[PATCH] D68651: [InstCombine] Signed saturation patterns

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

@hsaito We've put quite a bit of effort into making sure that saturating intrinsics optimize as well or better than expanded IR sequences, which is why they are indeed considered canonical IR. Whether intrinsics are canonical needs to be decided on a case by case basis, there is no general rule about it. For example the unsigned mul overflow intrinsic is considered canonical (for obvious reasons -- the alternatives tend to be much more expensive) and is formed in InstCombine. Unsigned add/sub overflow on the other hand are not, because they tends to optimize much worse than expanded IR. Those are formed in CGP instead.

It should be noted that the saturating add/sub intrinsics are not intended primarily as vector intrinsics -- even though that's the case where they will most typically select to hardware instructions rather than be expanded in DAG. The intrinsic representation is simply generally useful for optimization.


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