[PATCH] D108826: [SLP][LTO][WIP]Allow full SLP in LTO only at link time.

[Alexey Bataev via Phabricator via cfe-commits]

ABataev added a comment.

In D108826#2969594 <https://reviews.llvm.org/D108826#2969594>, @lebedev.ri wrote:

> In D108826#2969547 <https://reviews.llvm.org/D108826#2969547>, @ABataev wrote:
>
>> In D108826#2969471 <https://reviews.llvm.org/D108826#2969471>, @lebedev.ri wrote:
>>
>>> I think there is something really wrong with vectorzer passes in LTO pipelines.
>>> Can you say whether the problem you are observing is in ThinLTO, Full LTO, or both?
>>
>> I saw it in Full LTO but suppose we have a similar problem in ThinLTO. SLP vectorizer at compile-time tries to vectorize using small vectors at it may affect other optimizations at link time (e.g. after inlining we may try to vectorize using large vector sizes etc.). This is just a preliminary attempt to see how can we fix this early optimization in SLP.
>
> Aha, so full lto. That is consistent with the phase ordering dilemma @spatel discovered: D102002 <https://reviews.llvm.org/D102002>

Aha, do I understand correctly that he tries to add a flag(s) that we have a compile without LTO, compile at LTO and link at LTO? Or something else? Or he just tries to reorder passes depending whether we're in LTO or not in LTO?

> IMO workarounding it in the pass isn't the right course of action. Such workarounds tend to stick around.

I agree, I thought about a pipeline fix, this is just a temp solution to check how it affects the performance. It gets important especially for upcoming non-power-2 vectorization, which may cause regressions with LTO.


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