[PATCH] D125264: [ConstraintElimination] Simplify ssub(a, b) if a s>=b && a s>=0.

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

In D125264#3501812 <https://reviews.llvm.org/D125264#3501812>, @fhahn wrote:

> In D125264#3501773 <https://reviews.llvm.org/D125264#3501773>, @nikic wrote:
>
>> Would it be sufficient to only replace the `m_ExtractValue<1>`? I believe that's what we usually do. (Would also work if only the extractvalue is dominated.)
>
> And leave the removal of the actual intrinsic call to instcombine (or some other existing transform)? If that's what we do elsewhere I think it should work, the current placement in the regular optimization pipeline means we run instcombine shortly after ConstraintElimiantion.

Yeah, that's what I had in mind. Though checking now, it looks like both CVP (https://github.com/llvm/llvm-project/blob/72831a592edf1bdcca15354181867079a17d4f76/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp#L572) and SimplifyIndVars (https://github.com/llvm/llvm-project/blob/72831a592edf1bdcca15354181867079a17d4f76/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp#L423) do replace the with.overflow intrinsic itself, so maybe it's fine as is.


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