| Issue |
129401
|
| Summary |
Improve `llvm.ucmp.i8.i1` codegen
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
scottmcm
|
(Context: I was making a rustc PR and accidentally regressed `bool::cmp` by having it use `llvm.ucmp`, thus this bug that it would be nice if `ucmp` just was smart about it.)
`llvm.ucmp.i8.i1(a, b)` is actually the same as just `zext(a) - zext(b)`: <https://alive2.llvm.org/ce/z/oHq3bh>
But today they don't codegen the same: <https://llvm.godbolt.org/z/nxWdYhvTo>
```llvm
define noundef range(i8 -1, 2) i8 @src(i1 noundef zeroext %a, i1 noundef zeroext %b) unnamed_addr {
start:
%0 = call i8 @llvm.ucmp.i8.i1(i1 %a, i1 %b)
ret i8 %0
}
define noundef range(i8 -1, 2) i8 @tgt(i1 noundef zeroext %a, i1 noundef zeroext %b) unnamed_addr {
start:
%aa = zext i1 %a to i8
%bb = zext i1 %b to i8
%0 = sub nsw i8 %aa, %bb
ret i8 %0
}
```
on x64 gives
```asm
src: # @src
cmp dil, sil
seta al
sbb al, 0
ret
tgt: # @tgt
mov eax, edi
sub al, sil
ret
```
I don't know if it's better to InstSimplify the `ucmp` to `sext(b) + zext(a)` or to improve the codegen for the `i1` case, but either way, it'd be nice if the intrinsic worked optimally for `i1` in addition to the wider widths.
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