[PATCH] D109049: [SLP] Support llvm.isnan in vectorizer

[Serge Pavlov via Phabricator via llvm-commits]

sepavloff added a comment.

In D109049#2976449 <https://reviews.llvm.org/D109049#2976449>, @xbolva00 wrote:

> But there are concerns in https://reviews.llvm.org/D104854 whether the base change should not be reverted at all…

There is a discussion: https://lists.llvm.org/pipermail/llvm-dev/2021-August/152257.html, devoted to this topic. There are no additional feedback during the last week, so I would think the problem is closed.

There are no arguments against the intrinsic, this is **the only way** to implement `isnan`, which would satisfy IEEE-754 and C standard in the case when FP exceptions are not ignored.

There were some hesitation whether this intrinsic should be optimized out in `fast-math` mode. So far there are no sound arguments in favor of such optimization, but there is a reference to the relevant discussion in GCC mail list, which demonstrates difficulties that rise if the effect of `fast-math` is understood in "naive" way. No matter if we decided optimize out this intrinsic or not, necessity of the intrinsic is not doubted.


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https://reviews.llvm.org/D109049