
The relevant parts of the StackMap section for our example are:
|
|
|
|
.. code-block:: gas
|
|
|
|
# This describes the call site
|
|
# Stack Maps: callsite 2882400000
|
|
.quad 2882400000
|
|
.long .Ltmp1-test1
|
|
.short 0
|
|
# .. 8 entries skipped ..
|
|
# This entry describes the spill slot which is directly addressable
|
|
# off RSP with offset 0. Given the value was spilled with a pushq,
|
|
# that makes sense.
|
|
# Stack Maps: Loc 8: Direct RSP [encoding: .byte 2, .byte 8, .short 7, .int 0]
|
|
.byte 2
|
|
.byte 8
|
|
.short 7
|
|
.long 0
|
|
|
|
This example was taken from the tests for the :ref:`RewriteStatepointsForGC` utility pass. As such, it's full StackMap can be easily examined with the following command.
|
|
|
|
.. code-block:: bash
|
|
|
|
opt -rewrite-statepoints-for-gc test/Transforms/RewriteStatepointsForGC/basics.ll -S | llc -debug-only=stackmaps
|
|
|
|
|
|
|
|
|
|
|
|
Intrinsics
|
|
===========
|
|
|
|
'llvm.experimental.gc.statepoint' Intrinsic
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Syntax:
|
|
"""""""
|
|
|
|
::
|
|
|
|
declare i32
|
|
@llvm.experimental.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.
|
|
|
|
'llvm.experimental.gc.result' Intrinsic
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Syntax:
|
|
"""""""
|
|
|
|
::
|
|
|
|
declare type*
|
|
@llvm.experimental.gc.result(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``.
|
|
|
|
'llvm.experimental.gc.relocate' Intrinsic
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Syntax:
|
|
"""""""
|
|
|
|
::
|
|
|
|
declare <pointer type>
|
|
@llvm.experimental.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``.
|
|
|
|
.. _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 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.
|
|
|
|
.. _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 i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @foo, 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 i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @foo, 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(i32 %0, i32 9, i32 9)
|
|
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).
|
|
|
|
|
|
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 i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @do_safepoint, i32 0, i32 0, i32 0)
|
|
%safepoint_token1 = call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @foo, 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.
|
|
|
|
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
|
|
llvmdev mailing list for suggestions.
|
|
|
|
|
|
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 `llvmdev
|
|
<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_, and patches
|
|
should be sent to `llvm-commits
|
|
<http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits>`_ for review.
|
|
|