[llvm-commits] [llvm] r169452 - /llvm/trunk/lib/Analysis/ValueTracking.cpp

[Jakub Staszak]

If you change -0.0 into +0.0, what about
fsub -0.0, %x

I think we might get unoptimized code then.


On Thu, Dec 6, 2012 at 9:53 AM, Duncan Sands <baldrick at free.fr> wrote:

> Hi Michael,
>
>
> On 06/12/12 01:07, Michael Ilseman wrote:
>
>> Author: milseman
>> Date: Wed Dec  5 18:07:09 2012
>> New Revision: 169452
>>
>> URL: http://llvm.org/viewvc/llvm-**project?rev=169452&view=rev<http://llvm.org/viewvc/llvm-project?rev=169452&view=rev>
>> Log:
>> Have CannotBeNegativeZero() be aware of the nsz fast-math flag
>>
>
> does nsz mean that if there is a signed zero then the behaviour is
> undefined?
> Or does it mean negative zero can be treated like positive zero, and
> there's no
> need to get the sign of zero right?
>
> It seems to me that your patch pretty much assumes the first case.
> If it's instead the second then I think you would do better to have
> instcombine turn all instances of -0.0 into +0.0.
>
> Ciao, Duncan.
>
>
>
>> Modified:
>>      llvm/trunk/lib/Analysis/**ValueTracking.cpp
>>
>> Modified: llvm/trunk/lib/Analysis/**ValueTracking.cpp
>> URL: http://llvm.org/viewvc/llvm-**project/llvm/trunk/lib/**
>> Analysis/ValueTracking.cpp?**rev=169452&r1=169451&r2=**169452&view=diff<http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=169452&r1=169451&r2=169452&view=diff>
>> ==============================**==============================**
>> ==================
>> --- llvm/trunk/lib/Analysis/**ValueTracking.cpp (original)
>> +++ llvm/trunk/lib/Analysis/**ValueTracking.cpp Wed Dec  5 18:07:09 2012
>> @@ -1312,7 +1312,12 @@
>>
>>     const Operator *I = dyn_cast<Operator>(V);
>>     if (I == 0) return false;
>> -
>> +
>> +  // Check if the nsz fast-math flag is set
>> +  if (const FPMathOperator *FPO = dyn_cast<FPMathOperator>(I))
>> +    if (FPO->hasNoSignedZeros())
>> +      return true;
>> +
>>     // (add x, 0.0) is guaranteed to return +0.0, not -0.0.
>>     if (I->getOpcode() == Instruction::FAdd &&
>>         isa<ConstantFP>(I->getOperand(**1)) &&
>>
>>
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