| Issue |
130376
|
| Summary |
Missed use of movk on aarch64 when performing disjoint OR with shifted 16 bit immediate
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
neildhar
|
I've observed cases where clang emits a suboptimal sequence of `mov` + `orr` or when a `movk` would suffice:
The most straightforward case is as follows:
```
uint64_t foo(uint32_t* raw){
// load
uint64_t res = *raw;
// movk
res |= (uint64_t)0xfffd << 48;
// ret
return res;
}
```
For which clang emits:
```
ldr w8, [x0]
mov x9, #-844424930131968
orr x0, x8, x9
ret
```
The `mov` + `orr` could instead be a single `movk`.
A related (but as far as I can tell, distinct) case:
```
uint64_t bar(uint32_t* raw){
// load
uint64_t res = *raw;
// movk
res |= (uint64_t)0xfffd << 48;
// asr
res = (int64_t)res >> 3;
// ret
return res;
}
```
For which clang emits:
```
ldr w8, [x0]
mov x9, #175921860444160
movk x9, #65535, lsl #48
orr x0, x9, x8, lsr #3
ret
```
Based on a quick scan of the resulting IR, the additional consideration here is that the arithmetic shift is turned into a logical shift (and the constant is made correspondingly larger) before getting to ISel.
https://godbolt.org/z/h95E3zhhd
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