[llvm-dev] Enabling Loop Distribution Pass as default in the pipeline of new pass manager

[Jingu Kang via llvm-dev]

I appreciate your replies. I have seen below performance data.

For AArch64, the performance data from llvm-test-suite is as below.

Metric: exec_time

Program                                        results_base results_loop_dist diff 
 test-suite...ications/JM/lencod/lencod.test     3.95         4.29             8.8%
 test-suite...emCmp<5, GreaterThanZero, Mid>   1456.09      1574.29            8.1%
 test-suite...st:BM_BAND_LIN_EQ_LAMBDA/44217    22.83        24.50             7.3%
 test-suite....test:BM_BAND_LIN_EQ_RAW/44217    23.00        24.17             5.1%
 test-suite...st:BM_INT_PREDICT_LAMBDA/44217   589.54       616.70             4.6%
 test-suite...t:BENCHMARK_asin_novec_double_   330.25       342.17             3.6%
 test-suite...ow-dbl/GlobalDataFlow-dbl.test     2.58         2.67             3.3%
 test-suite...da.test:BM_PIC_2D_LAMBDA/44217   781.30       806.36             3.2%
 test-suite...est:BM_ENERGY_CALC_LAMBDA/5001    63.02        64.93             3.0%
 test-suite...gebra/kernels/syr2k/syr2k.test     6.53         6.73             3.0%
 test-suite...t/StatementReordering-flt.test     2.33         2.40             2.8%
 test-suite...sCRaw.test:BM_PIC_2D_RAW/44217   789.90       810.05             2.6%
 test-suite...s/gramschmidt/gramschmidt.test     1.44         1.48             2.5%
 test-suite...Raw.test:BM_HYDRO_1D_RAW/44217    38.42        39.37             2.5%
 test-suite....test:BM_INT_PREDICT_RAW/44217   597.73       612.34             2.4%
 Geomean difference                                                           -0.0%
        results_base  results_loop_dist        diff
count  584.000000     584.000000         584.000000
mean   2761.681991    2759.451499       -0.000020  
std    30145.555650   30124.858004       0.011093  
min    0.608782       0.608729          -0.116286  
25%    3.125425       3.106625          -0.000461  
50%    130.212207     130.582658         0.000004  
75%    602.708659     612.931769         0.000438  
max    511340.880000  511059.980000      0.087630

For AArch64, the performance data from SPEC benchmark is as below.

SPEC2006		
Benchmark		Improvement(%)
400.perlbench		-1.786911228
401.bzip2		-3.174199894
403.gcc		0.717990522
429.mcf		2.053027806
445.gobmk		0.775388165
456.hmmer		43.39308377
458.sjeng		0.133933093
462.libquantum		4.647923489
464.h264ref		-0.059568786
471.omnetpp		1.352515266
473.astar		0.362752409
483.xalancbmk		0.746580249
		
SPEC2017		
Benchmark		Improvement(%)
500.perlbench_r		0.415424516
502.gcc_r		-0.112915812
505.mcf_r		0.238633706
520.omnetpp_r		0.114830748
523.xalancbmk_r		0.460107636
525.x264_r		-0.401915964
531.deepsjeng_r		0.010064227
541.leela_r		0.394797504
557.xz_r		0.111781366

Thanks
JinGu Kang

> -----Original Message-----
> From: Michael Kruse <llvmdev at meinersbur.de>
> Sent: 17 June 2021 19:13
> To: Jingu Kang <Jingu.Kang at arm.com>
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] Enabling Loop Distribution Pass as default in the pipeline
> of new pass manager
> 
> The LoopDistribute pass doesn't do anything unless it sees
> llvm.loop.distribute.enable (`#pragma clang loop distribute(enable)`) because it
> does not have a profitability heuristic. It cannot say whether loop distribution is
> good for performance or not. What makes it improve hmmer is that the
> distributed loops can be vectoried.
> However, LoopDistribute is located before the vectorizer and cannot say in
> advance whether a distributed loop will be vectorized or not.
> If not, then it potentially only increased loop overhead.
> 
> To make -enable-loop-distribute on by default would mean that we could
> consider loop distribution to be usually beneficial without causing major
> regressions. We need a lot more data to support that conclusion.
> 
> Alternatively, we could consider loop-distribution a canonicalization.
> A later LoopFuse would do the profitability heuristic to re-fuse loops again if
> loop distribution did not gain anything.
> 
> Michael