[all-commits] [llvm/llvm-project] e094ab: [SelectionDAG] Expand [US]CMP using arithmetic on ...

[Volodymyr Vasylkun via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e094abde42634e38cda85a6024792f681fc58f32
      https://github.com/llvm/llvm-project/commit/e094abde42634e38cda85a6024792f681fc58f32
  Author: Volodymyr Vasylkun <vvmposeydon at gmail.com>
  Date:   2024-07-16 (Tue, 16 Jul 2024)

  Changed paths:
    M llvm/include/llvm/CodeGen/TargetLowering.h
    M llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
    M llvm/lib/Target/AArch64/AArch64ISelLowering.h
    M llvm/lib/Target/SystemZ/SystemZISelLowering.h
    A llvm/test/CodeGen/ARM/scmp.ll
    A llvm/test/CodeGen/ARM/ucmp.ll
    A llvm/test/CodeGen/LoongArch/scmp.ll
    A llvm/test/CodeGen/LoongArch/ucmp.ll
    A llvm/test/CodeGen/PowerPC/scmp.ll
    A llvm/test/CodeGen/PowerPC/ucmp.ll
    A llvm/test/CodeGen/RISCV/scmp.ll
    A llvm/test/CodeGen/RISCV/ucmp.ll
    A llvm/test/CodeGen/SystemZ/scmp.ll
    A llvm/test/CodeGen/SystemZ/ucmp.ll
    A llvm/test/CodeGen/Thumb/scmp.ll
    A llvm/test/CodeGen/Thumb/ucmp.ll
    A llvm/test/CodeGen/WebAssembly/scmp.ll
    A llvm/test/CodeGen/WebAssembly/ucmp.ll
    M llvm/test/CodeGen/X86/scmp.ll
    M llvm/test/CodeGen/X86/ucmp.ll

  Log Message:
  -----------
  [SelectionDAG] Expand [US]CMP using arithmetic on boolean values instead of selects (#98774)

The previous expansion of [US]CMP was done using two selects and two
compares. It produced decent code, but on many platforms it is better to
implement [US]CMP nodes by performing the following operation:

  ```
[us]cmp(x, y) = (x [us]> y) - (x [us]< y)
```

This patch adds this new expansion, as well as a hook in TargetLowering to allow some targets to still use the select-based approach. AArch64 and SystemZ are currently the only targets to prefer the former approach, but other targets may also start to use it if it provides for better codegen.



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