[PATCH] D118426: [InstCombine] Remove side effect of replaced constrained intrinsics

[Serge Pavlov via Phabricator via llvm-commits]

sepavloff added a comment.

In D118426#3337489 <https://reviews.llvm.org/D118426#3337489>, @kpn wrote:

> I'm still not clear on why, when a transform pass knows an instruction can be deleted, we are required to keep that instruction around anyway.

Are we really required? Or the transform pass merely does not know that the instruction can be deleted? InstCombine is just this case, at least in constant folding.

> It seems like these dangling, useless instructions would interfere with subsequent optimizations until finally they might or might not get removed here. And constant folding isn't sufficient to duplicate the logic of the transform pass that determined the instruction could be deleted.

InstCombine is called several times, it calls constant folding and instruction simplification functions. It looks like a good place to get rid of such useless instructions.

> For example, here's a small test that I'm working on for InstSimplify:
>
> ; NOTE: The fsub instruction is expected to remain, but the result isn't used.
> define float @nonneg_ebstrict(i32 %a) #0 {
>
>   %fpa = call float @llvm.experimental.constrained.uitofp.f32.i32(i32 %a, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
>   %sqra = call float @llvm.experimental.constrained.sqrt.f32(float %fpa, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
>   %sub = call nnan float @llvm.experimental.constrained.fsub.f32(float %sqra, float -0.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
>   ret float %sub
>
> }
>
> The input to the sqrt instruction will always be non-negative, so we know the output is valid, and we know the output will never be -0.0. This means the fsub can be safely eliminated. But the fsub will hang around forever, useless, because InstSimplify isn't allowed to tell anyone that it can be deleted, and nobody else can determine it again without duplicating InstSimplify.

InstSimplify is not the only place where peephole transformations may occur. InstCombine can also do such transformations and it is less limited. The described transformation could be a part of it.


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