[llvm-dev] Saving Compile Time in InstCombine

Fri Mar 17 18:12:27 PDT 2017

Honestly, I'm not a huge fan of this change as-is. The set of transforms
that were added behind ExpensiveChecks seems awfully strange and many would
not lead the reader to believe that they are expensive at all
(the SimplifyDemandedInstructionBits and foldICmpUsingKnownBits calls being
the obvious expensive routines).

The purpose of many of InstCombine's xforms is to canonicalize the IR to
make life easier for downstream passes and analyses.

InstCombine internally relies on knowing what transforms it may or may not
perform. This is important: canonicalizations may duel endlessly if we get
this wrong; the order of the combines is also important for exactly the
same reason (SelectionDAG deals with this problem in a different way with
its pattern complexity field).

Another concern with moving seemingly arbitrary combines under
ExpensiveCombines is that it will make it that much harder to understand
what is and is not canonical at a given point during the execution of the
optimizer.

I'd be much more interested in a patch which caches the result of
frequently called ValueTracking functionality like ComputeKnownBits,
ComputeSignBit, etc. which often doesn't change but is not intelligently
reused. I imagine that the performance win might be quite comparable. Such
a patch would have the benefit of keeping the set of available transforms
constant throughout the pipeline while bringing execution time down; I
wouldn't be at all surprised if caching the ValueTracking functions
resulted in a bigger time savings.

On Fri, Mar 17, 2017 at 5:49 PM, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> On 03/17/2017 04:30 PM, Mehdi Amini via llvm-dev wrote:
>
>
> On Mar 17, 2017, at 11:50 AM, Mikhail Zolotukhin via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> One of the most time-consuming passes in LLVM middle-end is InstCombine
> (see e.g. [1]). It is a very powerful pass capable of doing all the crazy
> stuff, and new patterns are being constantly introduced there. The problem
> is that we often use it just as a clean-up pass: it's scheduled 6 times in
> the current pass pipeline, and each time it's invoked it checks all known
> patterns. It sounds ok for O3, where we try to squeeze as much performance
> as possible, but it is too excessive for other opt-levels. InstCombine has
> an ExpensiveCombines parameter to address that - but I think it's underused
> at the moment.
>
>
> Yes, the “ExpensiveCombines” has been added recently (4.0? 3.9?) but I
> believe has always been intended to be extended the way you’re doing it. So
> I support this effort :)
>
>
> +1
>
> Also, did your profiling reveal why the other combines are expensive?
> Among other things, I'm curious if the expensive ones tend to spend a lot
> of time in ValueTracking (getting known bits and similar)?
>
>  -Hal
>
>
>
> CC: David for the general direction on InstCombine though.
>
>
> —
> Mehdi
>
>
>
>
> Trying to find out, which patterns are important, and which are rare, I
> profiled clang using CTMark and got the following coverage report:
> <InstCombine_covreport.html>
> (beware, the file is ~6MB).
>
> Guided by this profile I moved some patterns under the "if
> (ExpensiveCombines)" check, which expectedly happened to be neutral for
> runtime performance, but improved compile-time. The testing results are
> below (measured for Os).
>
> Performance Improvements - Compile Time Δ  Previous Current σ
> CTMark/sqlite3/sqlite3
> <http://michaelsmacmini.local/perf/v4/nts/2/graph?test.15=2> -1.55% 6.8155
> 6.7102 0.0081
> CTMark/mafft/pairlocalalign
> <http://michaelsmacmini.local/perf/v4/nts/2/graph?test.1=2> -1.05% 8.0407
> 7.9559 0.0193
> CTMark/ClamAV/clamscan
> <http://michaelsmacmini.local/perf/v4/nts/2/graph?test.7=2> -1.02% 11.3893
> 11.2734 0.0081
> CTMark/lencod/lencod
> <http://michaelsmacmini.local/perf/v4/nts/2/graph?test.10=2> -1.01%
> 12.8763 12.7461 0.0244
> CTMark/SPASS/SPASS
> <http://michaelsmacmini.local/perf/v4/nts/2/graph?test.5=2> -1.01% 12.5048
> 12.3791 0.0340
>
> Performance Improvements - Compile Time Δ  Previous Current σ
> External/SPEC/CINT2006/403.gcc/403.gcc
> <http://michaelsmacmini.local/perf/v4/nts/2/graph?test.14=2> -1.64%
> 54.0801 53.1930 -
> External/SPEC/CINT2006/400.perlbench/400.perlbench
> <http://michaelsmacmini.local/perf/v4/nts/2/graph?test.7=2> -1.25% 19.1481
> 18.9091 -
> External/SPEC/CINT2006/445.gobmk/445.gobmk
> <http://michaelsmacmini.local/perf/v4/nts/2/graph?test.15=2> -1.01%
> 15.2819 15.1274 -
>
>
> Do such changes make sense? The patch doesn't change O3, but it does
> change Os and potentially can change performance there (though I didn't see
> any changes in my tests).
>
> The patch is attached for the reference, if we decide to go for it, I'll
> upload it to phab:
>
> <0001-InstCombine-Move-some-infrequent-patterns-under-if-E.patch>
>
>
> Thanks,
> Michael
>
> [1]: http://lists.llvm.org/pipermail/llvm-dev/2016-December/108279.html
>
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>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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