[all-commits] [llvm/llvm-project] 175d29: [LoopUnroll] Add CSE to remove redundant loads aft...

[Florian Hahn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 175d2971020ceaad3e1adcf9bb92e4ebaaa449ee
      https://github.com/llvm/llvm-project/commit/175d2971020ceaad3e1adcf9bb92e4ebaaa449ee
  Author: Florian Hahn <flo at fhahn.com>
  Date:   2024-05-02 (Thu, 02 May 2024)

  Changed paths:
    M llvm/include/llvm/Analysis/MemorySSA.h
    M llvm/include/llvm/Transforms/Utils/UnrollLoop.h
    M llvm/lib/Analysis/MemorySSA.cpp
    M llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
    M llvm/lib/Transforms/Utils/LoopUnroll.cpp
    M llvm/test/Transforms/LoopUnroll/shifted-tripcount.ll
    M llvm/test/Transforms/LoopUnroll/unroll-loads-cse.ll
    M llvm/test/Transforms/PhaseOrdering/AArch64/extra-unroll-simplifications.ll

  Log Message:
  -----------
  [LoopUnroll] Add CSE to remove redundant loads after unrolling. (#83860)

This patch adds loadCSE support to simplifyLoopAfterUnroll. It is based
on EarlyCSE's implementation using ScopeHashTable and is using SCEV for
accessed pointers to check to find redundant loads after unrolling.

This applies to the late unroll pass only, for full unrolling those
redundant loads will be cleaned up by the regular pipeline.

The current approach constructs MSSA on-demand per-loop, but there is
still small but notable compile-time impact:

stage1-O3  +0.04%
stage1-ReleaseThinLTO +0.06%
stage1-ReleaseLTO-g +0.05%
stage1-O0-g +0.02%
stage2-O3 +0.09%
stage2-O0-g +0.04%
stage2-clang +0.02%


https://llvm-compile-time-tracker.com/compare.php?from=c089fa5a729e217d0c0d4647656386dac1a1b135&to=ec7c0f27cb5c12b600d9adfc8543d131765ec7be&stat=instructions:u

This benefits some workloads with runtime-unrolling disabled,
where users use pragmas to force unrolling, as well as with
runtime unrolling enabled.

On SPEC/MultiSource, this removes a number of loads after unrolling
on AArch64 with runtime unrolling enabled.

```
External/S...te/526.blender_r/526.blender_r    96
MultiSourc...rks/mediabench/gsm/toast/toast    39
SingleSource/Benchmarks/Misc/ffbench            4
External/SPEC/CINT2006/403.gcc/403.gcc         18
MultiSourc.../Applications/JM/ldecod/ldecod     4
MultiSourc.../mediabench/jpeg/jpeg-6a/cjpeg     6
MultiSourc...OE-ProxyApps-C/miniGMG/miniGMG     9
MultiSourc...e/Applications/ClamAV/clamscan     4
MultiSourc.../MallocBench/espresso/espresso     3
MultiSourc...dence-flt/LinearDependence-flt     2
MultiSourc...ch/office-ispell/office-ispell     4
MultiSourc...ch/consumer-jpeg/consumer-jpeg     6
MultiSourc...ench/security-sha/security-sha    11
MultiSourc...chmarks/McCat/04-bisect/bisect     3
SingleSour...tTests/2020-01-06-coverage-009    12
MultiSourc...ench/telecomm-gsm/telecomm-gsm    39
MultiSourc...lds-flt/CrossingThresholds-flt    24
MultiSourc...dence-dbl/LinearDependence-dbl     2
External/S...C/CINT2006/445.gobmk/445.gobmk     6
MultiSourc...enchmarks/mafft/pairlocalalign    53
External/S...31.deepsjeng_r/531.deepsjeng_r     3
External/S...rate/510.parest_r/510.parest_r    58
External/S...NT2006/464.h264ref/464.h264ref    29
External/S...NT2017rate/502.gcc_r/502.gcc_r    45
External/S...C/CINT2006/456.hmmer/456.hmmer     6
External/S...te/538.imagick_r/538.imagick_r    18
External/S.../CFP2006/447.dealII/447.dealII     4
MultiSourc...OE-ProxyApps-C++/miniFE/miniFE    12
External/S...2017rate/525.x264_r/525.x264_r    36
MultiSourc...Benchmarks/7zip/7zip-benchmark    33
MultiSourc...hmarks/ASC_Sequoia/AMGmk/AMGmk     2
MultiSourc...chmarks/VersaBench/8b10b/8b10b     1
MultiSourc.../Applications/JM/lencod/lencod   116
MultiSourc...lds-dbl/CrossingThresholds-dbl    24
MultiSource/Benchmarks/McCat/05-eks/eks        15
```

PR: https://github.com/llvm/llvm-project/pull/83860



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