[PATCH] D134354: [AMDGPU][GlobalISel] Support mad/fma_mix selection

[Jay Foad via Phabricator via llvm-commits]

foad added inline comments.


================
Comment at: llvm/lib/CodeGen/GlobalISel/Utils.cpp:665-667
+    return isKnownNeverNaN(DefMI->getOperand(1).getReg(), MRI, SNaN) &&
+           isKnownNeverNaN(DefMI->getOperand(2).getReg(), MRI, SNaN) &&
+           isKnownNeverNaN(DefMI->getOperand(3).getReg(), MRI, SNaN);
----------------
Pierre-vh wrote:
> foad wrote:
> > This isn't true. You can get a NaN result even if none of the inputs are NaNs, e.g. from +inf + -inf.
> I just copied the SDag implementation so that one would be wrong too then
> ```
>   case ISD::FMA:
>   case ISD::FMAD: {
>     if (SNaN)
>       return true;
>     return isKnownNeverNaN(Op.getOperand(0), SNaN, Depth + 1) &&
>            isKnownNeverNaN(Op.getOperand(1), SNaN, Depth + 1) &&
>            isKnownNeverNaN(Op.getOperand(2), SNaN, Depth + 1);
>   }
> ```
> 
> What is the alternative? Can this function not handle FMA/FMAD at all then?
D50804 fixed the sdag implementation for fadd/fmul etc but apparently not for fma/fmad :(


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  https://reviews.llvm.org/D134354/new/

https://reviews.llvm.org/D134354