[PATCH] D135633: [GlobalISel] Combine things like (z = x <= 0 ? z = x : z = 0) -> x & (x >> bw-1)

[Jessica Paquette via Phabricator via llvm-commits]

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If we have a signed comparison like this:

  %cmp = icmp sge i32 %x, 0
  %select = select i1 %cmp, i32 %x, i32 0
  ret i32 %select

We can represent it using an `and` + a right shift.

On AArch64, this allows us to do the cmp + select in a single instruction.

This also works with signed less-than operators, but instead we compliment the
shift first.

Saves 0.12% code size on CTMark/lencod @ -Os for AArch64. Other minor code size
savings as well with no regressions.

https://godbolt.org/z/7E4vYWah6


https://reviews.llvm.org/D135633

Files:
  llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
  llvm/include/llvm/CodeGen/TargetLowering.h
  llvm/include/llvm/Target/GlobalISel/Combine.td
  llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
  llvm/lib/Target/AArch64/AArch64ISelLowering.h
  llvm/test/CodeGen/AArch64/combine-select-to-and-shift.mir

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