| Issue |
71389
|
| Summary |
Constants are not reused for their inverse (andn/orn)
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Validark
|
I have this code:
```zig
export fn missed_andn(v: u64) u64 {
const mask: u64 = 0x7F7F7F7F7F7F7F7F;
const low_7_bits = v & mask;
const non_zero = low_7_bits + mask;
return non_zero & ~mask;
}
```
[On x86_64 Zen 4, I get](https://zig.godbolt.org/z/vsebWje1r):
```asm
missed_andn:
movabs rcx, 9187201950435737471
movabs rax, -9187201950435737472
and rdi, rcx
add rcx, rdi
and rax, rcx
ret
```
I think we could have used an `andn` instruction rather than load 2 constants.
I believe the the same principle applies on other architectures even without `andn`/`orn`, especially load-store architectures like the RISC-V SiFive U74, on which we get:
```asm
.LCPI0_0:
.quad -9187201950435737472
missed_andn:
lui a1, 522232
addiw a1, a1, -129
slli a2, a1, 32
lui a3, %hi(.LCPI0_0)
add a1, a1, a2
and a0, a0, a1
add a0, a0, a1
ld a1, %lo(.LCPI0_0)(a3)
and a0, a0, a1
ret
```
It could probably have been:
```asm
missed_andn:
lui a1, 522232
addiw a1, a1, -129
slli a2, a1, 32
add a1, a1, a2
and a0, a0, a1
add a0, a0, a1
not a1, a1
and a0, a0, a1
ret
```
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