[PATCH] D106362: [FPEnv][InstSimplify] Enable more folds for constrained fadd

[Sanjay Patel via Phabricator via llvm-commits]

spatel added reviewers: efriedma, scanon.
spatel added inline comments.


================
Comment at: llvm/lib/Analysis/InstructionSimplify.cpp:4908
+      (!RoundingModeMayBe(Rounding, RoundingMode::TowardNegative) &&
+       (ExBehavior != fp::ebStrict || FMF.noNaNs())))
+    if (match(Op1, m_NegZeroFP()))
----------------
sepavloff wrote:
> kpn wrote:
> > sepavloff wrote:
> > > Even if `ExBehavior != fp::ebStrict` this transformation is invalid if `X` is SNaN, the latter must be converted to QNaN.
> > > 
> > > 
> > Can I put this in a different patch? We have the same issue for constrained and non-constrained cases, and we don't have a good way to distinguish them -- nor should we.
> > 
> > Most of the required code is present in APFloat, but not all. Would it be OK for me to add the needed bits to APFloat and use them from InstSimplify in a different ticket?
> I am not sure I understand what you are going to put into another patch. The transformation:
> 
>     fadd X, -0 ==> X
> 
> is valid only if `X != SNaN`, otherwise we have:
> 
>     fadd SNaN, -0 ==> QNaN
> 
> It does not depend on whether FP environment is default or not, the code before your changes was already incorrect. Such transformation is valid if the operation has flag `nnan`, but not in general case. Code in APFloat hardly can help, as constant folding is made previously, if `X` is a constant here, it means it cannot be folded.
IIUC, you are saying SNaN vs. QNaN is more than just a part of the exception state. But that's not how I interpret the current LangRef:
https://llvm.org/docs/LangRef.html#floating-point-environment

...and that's why *none* of the transforms here are intentionally SNaN preserving/clearing. 

If we are going to change the behavior of the default FP env, the LangRef must be updated to make that clear. I don't see the motivation yet.


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  https://reviews.llvm.org/D106362/new/

https://reviews.llvm.org/D106362