[all-commits] [llvm/llvm-project] a21a7d: [X86] Optimize umax(X, 1) (NFC)

[kazutakahirata via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: a21a7ddf5ad1f34874cddb4d10cbd40b8ce1bef8
      https://github.com/llvm/llvm-project/commit/a21a7ddf5ad1f34874cddb4d10cbd40b8ce1bef8
  Author: Kazu Hirata <kazu at google.com>
  Date:   2023-03-06 (Mon, 06 Mar 2023)

  Changed paths:
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/test/CodeGen/X86/umax.ll

  Log Message:
  -----------
  [X86] Optimize umax(X,1) (NFC)

Without this patch:

  %cond = call i32 @llvm.umax.i32(i32 %X, i32 1)

is compiled as:

  83 ff 02                   cmp    $0x2,%edi
  b8 01 00 00 00             mov    $0x1,%eax
  0f 43 c7                   cmovae %edi,%eax

With this patch, the compiler generates:

  89 f8                      mov    %edi,%eax
  83 ff 01                   cmp    $0x1,%edi
  83 d0 00                   adc    $0x0,%eax

saving 3 bytes.  We should be able to save 5 bytes in larger functions
where the mov is unnecessary.

This patch converts the specific cmov pattern to cmp $1 followed by
adc $0.

This patch partially fixes:

https://github.com/llvm/llvm-project/issues/60374

The LLVM IR optimizer is yet to canonicalize max expressions to
actual @llvm.umax.

Differential Revision: https://reviews.llvm.org/D144451