[PATCH] D84108: [SimplifyCFG][LoopRotate] SimplifyCFG: disable common instruction hoisting by default, enable late in pipeline

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D84108#2227371 <https://reviews.llvm.org/D84108#2227371>, @echristo wrote:

> One more follow up here:
>
> One set of benchmarks we're seeing this in are also in the llvm test harness:
>
> llvm_multisource_tsvc.TSVC-ControlFlow-flt and llvm_multisource_misc.Ptrdist-yacr2
>
> in an FDO mode (so we have some decent branch hints for the code).
>
> You should be able to duplicate at least some of the slowdown there.

That sadly doesn't tell me much.
Can you please provide the reproduction steps, and ideally the IR with/without this change?

In D84108#2227365 <https://reviews.llvm.org/D84108#2227365>, @echristo wrote:

> So, we're seeing several significant (20-30%) regressions due to this in various different library benchmarks. Usually around things that are compression/decompression loops, but also other places.
>
> I'm uncertain what we can do, but I think this might need a wider range of discussion rather than the phabricator review.
>
> Would you mind terribly reverting and starting a thread on llvm-dev about this? I think that'll give us an opportunity to weigh some of the tradeoffs in a wider space.
>
> -eric




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