[llvm] r322125 - [IPSCCP] Remove calls without side effects
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Tue Jan 9 22:40:21 PST 2018
I think I'm missing something then. Why would any trivially dead
instruction not be safe to remove in SCCP after we've done the constant
folding of the result?
Philip
On 01/09/2018 04:09 PM, Davide Italiano wrote:
> 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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