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)
+ %0 = call i32 (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj)
+ %obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(i32 %0, i32 7, i32 7)
ret i8 addrspace(1)* %obj.relocated
}
opt -rewrite-statepoints-for-gc test/Transforms/RewriteStatepointsForGC/basics.ll -S | llc -debug-only=stackmaps
+GC Transitions
+^^^^^^^^^^^^^^^^^^
+
+As a practical consideration, many garbage-collected systems allow code that is
+collector-aware ("managed code") to call code that is not collector-aware
+("unmanaged code"). It is common that such calls must also be safepoints, since
+it is desirable to allow the collector to run during the execution of
+unmanaged code. Futhermore, it is common that coordinating the transition from
+managed to unmanaged code requires extra code generation at the call site to
+inform the collector of the transition. In order to support these needs, a
+statepoint may be marked as a GC transition, and data that is necessary to
+perform the transition (if any) may be provided as additional arguments to the
+statepoint.
+
+ Note that although in many cases statepoints may be inferred to be GC
+ transitions based on the function symbols involved (e.g. a call from a
+ function with GC strategy "foo" to a function with GC strategy "bar"),
+ indirect calls that are also GC transitions must also be supported. This
+ requirement is the driving force behing the decision to require that GC
+ transitions are explicitly marked.
+
+Let's revisit the sample given above, this time treating the call to ``@foo``
+as a GC transition. Depending on our target, the transition code may need to
+access some extra state in order to inform the collector of the transition.
+Let's assume a hypothetical GC--somewhat unimaginatively named "hypothetical-gc"
+--that requires that a TLS variable must be written to before and after a call
+to unmanaged code. The resulting relocation sequence is:
+.. code-block:: llvm
+
+ @flag = thread_local global i32 0, align 4
+
+ define i8 addrspace(1)* @test1(i8 addrspace(1) *%obj)
+ gc "hypothetical-gc" {
+
+ %0 = call i32 (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 1, i32* @Flag, i32 0, i8 addrspace(1)* %obj)
+ %obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(i32 %0, i32 7, i32 7)
+ ret i8 addrspace(1)* %obj.relocated
+ }
+
+During lowering, this will result in a instruction selection DAG that looks
+something like:
+
+::
+
+ CALLSEQ_START
+ ...
+ GC_TRANSITION_START (lowered i32 *@Flag), SRCVALUE i32* Flag
+ STATEPOINT
+ GC_TRANSITION_END (lowered i32 *@Flag), SRCVALUE i32 *Flag
+ ...
+ CALLSEQ_END
+
+In order to generate the necessary transition code, the backend for each target
+supported by "hypothetical-gc" must be modified to lower ``GC_TRANSITION_START``
+and ``GC_TRANSITION_END`` nodes appropriately when the "hypothetical-gc"
+strategy is in use for a particular function. Assuming that such lowering has
+been added for X86, the generated assembly would be:
+
+.. code-block:: gas
+
+ .globl test1
+ .align 16, 0x90
+ pushq %rax
+ movl $1, %fs:Flag@TPOFF
+ callq foo
+ movl $0, %fs:Flag@TPOFF
+ .Ltmp1:
+ movq (%rsp), %rax # This load is redundant (oops!)
+ popq %rdx
+ retq
+
+Note that the design as presented above is not fully implemented: in particular,
+strategy-specific lowering is not present, and all GC transitions are emitted as
+as single no-op before and after the call instruction. These no-ops are often
+removed by the backend during dead machine instruction elimination.
Intrinsics
::
declare i32
- @llvm.experimental.gc.statepoint(func_type <target>,
- i64 <#call args>. i64 <unused>,
+ @llvm.experimental.gc.statepoint(i64 <id>, i32 <num patch bytes>,
+ func_type <target>,
+ i64 <#call args>, i64 <flags>,
... (call parameters),
+ i64 <# transition args>, ... (transition parameters),
i64 <# deopt args>, ... (deopt parameters),
... (gc parameters))
Operands:
"""""""""
+The 'id' operand is a constant integer that is reported as the ID
+field in the generated stackmap. LLVM does not interpret this
+parameter in any way and its meaning is up to the statepoint user to
+decide. Note that LLVM is free to duplicate code containing
+statepoint calls, and this may transform IR that had a unique 'id' per
+lexical call to statepoint to IR that does not.
+
+If 'num patch bytes' is non-zero then the call instruction
+corresponding to the statepoint is not emitted and LLVM emits 'num
+patch bytes' bytes of nops in its place. LLVM will emit code to
+prepare the function arguments and retrieve the function return value
+in accordance to the calling convention; the former before the nop
+sequence and the latter after the nop sequence. It is expected that
+the user will patch over the 'num patch bytes' bytes of nops with a
+calling sequence specific to their runtime before executing the
+generated machine code. There are no guarantees with respect to the
+alignment of the nop sequence. Unlike :doc:`StackMaps` statepoints do
+not have a concept of shadow bytes. Note that semantically the
+statepoint still represents a call or invoke to 'target', and the nop
+sequence after patching is expected to represent an operation
+equivalent to a call or invoke to 'target'.
+
The 'target' operand is the function actually being called. The
target can be specified as either a symbolic LLVM function, or as an
arbitrary Value of appropriate function type. Note that the function
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 'flags' operand is used to specify extra information about the
+statepoint. This is currently only used to mark certain statepoints
+as GC transitions. This operand is a 64-bit integer with the following
+layout, where bit 0 is the least significant bit:
+
+ +-------+---------------------------------------------------+
+ | Bit # | Usage |
+ +=======+===================================================+
+ | 0 | Set if the statepoint is a GC transition, cleared |
+ | | otherwise. |
+ +-------+---------------------------------------------------+
+ | 1-63 | Reserved for future use; must be cleared. |
+ +-------+---------------------------------------------------+
The 'call parameters' arguments are simply the arguments which need to
be passed to the call target. They will be lowered according to the
specified in '# call args'. The types must match the signature of
'target'.
+The 'transition parameters' arguments contain an arbitrary list of
+Values which need to be passed to GC transition code. They will be
+lowered and passed as operands to the appropriate GC_TRANSITION nodes
+in the selection DAG. It is assumed that these arguments must be
+available before and after (but not necessarily during) the execution
+of the callee. The '# transition args' field indicates how many operands
+are to be interpreted as 'transition parameters'.
+
The 'deopt parameters' arguments contain an arbitrary list of Values
which is meaningful to the runtime. The runtime may read any of these
values, but is assumed not to modify them. If the garbage collector
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
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.
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
+a verifier. Please ask on llvm-dev if you're interested in
experimenting with the current version.
.. _statepoint-utilities:
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)
+ call i32 (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
}
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)
+ %0 = call i32 (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(i32 %0, i32 12, i32 12)
ret i8 addrspace(1)* %obj.relocated
}
.. 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)
+ %safepoint_token = call i32 (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 i32 (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
}
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
-llvmdev mailing list for suggestions.
+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
=====================
<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
+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.cs.uiuc.edu/mailman/listinfo/llvm-commits>`_ for review.
+<http://lists.llvm.org/mailman/listinfo/llvm-commits>`_ for review.
projects / oota-llvm.git / blobdiff