| Issue |
86874
|
| Summary |
Missed peephole opt: `sub nuw 15, x` => `xor x, 15`
|
| Labels |
llvm:instcombine,
missed-optimization
|
| Assignees |
|
| Reporter |
Kmeakin
|
This transformation is beneficial because materializing the `15` argument to `sub` requires an extra instruction, but the `15` argument to `xor` can fit directly into the `xor` instruction
Rust example (https://godbolt.org/z/h7aTc5sca):
```rust
#![feature(unchecked_math)]
#[no_mangle]
pub fn src(x: u8) -> u8 {
unsafe { 15_u8.unchecked_sub(x) }
}
#[no_mangle]
pub fn tgt(x: u8) -> u8 {
x ^ 15
}
```
alive proof: https://alive2.llvm.org/ce/z/YKCPCA
```llvm
define noundef i8 @src(i8 noundef %x) unnamed_addr #0 {
%_0 = sub nuw i8 15, %x
ret i8 %_0
}
define noundef i8 @tgt(i8 noundef %x) unnamed_addr #0 {
%_0 = xor i8 %x, 15
ret i8 %_0
}
```
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