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

Wed Sep 1 03:26:56 PDT 2021

lebedev.ri added a comment.

In D109049#2976512 <https://reviews.llvm.org/D109049#2976512>, @sepavloff wrote:

> 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.

Au contraire.
It looks like goalpost shifting to me.
What is going on the is basic design disscussion, with no clear favorite,
even though a design has already been "selected" and committed.
I would like to once again urge you to restart this process.

> 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/new/

https://reviews.llvm.org/D109049