[PATCH] D80460: [AArch64][GlobalISel] When generating SUBS for compares, don't write to wzr/xzr.

[Amara Emerson via Phabricator via llvm-commits]

aemerson created this revision.
aemerson added a reviewer: paquette.
aemerson added a project: LLVM.
Herald added subscribers: danielkiss, hiraditya, kristof.beyls, rovka.

Although writing to wzr/xzr is correct since we don't care about the result of the sub, only the flags, doing so causes tail merge blocks to fail.

Writing to an unused virtual register instead allows the optimization to fire, improving performance significantly on 256.bzip2.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D80460

Files:
  llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/opt-fold-compare.mir
  llvm/test/CodeGen/AArch64/GlobalISel/select-arith-immed-compare.mir
  llvm/test/CodeGen/AArch64/GlobalISel/select-cmp.mir
  llvm/test/CodeGen/AArch64/GlobalISel/select.mir
  llvm/test/CodeGen/AArch64/GlobalISel/tbnz-slt.mir
  llvm/test/CodeGen/AArch64/GlobalISel/tbz-sgt.mir

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