Extend the statepoint intrinsic to allow statepoints to be marked as transitions from GC-aware code to code that is not GC-aware.

This changes the shape of the statepoint intrinsic from:

  @llvm.experimental.gc.statepoint(anyptr target, i32 # call args, i32 unused, ...call args, i32 # deopt args, ...deopt args, ...gc args)

to:

  @llvm.experimental.gc.statepoint(anyptr target, i32 # call args, i32 flags, ...call args, i32 # transition args, ...transition args, i32 # deopt args, ...deopt args, ...gc args)

This extension offers the backend the opportunity to insert (somewhat) arbitrary code to manage the transition from GC-aware code to code that is not GC-aware and back.

In order to support the injection of transition code, this extension wraps the STATEPOINT ISD node generated by the usual lowering lowering with two additional nodes: GC_TRANSITION_START and GC_TRANSITION_END. The transition arguments that were passed passed to the intrinsic (if any) are lowered and provided as operands to these nodes and may be used by the backend during code generation.

Eventually, the lowering of the GC_TRANSITION_{START,END} nodes should be informed by the GC strategy in use for the function containing the intrinsic call; for now, these nodes are instead replaced with no-ops.

Differential Revision: http://reviews.llvm.org/D9501

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236888 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 104 insertions, 9 deletions

diff --git a/docs/Statepoints.rst b/docs/Statepoints.rst
index 9741c93cc81..438171cb932 100644
--- a/docs/Statepoints.rst
+++ b/docs/Statepoints.rst
@@ -142,8 +142,8 @@ resulting relocation sequence is:
 
   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 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @foo, 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 4, i32 4)
     ret i8 addrspace(1)* %obj.relocated
   }
 
@@ -207,7 +207,82 @@ This example was taken from the tests for the :ref:`RewriteStatepointsForGC` uti
   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 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(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 4, i32 4)
+    ret i8 addrspace(1)* %obj.relocated
+  }
+
+During lowering, this will result in a instruction selection DAG that looks
+something like:
+
+.. code-block::
+
+  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
@@ -223,8 +298,9 @@ Syntax:
 
       declare i32
         @llvm.experimental.gc.statepoint(func_type <target>, 
-                       i64 <#call args>. i64 <unused>, 
+                       i64 <#call args>. i64 <flags>,
                        ... (call parameters),
+                       i64 <# transition args>, ... (transition parameters),
                        i64 <# deopt args>, ... (deopt parameters),
                        ... (gc parameters))
 
@@ -247,8 +323,19 @@ 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 '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
@@ -257,6 +344,14 @@ instruction.  The number of arguments must exactly match what is
 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
@@ -471,7 +566,7 @@ As an example, given this code:
 
   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 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(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
   }
 
@@ -481,7 +576,7 @@ The pass would produce this IR:
 
   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)
+    %0 = call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(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 9, i32 9)
     ret i8 addrspace(1)* %obj.relocated
   }
@@ -535,8 +630,8 @@ 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)
+    %safepoint_token = call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @do_safepoint, i32 0, 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, i32 0)
     ret void
   }