[llvm-dev] Saving Compile Time in InstCombine

[Mikhail Zolotukhin via llvm-dev]

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.

Trying to find out, which patterns are important, and which are rare, I profiled clang using CTMark and got the following coverage report:

(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:




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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