| Issue |
131882
|
| Summary |
constprop in bitwise operations pessimizes constant selection
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
purplesyringa
|
[godbolt](https://godbolt.org/z/8Y5xzGWcd)
```cpp
#include <utility>
std::pair<long, long> f(long n) {
const long C = 0x7ba52da77e55f70b;
return {n & C, (n * 2) & C};
}
```
LLVM seems dead set on loading two distinct constants because it knows `n * 2` is even, and so the mask can be "simplified" to `0x7ba52da77e55f70a` in one of the cases:
```asm
f(long):
movabs rcx, 8909577635224745738
lea rax, [rcx + 1]
and rax, rdi
lea rdx, [rdi + rdi]
and rdx, rcx
ret
```
While sharing the constant would work just fine:
```asm
f(long):
movabs rax, 8909577635224745739
lea rdx, [rdi + rdi]
and rdx, rax
and rax, rdi
ret
```
And LLVM *can* produce this codegen if I [disable constprop of `C`](https://godbolt.org/z/oE1n9PWx1).
Applies to x86-64, aarch64, probably everything else under the sun.
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