[llvm-dev] Saving Compile Time in InstCombine

[Reid Kleckner via llvm-dev]

I had an idea that llvm.dbg.value should be variadic. I was staring at some
program output, and I noticed that debug values tend to group together
around inline call sites. It might be interesting to shorten the
instruction stream by extending the dbg.value operand list to describe
multiple variables and expressions.

On Fri, Apr 14, 2017 at 2:19 PM, Mikulin, Dmitry via llvm-dev <
llvm-dev at lists.llvm.org> 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.
>
>
> > ComputeKnownBits takes about the same time and other visitors have
> elevated times I would guess due to the need to propagate debug info.
> >
> >
> >>
> >>> I wanted to see what transformations InstCombine actually performs.
> Using -debug option turned out not to be very scalable. Never mind the
> large output size of the trace, running "opt -debug -instcombine” on
> anything other than a small IR is excruciatingly slow. Out of curiosity I
> profiled it too: 96% of the time is spent decoding and printing
> instructions. Is this a known problem? If so, what are the alternatives for
> debugging large scale problem? If not, it’s possibly another item to add to
> the to-do list.
> >>>
> >>
> >> You may consider adding statistics (those should be much more
> >> scalable) although more coarse.
> >>
> >> Thanks!
> >>
> >> --
> >> Davide
> >>
> >> "There are no solved problems; there are only problems that are more
> >> or less solved" -- Henri Poincare
> >>
> >
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