| Issue |
172176
|
| Summary |
[missed-opt] [instcombine] fshl/fshr not pushed into constant-argument phi
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
purplesyringa
|
Rust reproducer because I don't remember how to trigger funnel shifts with C intrinsics.
[Godbolt](https://godbolt.org/z/qWs9W6r5f)
```rust
#[unsafe(no_mangle)]
pub fn fshl(cond: bool) -> u32 {
if cond { 0xd2c31701u32 } else { 0xfd46843fu32 }.rotate_left(3)
}
#[unsafe(no_mangle)]
pub fn fshr(cond: bool) -> u32 {
if cond { 0xd2c31701u32 } else { 0xfd46843fu32 }.rotate_right(3)
}
```
```asm
fshl:
test edi, edi
mov ecx, -758966527
mov eax, -45710273
cmovne eax, ecx
rol eax, 3
ret
fshr:
test edi, edi
mov ecx, -758966527
mov eax, -45710273
cmovne eax, ecx
rol eax, 29
ret
```
I would like the `rol` to be absent and inlined into the constants.
IR:
```llvm
define noundef i32 @fshl(i1 noundef zeroext %0) unnamed_addr {
br i1 %0, label %3, label %2
2: ; preds = %1
br label %4
3: ; preds = %1
br label %4
4: ; preds = %3, %2
%5 = phi i32 [ -758966527, %3 ], [ -45710273, %2 ]
%6 = call i32 @llvm.fshl.i32(i32 %5, i32 %5, i32 3)
ret i32 %6
}
define noundef i32 @fshr(i1 noundef zeroext %0) unnamed_addr {
br i1 %0, label %3, label %2
2: ; preds = %1
br label %4
3: ; preds = %1
br label %4
4: ; preds = %3, %2
%5 = phi i32 [ -758966527, %3 ], [ -45710273, %2 ]
%6 = call i32 @llvm.fshr.i32(i32 %5, i32 %5, i32 3)
ret i32 %6
}
```
This applies specifically to funnel shifts, not normal shifts.
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