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

Mehdi AMINI via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Tue Apr 23 11:07:36 PDT 2019

mehdi_amini added inline comments.

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
tejohnson wrote:
> 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).
(I fully agree it is orthogonal, I just spotted this as a separate bug indeed, thanks for fixing!)

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