[llvm] r322125 - [IPSCCP] Remove calls without side effects

[Chris Bieneman via llvm-commits]

Author: cbieneman
Date: Tue Jan  9 13:58:46 2018
New Revision: 322125

URL: http://llvm.org/viewvc/llvm-project?rev=322125&view=rev
Log:
[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

Added:
    llvm/trunk/test/Transforms/IPConstantProp/remove-call-inst.ll
      - copied, changed from r322124, llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll
Modified:
    llvm/trunk/include/llvm/IR/Instruction.h
    llvm/trunk/lib/IR/Instruction.cpp
    llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
    llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll

Modified: llvm/trunk/include/llvm/IR/Instruction.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/Instruction.h?rev=322125&r1=322124&r2=322125&view=diff
==============================================================================
--- llvm/trunk/include/llvm/IR/Instruction.h (original)
+++ llvm/trunk/include/llvm/IR/Instruction.h Tue Jan  9 13:58:46 2018
@@ -535,6 +535,14 @@ public:
   /// matters, isSafeToSpeculativelyExecute may be more appropriate.
   bool mayHaveSideEffects() const { return mayWriteToMemory() || mayThrow(); }
 
+  /// Return true if the instruction can be removed if the result is unused.
+  ///
+  /// When constant folding some instructions cannot be removed even if their
+  /// results are unused. Specifically terminator instructions and calls that
+  /// may have side effects cannot be removed without semantically changing the
+  /// generated program.
+  bool isSafeToRemove() const;
+  
   /// Return true if the instruction is a variety of EH-block.
   bool isEHPad() const {
     switch (getOpcode()) {

Modified: llvm/trunk/lib/IR/Instruction.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Instruction.cpp?rev=322125&r1=322124&r2=322125&view=diff
==============================================================================
--- llvm/trunk/lib/IR/Instruction.cpp (original)
+++ llvm/trunk/lib/IR/Instruction.cpp Tue Jan  9 13:58:46 2018
@@ -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))

Modified: llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SCCP.cpp?rev=322125&r1=322124&r2=322125&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/SCCP.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/SCCP.cpp Tue Jan  9 13:58:46 2018
@@ -1900,7 +1900,7 @@ static bool runIPSCCP(Module &M, const D
         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;

Copied: llvm/trunk/test/Transforms/IPConstantProp/remove-call-inst.ll (from r322124, llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll)
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IPConstantProp/remove-call-inst.ll?p2=llvm/trunk/test/Transforms/IPConstantProp/remove-call-inst.ll&p1=llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll&r1=322124&r2=322125&rev=322125&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll (original)
+++ llvm/trunk/test/Transforms/IPConstantProp/remove-call-inst.ll Tue Jan  9 13:58:46 2018
@@ -6,7 +6,7 @@
 
 ; CHECK: define i32 @main() #0 {
 ; CHECK-NEXT: entry:
-; CHECK-NEXT: %call2 = tail call i32 @wwrite(i64 0) [[NUW:#[0-9]+]]
+; CHECK-NOT: call
 ; CHECK-NEXT: ret i32 123
 
 define i32 @main() noreturn nounwind {
@@ -31,4 +31,3 @@ return:
 
 ; CHECK: attributes #0 = { noreturn nounwind }
 ; CHECK: attributes #1 = { nounwind readnone }
-; CHECK: attributes [[NUW]] = { nounwind }

Modified: llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll?rev=322125&r1=322124&r2=322125&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll (original)
+++ llvm/trunk/test/Transforms/IPConstantProp/user-with-multiple-uses.ll Tue Jan  9 13:58:46 2018
@@ -15,7 +15,7 @@ entry:
   ret i32 %call2
 }
 
-define internal i32 @wwrite(i64 %i) nounwind readnone {
+define internal i32 @wwrite(i64 %i) nounwind {
 entry:
   switch i64 %i, label %sw.default [
     i64 3, label %return
@@ -30,5 +30,4 @@ return:
 }
 
 ; CHECK: attributes #0 = { noreturn nounwind }
-; CHECK: attributes #1 = { nounwind readnone }
-; CHECK: attributes [[NUW]] = { nounwind }
+; CHECK: attributes #1 = { nounwind }