[PATCH] D61022: [ThinLTO] Pass down opt level to LTO backend and handle -O0 LTO in new PM

Tue Apr 23 09:41:13 PDT 2019

tejohnson marked an inline comment as done.
tejohnson added inline comments.

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Comment at: llvm/test/tools/gold/X86/opt-level.ll:53
+  ; CHECK-O1-OLDPM: select
+  ; The new PM does not do as many optimizations at O1
+  ; CHECK-O1-NEWPM: phi
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tejohnson wrote:
> mehdi_amini wrote:
> > This is intended? I'm surprised the two PMs don't have the same list of passes for a given opt level?
> I'm really not sure. I did compare the post-link LTO pipelines of both PMs at O0/O1/O2 and confirmed that the old PM is doing many more passes than the new PM at O1. Probably a question for @chandlerc ? In any case, I didn't want to address it here since it is orthogonal.
Some more info:

This is the regular LTO post-link pipeline, ThinLTO post-link pipeline uses essentially the same as a normal opt pipeline so would be consistent at -O1.

The optimization missing from the new PM regular LTO post link pipeline that affects this test is SimplifyCFG. This and a few other optimizations are added in the old PM at O1 via PassManagerBuilder::addLateLTOOptimizationPasses. Note that PassManagerBuilder::addLTOOptimizationPasses does exit early at -O1 (similar to where we exit early in the new PM for the regular LTO post link compilation). But the "late" LTO optimization passes are added unconditionally above -O0. Is this correct/desired? There isn't an equivalent in the new PM.

Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D61022/new/

https://reviews.llvm.org/D61022