[PATCH] D85709: [InstSimplify] Implement Instruction simplification for X/sqrt(X) to sqrt(X).

[Cameron McInally via Phabricator via llvm-commits]

cameron.mcinally added a comment.

In D85709#2211204 <https://reviews.llvm.org/D85709#2211204>, @spatel wrote:

> After looking at the codegen, I'm not sure if we can do this transform in IR with the expected performance in codegen because the transform loses information:
> https://godbolt.org/z/7b84rG
>
> The codegen for the case of "sqrt(x)" has to account for a 0.0 input. Ie, we filter out a 0.0 (or potentially denorm) input to avoid the NAN answer that we would get from "0.0 / 0.0". But the codegen for the case of "x/sqrt(x)" does not have to do that - NAN is the correct answer for a 0.0 input, so the code has implicitly signaled to us that 0.0 is not a valid input when compiled with -ffast-math (we can ignore possible NANs).
>
> It might help to see the motivating code that produces the x/sqrt(x) pattern to see if there's something else we should be doing there.

Excellent catch, Sanjay. The sqrt(x) -> x*rsqrt(x) transform is not safe for x==0.


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