| Issue |
147176
|
| Summary |
[InstCombine] Unprofitable transformation
|
| Labels |
llvm:instcombine,
missed-optimization
|
| Assignees |
|
| Reporter |
nikic
|
https://llvm.godbolt.org/z/PP1q1hTYE
```llvm
define i8 @src(i8 noundef %_23) {
%_17 = lshr i8 %_23, 4
%_18 = and i8 %_23, 15
%_16 = add nuw nsw i8 %_17, %_18
ret i8 %_16
}
define i8 @tgt(i8 noundef %_23) {
%_17 = lshr i8 %_23, 4
%1 = mul i8 %_17, -15
%_16 = add i8 %1, %_23
ret i8 %_16
}
```
This goes against our usual guideline to avoid introducing mul out of thin air, and also increases the length of the dependence chain.
From: https://github.com/rust-lang/rust/issues/143513
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