[llvm] 829d377 - [InstSimplify] Don't simplify musttail calls

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2020-03-09T18:46:56+01:00
New Revision: 829d377a98fd3ee087935ea215677e49c8b51b27

URL: https://github.com/llvm/llvm-project/commit/829d377a98fd3ee087935ea215677e49c8b51b27
DIFF: https://github.com/llvm/llvm-project/commit/829d377a98fd3ee087935ea215677e49c8b51b27.diff

LOG: [InstSimplify] Don't simplify musttail calls

As pointed out by jdoerfert on D75815, we must be careful when
simplifying musttail calls: We can only replace the return value
if we can eliminate the call entirely. As we can't make this
guarantee for all consumers of InstSimplify, this patch disables
simplification of musttail calls. Without this patch, musttail
simplification currently results in module verification errors.

Differential Revision: https://reviews.llvm.org/D75824

Added: 
    

Modified: 
    llvm/lib/Analysis/InstructionSimplify.cpp
    llvm/test/Transforms/InstSimplify/call.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 5ad09ebadc58..3b80e535f85b 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -5330,6 +5330,11 @@ static Value *simplifyIntrinsic(CallBase *Call, const SimplifyQuery &Q) {
 Value *llvm::SimplifyCall(CallBase *Call, const SimplifyQuery &Q) {
   Value *Callee = Call->getCalledValue();
 
+  // musttail calls can only be simplified if they are also DCEd.
+  // As we can't guarantee this here, don't simplify them.
+  if (Call->isMustTailCall())
+    return nullptr;
+
   // call undef -> undef
   // call null -> undef
   if (isa<UndefValue>(Callee) || isa<ConstantPointerNull>(Callee))
@@ -5512,6 +5517,9 @@ Value *llvm::SimplifyInstruction(Instruction *I, const SimplifyQuery &SQ,
     break;
   case Instruction::Call: {
     Result = SimplifyCall(cast<CallInst>(I), Q);
+    // Don't perform known bits simplification below for musttail calls.
+    if (cast<CallInst>(I)->isMustTailCall())
+      return Result;
     break;
   }
   case Instruction::Freeze:

diff  --git a/llvm/test/Transforms/InstSimplify/call.ll b/llvm/test/Transforms/InstSimplify/call.ll
index c5706ec66649..108de9082a70 100644
--- a/llvm/test/Transforms/InstSimplify/call.ll
+++ b/llvm/test/Transforms/InstSimplify/call.ll
@@ -1007,3 +1007,30 @@ define i32 @returned_var_arg(i32 %arg) {
   %x = call i32 @passthru_i32(i32 %arg)
   ret i32 %x
 }
+
+define i32 @returned_const_int_arg_musttail(i32 %arg) {
+; CHECK-LABEL: @returned_const_int_arg_musttail(
+; CHECK-NEXT:    [[X:%.*]] = musttail call i32 @passthru_i32(i32 42)
+; CHECK-NEXT:    ret i32 [[X]]
+;
+  %x = musttail call i32 @passthru_i32(i32 42)
+  ret i32 %x
+}
+
+define i32 @returned_var_arg_musttail(i32 %arg) {
+; CHECK-LABEL: @returned_var_arg_musttail(
+; CHECK-NEXT:    [[X:%.*]] = musttail call i32 @passthru_i32(i32 [[ARG:%.*]])
+; CHECK-NEXT:    ret i32 [[X]]
+;
+  %x = musttail call i32 @passthru_i32(i32 %arg)
+  ret i32 %x
+}
+
+define i32 @call_undef_musttail() {
+; CHECK-LABEL: @call_undef_musttail(
+; CHECK-NEXT:    [[X:%.*]] = musttail call i32 undef()
+; CHECK-NEXT:    ret i32 [[X]]
+;
+  %x = musttail call i32 undef()
+  ret i32 %x
+}