[PATCH] D158291: [PoC][WIP] Add an AArch64 specific pass for loop idiom recognition

[Kerry McLaughlin via Phabricator via llvm-commits]

kmclaughlin created this revision.
Herald added subscribers: ctetreau, arphaman, hiraditya, kristof.beyls.
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kmclaughlin requested review of this revision.
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This pass looks for loops such as the following:

  while (i != max_len)
      if (a[i] != b[i])
          break;

Although similar to a memcmp, this is slightly difference because instead of returning
the difference between the values of the first non-matching pair of bytes, it returns
the index of the first mismatch. As such, we are not able to lower this to a memcmp call.
Replacing this pattern with a specialised predicated SVE loop gives a significant
performance improvement for AArch64.

This patch introduces a new pass which identifies this pattern and replaces it with the
SVE loop. It is intended as a short-term solution until this is handled in the vectoriser.

A new intrinsic is created in this patch for counting the trailing zero elements in a
vector which has generic lowering in SelectionDAGBuilder. For AArch64 where SVE is
enabled, this is replaced with brkb & cntp instructions.

Note: This is a work in progress, see discussion on Discourse:
https://discourse.llvm.org/t/aarch64-target-specific-loop-idiom-recognition/72383


https://reviews.llvm.org/D158291

Files:
  llvm/include/llvm/CodeGen/TargetLowering.h
  llvm/include/llvm/IR/Intrinsics.td
  llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  llvm/lib/Passes/PassBuilderPipelines.cpp
  llvm/lib/Target/AArch64/AArch64.h
  llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
  llvm/lib/Target/AArch64/AArch64ISelLowering.h
  llvm/lib/Target/AArch64/AArch64LoopIdiomRecognize.cpp
  llvm/lib/Target/AArch64/AArch64LoopIdiomRecognize.h
  llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
  llvm/lib/Target/AArch64/AArch64TargetMachine.h
  llvm/lib/Target/AArch64/CMakeLists.txt
  llvm/lib/Target/AArch64/SVEInstrFormats.td
  llvm/test/CodeGen/AArch64/intrinsic-cttz-elts.ll
  llvm/test/Other/new-pm-defaults.ll
  llvm/test/Transforms/LoopIdiom/AArch64/byte-compare-index.ll
  llvm/utils/gn/secondary/llvm/lib/Target/AArch64/BUILD.gn

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