[llvm-dev] Saving Compile Time in InstCombine (original) (raw)

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.

~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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