[PATCH] D100242: [SystemZ / TII] Peephole optimization of zero-extension of i1.

Sat Apr 10 03:12:06 PDT 2021

jonpa created this revision.
jonpa added reviewers: uweigand, qcolombet.
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This is yet another attempt to eliminate unnecessary loads of immediates in case where it is already known by the preceding comparison (https://reviews.llvm.org/D98905, https://reviews.llvm.org/D100039).

**SystemZ:**

- Added isSelect flag on LOCHIMux and LOCGHI.
- Implemented analyzeSelect() and optimizeSelect() for them.

**TargetInstrInfo - analyzeSelect() and optimizeSelect():**

Changed the handling of optimizeSelect() so that target can return a modified instrution in which case it is *not* deleted.

If (as it appears to me) PeepholeOptimizer.cpp is the only user of these hooks (and there are no downstream out-of-tree targets that have requested this), maybe we could merge these two hooks? It seems this could more or less be just one 'optimizeSelect()' method as there appears to be no use for the arguments to analyzeSelect(), or?

If the arguments to analyzeSelect() are indeed needed to be filled out, the current patch makes sense, by doing a careful analysis in that method. Otherwise, it is a waste as it has to be redone in optimizeSelect() (It would probably be better to return true from analyzeSelect() from the interesting opcodes and then do the work in optimizeSelect()).

It seems to me that handling this in PeepholeOptimizer is probably the right approach...

**Benchmarks:**

I tried four combinations of two options: "single use of compare operand" and "find LHIMux/LGHI via MRI if not found locally" (experimental options in the patch):

  master <> "multiple users" + "only cases with local LHIMux/LGHI"
  lhi            :               225040               222044    -2996
  lghi           :               445603               444910     -693
  lr             :                61869                62276     +407
  lgr            :               853946               854211     +265
  ...

  master <> "single uer" + "only cases with local LHIMux/LGHI"
  lhi            :               225040               222702    -2338
  lghi           :               445603               445263     -340
  lgr            :               853946               854083     +137
  lr             :                61869                61928      +59
  ...

  master <> "multiple users" + "use MRI to find LHIMux/LGHI"
  lhi            :               225040               220319    -4721
  lghi           :               445603               443104    -2499
  lr             :                61869                62808     +939
  lgr            :               853946               854436     +490
  ...

  master <> "single user" + "use MRI to find LHIMux/LGHI"
  lhi            :               225040               221788    -3252
  lghi           :               445603               443556    -2047
  lgr            :               853946               854352     +406
  lr             :                61869                61942      +73
  ...

Initial measurements do not show any bigger performance changes either way...

https://reviews.llvm.org/D100242

Files:
  llvm/include/llvm/CodeGen/TargetInstrInfo.h
  llvm/lib/CodeGen/PeepholeOptimizer.cpp
  llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
  llvm/lib/Target/SystemZ/SystemZInstrInfo.h
  llvm/lib/Target/SystemZ/SystemZInstrInfo.td
  llvm/test/CodeGen/SystemZ/setcc-05.ll

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