[PATCH] D138814: [InstCombine] Combine a/lshr of add -> uadd.with.overflow

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

In D138814#3980701 <https://reviews.llvm.org/D138814#3980701>, @Pierre-vh wrote:

> In D138814#3973599 <https://reviews.llvm.org/D138814#3973599>, @nikic wrote:
>
>> FWIW our historical stance has always been that uadd.with.overflow is non-canonical, and the canonical pattern is `a + b < a` (for non-constant `b`). uadd.with.overflow generally has worse optimization support, which is why we only form it during CGP for backend purposes.
>
> Interesting, not sure what other reviewers think?
> Maybe adding a TII hook so targets can enable/disable the combine is a good idea? e.g. something like `allowUAddoCanonicalForm`?

I don't think there's any need for target dependence here. You just need to produce `a + b < a` instead of `extract(uaddo(a, b), 1)`. The uaddo will be formed by the backend.


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