[llvm-dev] [RFC] Use of saturating intrinsics

[Roman Lebedev via llvm-dev]

On Thu, Oct 10, 2019 at 6:18 PM David Green via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello all again, take 2.
>
> Over in D68651 I would like to make code that attempt to saturate an value (using higher bitwidth integers) use a saturating intrinsic instead. Something like this:
> https://godbolt.org/z/9knBnP
>
> As can be seen, the unsigned cases are already being matched to llvm.uadd.sat intrinsics. I am hoping to extend that to the signed cases. This has numerous benefits including simpler vectorization, cost-modelling and matching in the backend. The current forms of the saturating intrinsics extend into a higher type, which can be awkward to deal with in some cases (i64's are often not legal types, for example).
>
> The intrinsic seems to produce as good or better code in almost all the cases I have tried (partly thanks to patches like D68643 and the many other optimisation over the years into the backend to efficiently optimise these saturating intrinsics, even when they are not natively available.)
>
> As noted in the review, this does bring up the question of what we should consider canonical. I believe that these intrinsics should be (especially over changing types!).
In this particular case for this particular intrinsic i do believe the
reasoning is sound.

In https://reviews.llvm.org/D68651 Hideki Saito does raise a point
about whether this canonicalization hinders some other optimizations.
Does it appear to regress anything?

> Let us know if you disagree.
>
> Cheers
> Dave
Roman

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