[llvm-dev] Saving Compile Time in InstCombine

[Craig Topper via llvm-dev]

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.

~Craig

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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