| Issue |
171750
|
| Summary |
shl nuw + zext adds unnecessary masking
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
purplesyringa
|
[Godbolt](https://godbolt.org/z/jGcYTYv3d)
```cpp
#include <stdint.h>
uint32_t f(uint16_t x) {
if (x >= 8192) {
__builtin_unreachable();
}
return (uint16_t)(x << 3);
}
uint32_t g(uint16_t x) {
if (x >= 8192) {
__builtin_unreachable();
}
return (uint16_t)(x << 3) | ((uint32_t)x << 16);
}
```
Expected behavior: `x` fits in 13 bits, so `x << 3` fits in 16 bits and the cast can be optimized out. Current lowering:
```asm
f(unsigned short):
shl edi, 3
movzx eax, di ; unnecessary
ret
g(unsigned short):
mov eax, edi
and eax, 8191 ; unnecessary
shl edi, 16
lea eax, [rdi + 8*rax]
ret
```
IR recognizes shift as non-wrapping with `shl nuw`, so this is just a missed opt in isel.
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