[PATCH] D65060: [LICM] Make Loop ICM profile aware

[Xinliang David Li via llvm-commits]

+Hiroshi Yamauchi <yamauchi at google.com> who had to deal with similar issues
with profile guided size optimization, IIRC.

On Tue, Aug 13, 2019 at 10:37 PM Chandler Carruth via Phabricator <
reviews at reviews.llvm.org> wrote:

> chandlerc added a comment.
>
> This approach is broken for another reason, which also motivated the
> LoopSink approach David mentioned.
>
> BlockFrequencyInfo isn't preserved by loop passes. This is deeply
> problematic. In the legacy pass manager, this almost works, but can result
> in wildly inconsistent behavior when half way through a loop pipeline BFI
> gets invalidated and/or where it can partition the loop pass pipeline in
> weird ways breaking intended iteration order of the loop pass pipeline.
> With the new pass manager, I suspect this produces non-deterministic
> behavior based on what order we visit things causing BFI to be present or
> not when we happen to reach the loop pass manager, and happening to change
> the behavior. Even worse, if BFI *happens* to be available at the start of
> the loop pass pipeline in the new pass manager, it will continue to be used
> even after passes restructure the basic blocks invalidating it. This can
> lead to arbitrarily bad behavior.
>
> To do something like this, we really need to teach all the loop passes in
> the same loop pass pipeline as LICM that is set up this way to update and
> preserve BFI. This will fix the weird behavior w/ the legacy PM and it will
> allow you to add the analysis as one of the guaranteed loop analyses like
> MemorySSA and others.
>
> This is exactly what we had to do for MemorySSA and we'll have to do the
> same thing here.
>
> To be clear, we've already seen this cause stage2/3 differences and other
> problems when PGO is enabled (crashes, miscompiles).
>
> Hope my explanation helps, but we'll need to revert this ASAP.
>
>
> Repository:
>   rL LLVM
>
> CHANGES SINCE LAST ACTION
>   https://reviews.llvm.org/D65060/new/
>
> https://reviews.llvm.org/D65060
>
>
>
>
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