[PATCH] D96388: [AArch64][GlobalISel] Fold G_ADD into the cset for G_ICMP

[Jessica Paquette via Phabricator via llvm-commits]

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When we have a G_ADD which is fed by a G_ICMP on one side, we can fold it into the cset for the G_ICMP.

e.g. Given

  %cmp = G_ICMP ... %x, %y
  %add = G_ADD %cmp, %z

We would normally emit a cmp, cset, and add.

However, `%add` is either `%z` or `%z + 1`. So, we can just use `%z` as the source of the cset rather than wzr, saving an instruction.

This would probably be cleaner in AArch64PostLegalizerLowering, but we'd need to change the way we represent G_ICMP to do that, I think. For now, it's easiest to implement in selection.

This is a 0.1% code size improvement on CTMark/pairlocalalign at -Os.

Example: https://godbolt.org/z/7KdrP8


https://reviews.llvm.org/D96388

Files:
  llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/select-cmp.mir

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