[IPSCCP] Remove calls without side effects

Summary:
When performing constant propagation for call instructions we have historically replaced all uses of the return from a call, but not removed the call itself. This is required for correctness if the calls have side effects, however the compiler should be able to safely remove calls that don't have side effects.

This allows the compiler to completely fold away calls to functions that have no side effects if the inputs are constant and the output can be determined at compile time.

Reviewers: davide, sanjoy, bruno, dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38856

llvm-svn: 322125

2 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index 5f2a6146ad8..215c6907256 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -589,6 +589,11 @@ bool Instruction::mayThrow() const {
   return isa<ResumeInst>(this);
 }
 
+bool Instruction::isSafeToRemove() const {
+  return (!isa<CallInst>(this) || !this->mayHaveSideEffects()) &&
+         !isa<TerminatorInst>(this);
+}
+
 bool Instruction::isAssociative() const {
   unsigned Opcode = getOpcode();
   if (isAssociative(Opcode))
diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
index df6053ac201..b6d034e9fb9 100644
--- a/llvm/lib/Transforms/Scalar/SCCP.cpp
+++ b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -1900,7 +1900,7 @@ static bool runIPSCCP(Module &M, const DataLayout &DL,
         if (Inst->getType()->isVoidTy())
           continue;
         if (tryToReplaceWithConstant(Solver, Inst)) {
-          if (!isa<CallInst>(Inst) && !isa<TerminatorInst>(Inst))
+          if (Inst->isSafeToRemove())
             Inst->eraseFromParent();
           // Hey, we just changed something!
           MadeChanges = true;