[llvm] r322125 - [IPSCCP] Remove calls without side effects
Davide Italiano via llvm-commits
llvm-commits at lists.llvm.org
Tue Jan 9 16:09:04 PST 2018
hmm, the original proposal was that of keeping this function private
to SCCP as it has slightly different requirement
(`isInstructionTriviallyDead()` wasn't enough in this case).
Anyway, happy either way (if this gets merged as Philip suggested).
On Tue, Jan 9, 2018 at 4:01 PM, Philip Reames via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> This appears to be duplicating functionality from
> isInstructionTriviallyDead. Please merge them.
>
> Philip
>
>
>
> On 01/09/2018 01:58 PM, Chris Bieneman via llvm-commits wrote:
>>
>> 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 }
>>
>>
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--
Davide
"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare
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