| Issue |
131457
|
| Summary |
[AArch64] Opportunity to materialise constants using `sub 0, imm, lsl 12`
|
| Labels |
backend:AArch64,
llvm:optimizations,
missed-optimization
|
| Assignees |
|
| Reporter |
Kmeakin
|
Instead of materialising `-(4095 << 12)` with a MOVZ+MOVK pair, it can be done in a single instruction by subtracting from the zero register:
```c
uint32_t src32(uint32_t x) { return 0 - (4095 << 12); }
uint32_t tgt32(uint32_t x) { return x - (4095 << 12); }
uint64_t src64(uint64_t x) { return 0 - (4095 << 12); }
uint64_t tgt64(uint64_t x) { return x - (4095 << 12); }
```
```asm
src32:
mov w0, #4096
movk w0, #65280, lsl #16
ret
tgt32:
sub w0, w0, #4095, lsl #12
ret
src64:
mov x0, #-61440
movk x0, #65280, lsl #16
ret
tgt64:
sub x0, x0, #4095, lsl #12
ret
```
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