[llvm-dev] Enabling Loop Distribution Pass as default in the pipeline of new pass manager

[Jingu Kang via llvm-dev]

Hi All,

My colleague Sanne has found performance improvement with '-enable-loop-distribute' option from hmmer on SPEC2006.

On the hmmer, there is a loop with dependence. The Loop Distribute pass splits the loop into three sperate loops. One loop has still dependence, another is vectorizable, the other is vectorizable after running LoopBoundSplit pass which needs to be updated a bit. On AArch64, we have seen 40% improvement with enabling Loop Distribute pass and 80% improvement with enabling Loop Distribute pass and LoopBoundSplit from hmmer on SPEC2006.

>From llvm-test-suite and spec benchmarks, I have not seen any performance degradation with enabling the Loop Distribute pass because almost all tests are not handled by Loop Distribute pass with mainly below messages. I think the messages are reasonable.

Skipping; memory operations are safe for vectorization
Skipping; no unsafe dependences to isolate
Skipping; multiple exit blocks

For compile time, there is no big change because the almost all tests are not handled by the pass due to mainly above three reasons which comes from cached analysis information.

At this moment, we can enable the pass with metadata or command line option. If possible, can we enable the Loop Distribute pass as default in the pipeline of new pass manager please?

Thanks
JinGu Kang
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