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

Tue Apr 23 10:34:26 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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chandlerc wrote:
> tejohnson wrote:
> > 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.
> I don't think it was intentional, but I'm not sure I would directly replicate it if it requires adding an entirely new customization point. Is their some simpler way of getting equivalent results at O1?
Yeah we can presumably just add those optimizations to the -O1 early exit path in PassBuilder::buildLTODefaultPipeline. I can send a patch (but would like to get this one in as it is a bugfix and orthogonal).

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