[llvm] r199420 - Teach InstCombine that (fmul X, -1.0) can be simplified to (fneg X), which LLVM expresses as (fsub -0.0, X).

[Owen Anderson]

Multiplying by 1.0 (or -1.0) is always an exact operation.  Adding (or subtracting from) -0.0 is also always an exact operation.

—Owen

On Jan 16, 2014, at 2:19 PM, Philip Reames <listmail at philipreames.com> wrote:

> Owen,
> 
> Could you give a quick justification on why this is a correct optimization?  I'm not saying it's wrong, but given the complexity of floating point it doesn't seem obviously correct either.
> 
> The case which has me hesitating is where X is a really large number (large exponent).  Multiplication clearly gets an exact answer.  Does the addition?  Adding small and large number is a notoriously error prone thing in floating point.  Another concerning case might be where X is undef.  Does the transformation increase the set of possible outputs?
> 
> I'm very open to being convinced this is correct; I'm simply raising a concern since I didn't see any justification given.
> 
> Philip
> 
> On 1/16/14 12:36 PM, Owen Anderson wrote:
>> Author: resistor
>> Date: Thu Jan 16 14:36:42 2014
>> New Revision: 199420
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=199420&view=rev
>> Log:
>> Teach InstCombine that (fmul X, -1.0) can be simplified to (fneg X), which LLVM expresses as (fsub -0.0, X).
>> 
>> Modified:
>>     llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
>>     llvm/trunk/test/Transforms/InstCombine/fmul.ll
>> 
>> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp?rev=199420&r1=199419&r2=199420&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp (original)
>> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp Thu Jan 16 14:36:42 2014
>> @@ -426,6 +426,16 @@ Instruction *InstCombiner::visitFMul(Bin
>>          return NV;
>>        ConstantFP *C = dyn_cast<ConstantFP>(Op1);
>> +
>> +    // (fmul X, -1.0) --> (fsub -0.0, X)
>> +    if (C && C->isExactlyValue(-1.0)) {
>> +      Instruction *RI = BinaryOperator::CreateFSub(
>> +        ConstantFP::getNegativeZero(C->getType()),
>> +        Op0);
>> +      RI->copyFastMathFlags(&I);
>> +      return RI;
>> +    }
>> +
>>      if (C && AllowReassociate && C->getValueAPF().isFiniteNonZero()) {
>>        // Let MDC denote an expression in one of these forms:
>>        // X * C, C/X, X/C, where C is a constant.
>> 
>> Modified: llvm/trunk/test/Transforms/InstCombine/fmul.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/fmul.ll?rev=199420&r1=199419&r2=199420&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/InstCombine/fmul.ll (original)
>> +++ llvm/trunk/test/Transforms/InstCombine/fmul.ll Thu Jan 16 14:36:42 2014
>> @@ -93,3 +93,15 @@ for.body:
>>  for.end:                                          ; preds = %for.cond
>>    ret void
>>  }
>> +
>> +; X * -1.0 => -0.0 - X
>> +define float @test9(float %x) {
>> +  %mul = fmul float %x, -1.0
>> +  ret float %mul
>> +
>> +; CHECK-LABEL: @test9(
>> +; CHECK-NOT: fmul
>> +; CHECK: fsub
>> +}
>> +
>> +
>> 
>> 
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