[llvm-dev] Saving Compile Time in InstCombine

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Fri Apr 14 14:31:12 PDT 2017

I think the easy way to ignore them is to add visitDbgDeclareInst and
visitDbgValueInst to the InstCombine visitor tree and then just return
nullptr from them.


On Fri, Apr 14, 2017 at 2:23 PM, Davide Italiano via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Fri, Apr 14, 2017 at 2:19 PM, Mikulin, Dmitry
> <dmitry.mikulin at sony.com> wrote:
> >
> >>> Is this a run with debug info? i.e. are you passing -g to the per-TU
> >>> pipeline? I'm inclined to think this is mostly an additive effect
> >>> adding matchers here and there that don't really hurt small testcases
> >>> but we pay the debt over time (in particular for LTO). Side note, I
> >>> noticed (and others did as well) that instcombine is way slower with
> >>> `-g` on (one of the reasons could be we walking much longer use lists,
> >>> due to the dbg use). Do you have numbers of instcombine ran on IR with
> >>> and without debug info?
> >>
> >> I do have the numbers for the same app with and without debug info. The
> results above are for the no-debug version.
> >>
> >> Total execution time of -O3 is 34% slower with debug info. The size of
> the debug IR is 162M vs 39M no-debug. Both profiles look relatively similar
> with the exception of bit code writer and verifier taking a larger share in
> the -g case.
> >>
> >> Looking at InstCombine, it’s 23% slower. One notable thing is that
> CallInst takes significantly larger share with -g: 5s vs 13s, which
> translates to about half of the InstCombine slowdown. Need to understand
> why.
> >
> > Ah, it’s all those calls to @llvm.dbg.* functions. I’ll explore if they
> can be safely ignored by InstCombine.
> >
> >
> I took a look and saw no immediate problems, also discussed with David
> Majnemer on IRC, who thinks we should just bail out early.
> --
> Davide
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
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