Dávid Bolvanský 09515f2c20 [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

2023-06-01 20:56:44 +02:00

86 KiB

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