[all-commits] [llvm/llvm-project] 842d0b: [x86] Improve select lowering for smin(x, 0) & sma...

[Wei Xiao via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 842d0bf93176b9cb1e0d6894a2bbfb32ad33ebb8
      https://github.com/llvm/llvm-project/commit/842d0bf93176b9cb1e0d6894a2bbfb32ad33ebb8
  Author: Wei Xiao <wei3.xiao at intel.com>
  Date:   2022-04-07 (Thu, 07 Apr 2022)

  Changed paths:
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/test/CodeGen/X86/select-smin-smax.ll

  Log Message:
  -----------
  [x86] Improve select lowering for smin(x, 0) & smax(x, 0)

smin(x, 0):
  (select (x < 0), x, 0) -> ((x >> (size_in_bits(x)-1))) & x

smax(x, 0):
  (select (x > 0), x, 0) -> (~(x >> (size_in_bits(x)-1))) & x
  The comparison is testing for a positive value, we have to invert the sign
  bit mask, so only do that transform if the target has a bitwise 'and not'
  instruction (the invert is free).

The transform is performed only when CMP has a single user to avoid
increasing total instruction number.

https://alive2.llvm.org/ce/z/euUnNm
https://alive2.llvm.org/ce/z/37339J

Differential Revision: https://reviews.llvm.org/D123109