[all-commits] [llvm/llvm-project] 09515f: [SDAG] Preserve unpredictable metadata, teach X86C...

[Dávid Bolvanský via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 09515f2c20111628ce81ad5f40e12e5f6af5ed2f
      https://github.com/llvm/llvm-project/commit/09515f2c20111628ce81ad5f40e12e5f6af5ed2f
  Author: Dávid Bolvanský <david.bolvansky at gmail.com>
  Date:   2023-06-01 (Thu, 01 Jun 2023)

  Changed paths:
    M llvm/include/llvm/CodeGen/MachineInstr.h
    M llvm/include/llvm/CodeGen/SelectionDAGNodes.h
    M llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
    M llvm/lib/CodeGen/MIRParser/MILexer.cpp
    M llvm/lib/CodeGen/MIRParser/MILexer.h
    M llvm/lib/CodeGen/MIRParser/MIParser.cpp
    M llvm/lib/CodeGen/MIRPrinter.cpp
    M llvm/lib/CodeGen/MachineInstr.cpp
    M llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
    M llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
    M llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
    M llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
    M llvm/lib/Target/PowerPC/PPCInstrInfo.h
    M llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
    M llvm/lib/Target/SPIRV/SPIRVPreLegalizer.cpp
    M llvm/lib/Target/X86/X86CmovConversion.cpp
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/lib/Target/X86/X86InstrInfo.cpp
    M llvm/test/CodeGen/X86/x86-cmov-converter.ll

  Log Message:
  -----------
  [SDAG] Preserve unpredictable metadata, teach X86CmovConversion to respect this metadata

Sometimes an developer would like to have more control over cmov vs branch. We have unpredictable metadata in LLVM IR, but currently it is ignored by X86 backend. Propagate this metadata and avoid cmov->branch conversion in X86CmovConversion for cmov with this metadata.

Example:

```
int MaxIndex(int n, int *a) {
    int t = 0;
    for (int i = 1; i < n; i++) {
        // cmov is converted to branch by X86CmovConversion
        if (a[i] > a[t]) t = i;
    }
    return t;
}

int MaxIndex2(int n, int *a) {
    int t = 0;
    for (int i = 1; i < n; i++) {
        // cmov is preserved
        if (__builtin_unpredictable(a[i] > a[t])) t = i;
    }
    return t;
}
```

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D118118