[Statepoints] Let patchable statepoints have a symbolic call target.

Summary:
As added initially, statepoints required their call targets to be a
constant pointer null if ``numPatchBytes`` was non-zero.  This turns out
to be a problem ergonomically, since there is no way to mark patchable
statepoints as calling a (readable) symbolic value.

This change remove the restriction of requiring ``null`` call targets
for patchable statepoints, and changes PlaceSafepoints to maintain the
symbolic call target through its transformation.

Reviewers: reames, swaroop.sridhar

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11550

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243502 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Statepoints.rst

@@ -331,14 +331,16 @@ 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.
+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
 type must match the signature of the callee and the types of the 'call
-parameters' arguments.  If 'num patch bytes' is non-zero then 'target'
-has to be the constant pointer null of the appropriate function type.
+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

lib/CodeGen/SelectionDAG/StatepointLowering.cpp

@@ -289,7 +289,23 @@ lowerCallFromStatepoint(ImmutableStatepoint ISP, MachineBasicBlock *LandingPad,
 
   ImmutableCallSite CS(ISP.getCallSite());
 
-  SDValue ActualCallee = Builder.getValue(ISP.getCalledValue());
+  SDValue ActualCallee;
+
+  if (ISP.getNumPatchBytes() > 0) {
+    // If we've been asked to emit a nop sequence instead of a call instruction
+    // for this statepoint then don't lower the call target, but use a constant
+    // `null` instead.  Not lowering the call target lets statepoint clients get
+    // away without providing a physical address for the symbolic call target at
+    // link time.
+
+    const auto &TLI = Builder.DAG.getTargetLoweringInfo();
+    const auto &DL = Builder.DAG.getDataLayout();
+
+    unsigned AS = ISP.getCalledValue()->getType()->getPointerAddressSpace();
+    ActualCallee = Builder.DAG.getConstant(0, Builder.getCurSDLoc(),
+                                           TLI.getPointerTy(DL, AS));
+  } else
+    ActualCallee = Builder.getValue(ISP.getCalledValue());
 
   assert(CS.getCallingConv() != CallingConv::AnyReg &&
          "anyregcc is not supported on statepoints!");

lib/IR/Verifier.cpp

@@ -1544,12 +1544,6 @@ void Verifier::VerifyStatepoint(ImmutableCallSite CS) {
          "gc.statepoint callee must be of function pointer type", &CI, Target);
   FunctionType *TargetFuncType = cast<FunctionType>(PT->getElementType());
 
-  if (NumPatchBytes)
-    Assert(isa<ConstantPointerNull>(Target->stripPointerCasts()),
-           "gc.statepoint must have null as call target if number of patchable "
-           "bytes is non zero",
-           &CI);
-
   const Value *NumCallArgsV = CS.getArgument(3);
   Assert(isa<ConstantInt>(NumCallArgsV),
          "gc.statepoint number of arguments to underlying call "

lib/Transforms/Scalar/PlaceSafepoints.cpp

@@ -917,15 +917,10 @@ static Value *ReplaceWithStatepoint(const CallSite &CS, /* to replace */
       CS.getInstruction()->getContext(), AttributeSet::FunctionIndex,
       AttrsToRemove);
 
-  Value *StatepointTarget = NumPatchBytes == 0
-                                ? CS.getCalledValue()
-                                : ConstantPointerNull::get(cast<PointerType>(
-                                      CS.getCalledValue()->getType()));
-
   if (CS.isCall()) {
     CallInst *ToReplace = cast<CallInst>(CS.getInstruction());
     CallInst *Call = Builder.CreateGCStatepointCall(
-        ID, NumPatchBytes, StatepointTarget,
+        ID, NumPatchBytes, CS.getCalledValue(),
         makeArrayRef(CS.arg_begin(), CS.arg_end()), None, None,
         "safepoint_token");
     Call->setTailCall(ToReplace->isTailCall());
@@ -951,7 +946,7 @@ static Value *ReplaceWithStatepoint(const CallSite &CS, /* to replace */
     // original block.
     Builder.SetInsertPoint(ToReplace->getParent());
     InvokeInst *Invoke = Builder.CreateGCStatepointInvoke(
-        ID, NumPatchBytes, StatepointTarget, ToReplace->getNormalDest(),
+        ID, NumPatchBytes, CS.getCalledValue(), ToReplace->getNormalDest(),
         ToReplace->getUnwindDest(), makeArrayRef(CS.arg_begin(), CS.arg_end()),
         None, None, "safepoint_token");
 

test/Transforms/PlaceSafepoints/patchable-statepoints.ll

@@ -22,7 +22,7 @@ define void @test_num_patch_bytes() gc "statepoint-example" personality i32 ()*
 ; CHECK-LABEL: @test_num_patch_bytes(
 entry:
 ; CHECK-LABEL: entry:
-; CHECK: invoke i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 99, void ()* null,
+; CHECK: invoke i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 99, void ()* @f,
   invoke void @f()  "statepoint-num-patch-bytes"="99" to label %normal_return unwind label %exceptional_return
 
 normal_return:

test/Verifier/invalid-patchable-statepoint.ll

@@ -1,14 +0,0 @@
-; RUN: not opt -verify 2>&1 < %s | FileCheck %s
-
-; CHECK: gc.statepoint must have null as call target if number of patchable bytes is non zero
-
-define i1 @invalid_patchable_statepoint() gc "statepoint-example" {
-entry:
-  %safepoint_token = tail call i32 (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 3, i1 ()* @func, i32 0, i32 0, i32 0, i32 0)
-  %call1 = call i1 @llvm.experimental.gc.result.i1(i32 %safepoint_token)
-  ret i1 %call1
-}
-
-declare i32 @llvm.experimental.gc.statepoint.p0f_i1f(i64, i32, i1 ()*, i32, i32, ...)
-declare i1 @llvm.experimental.gc.result.i1(i32)
-declare i1 @func()