[llvm-dev] Saving Compile Time in InstCombine

Mon Mar 20 21:51:56 PDT 2017

> On Mar 17, 2017, at 6:12 PM, David Majnemer via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 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.

As we get further along with compile-time improvements one question we need to ask ourselves more frequently is about the effectiveness of optimizations/passes. For example -  in this case - how can we make an educated assessment that running the combiner N times is a good cost/benefit investment of compute resources? The questions below are meant to figure out what technologies/instrumentations/etc could help towards a more data-driven decision process when it comes to the effectiveness of optimizations. Instcombiner might just be an inspirational use case to see what is possible in that direction.

The combiner is invoked in full multiple times. But is it really necessary to run all of it for that purpose? After instcombine is run once is there a mapping from transformation -> combines? I suspect most transformations could invoke a subset of combines to re-canonicalize. Or, if there was a (cheap) verifier for canonical IR, it could invoke a specific canonicalization routine. Instrumenting the instcombiner and checking which patterns actually kick in (for different invocations)  might give insight into how the combiner could be structured and so that only a subset of pattern need to be checked.
> 
> 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).

Can you elaborate on this “duel endlessly” with specific examples? This is out of curiosity. There must be verifiers that check that this cannot happen. Or an implementation strategy that guarantees that. Global isel will run into the same/similar question when it gets far enough to replace SD.
> 
> 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.

Can you back this up with measurements? Caching schemes are tricky. Is there a way to evaluate when the results of ComputeKnownBits etc is actually effective meaining the result is used and gives faster instructions? E.g. it might well be that only the first instance of inst_combine benefits from computing the bits. 

> 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 <mailto: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 <mailto: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 <http://lists.llvm.org/pipermail/llvm-dev/2016-December/108279.html>
>>> 
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>> 
>> 
>> 
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> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
> 
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