[all-commits] [llvm/llvm-project] 35b398: [Passes] Run peeling as part of simple/full loop u...

[Florian Hahn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 35b3989a30eefa66cd6edca4c6e1ec061c05ad96
      https://github.com/llvm/llvm-project/commit/35b3989a30eefa66cd6edca4c6e1ec061c05ad96
  Author: Florian Hahn <flo at fhahn.com>
  Date:   2021-01-26 (Tue, 26 Jan 2021)

  Changed paths:
    M llvm/include/llvm/Transforms/Scalar.h
    M llvm/include/llvm/Transforms/Scalar/LoopUnrollPass.h
    M llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
    M llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
    M llvm/test/Transforms/PhaseOrdering/X86/peel-before-lv-to-enable-vectorization.ll

  Log Message:
  -----------
  [Passes] Run peeling as part of simple/full loop unrolling.

Loop peeling removes conditions from loop bodies that become invariant
after a small number of iterations. When triggered, this leads to fewer
compares and possibly PHIs in loop bodies, enabling further
optimizations. The current cost-model of loop peeling should be quite
conservative/safe, i.e. only peel if a condition in the loop becomes
known after peeling.

For example, see PR47671, where loop peeling enables vectorization by
removing a PHI the vectorizer does not understand. Granted, the
loop-vectorizer could also be taught about constant PHIs, but loop
peeling is likely to enable other optimizations as well.

This has an impact on quite a few benchmarks from
MultiSource/SPEC2000/SPEC2006 on X86 with -O3 -flto, for example

    Same hash: 186 (filtered out)
    Remaining: 51
    Metric: loop-vectorize.LoopsVectorized

    Program                                        base   patch  diff
     test-suite...ve-susan/automotive-susan.test     8.00   9.00 12.5%
     test-suite...nal/skidmarks10/skidmarks.test    35.00  31.00 -11.4%
     test-suite...lications/sqlite3/sqlite3.test    41.00  43.00  4.9%
     test-suite...s/ASC_Sequoia/AMGmk/AMGmk.test    25.00  26.00  4.0%
     test-suite...006/450.soplex/450.soplex.test    88.00  89.00  1.1%
     test-suite...TimberWolfMC/timberwolfmc.test   120.00 119.00 -0.8%
     test-suite.../CINT2006/403.gcc/403.gcc.test   215.00 216.00  0.5%
     test-suite...006/447.dealII/447.dealII.test   957.00 958.00  0.1%
     test-suite...ternal/HMMER/hmmcalibrate.test    75.00  75.00  0.0%

    Same hash: 186 (filtered out)
    Remaining: 51
    Metric: loop-vectorize.LoopsAnalyzed

    Program                                        base    patch   diff
     test-suite...ks/Prolangs-C/agrep/agrep.test   440.00  434.00  -1.4%
     test-suite...nal/skidmarks10/skidmarks.test   312.00  308.00  -1.3%
     test-suite...marks/7zip/7zip-benchmark.test   6399.00 6323.00 -1.2%
     test-suite...lications/minisat/minisat.test   134.00  135.00   0.7%
     test-suite...rks/FreeBench/pifft/pifft.test   295.00  297.00   0.7%
     test-suite...TimberWolfMC/timberwolfmc.test   1879.00 1869.00 -0.5%
     test-suite...pplications/treecc/treecc.test   689.00  691.00   0.3%
     test-suite...T2000/300.twolf/300.twolf.test   1593.00 1597.00  0.3%
     test-suite.../Benchmarks/Bullet/bullet.test   1394.00 1392.00 -0.1%
     test-suite...ications/JM/ldecod/ldecod.test   1431.00 1429.00 -0.1%
     test-suite...6/464.h264ref/464.h264ref.test   2229.00 2230.00  0.0%
     test-suite...lications/sqlite3/sqlite3.test   2590.00 2589.00 -0.0%
     test-suite...ications/JM/lencod/lencod.test   2732.00 2733.00  0.0%
     test-suite...006/453.povray/453.povray.test   3395.00 3394.00 -0.0%

Note the -11% regression in number of loops vectorized for skidmarks. I
suspect this corresponds to the fact that those loops are gone now (see
the reduction in number of loops analyzed by LV).

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D88471