[all-commits] [llvm/llvm-project] c19e90: [AArch64] Signed comparison using CMN is safe when...

[AZero13 via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c19e900ce8b422f6b8c028fbbd9ef7e9d3720236
      https://github.com/llvm/llvm-project/commit/c19e900ce8b422f6b8c028fbbd9ef7e9d3720236
  Author: AZero13 <gfunni234 at gmail.com>
  Date:   2025-06-12 (Thu, 12 Jun 2025)

  Changed paths:
    M llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
    M llvm/test/CodeGen/AArch64/cmp-to-cmn.ll

  Log Message:
  -----------
  [AArch64] Signed comparison using CMN is safe when the subtraction is nsw (#141993)

nsw means no signed wrap, and 0 - INT_MIN is a signed wrap.

Now, this is going to be a point I need to get out of the way:

So is it okay to always transform a > -b into cmn if it is a signed
comparison, even if b is INT_MIN because -INT_MIN is undefined, at least
in C, because unless fwrapv is specified, opt puts nsw on signed integer
operations, allowing for more folds anyway.



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