[llvm] r322125 - [IPSCCP] Remove calls without side effects
Davide Italiano via llvm-commits
llvm-commits at lists.llvm.org
Wed Jan 10 07:22:40 PST 2018
Please refer to the review for details.
Copy/pasting Danny's comment here for your convenience:
"That would not return true for anything with side effects. So it will
not handle the atomic loads and stores cases. mayHaveSideEffects will
return true for anything that might write to memory. mayWriteToMemory
will return true for any store, and any ordered load.
In fact, even getModRefInfo wouldn't help you with that here, it would
do the same, and can't be overridden by anything more powerful right
now (we only have an AA interface for callsites, the regular
instructions are all single implementation in AliasAnalysis.cpp)
So TL; DR if you wanted to use isInstructionTriviallyDead, it would be
something like isInstructionTriviallyDead || is a load || is a store
|| atomicrmw || cmpxchng || ...."
--
Davide
On Tue, Jan 9, 2018 at 10:40 PM, Philip Reames
<listmail at philipreames.com> wrote:
> 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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>>>
>>>
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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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