| Issue |
150954
|
| Summary |
x86 missed optimization for __builtin_clz()^31
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Alcaro
|
```c
#include <stdint.h>
uint8_t b(uint32_t in)
{
uint8_t ret = __builtin_clz(in) ^ 31;
return ret;
}
uint8_t c(uint32_t in)
{
uint8_t ret = __builtin_clz(in) ^ 31;
return ret + 1;
}
```
Expected: Since b optimizes to a single `bsr eax,edi`, c should optimize to at most one instruction more.
Actual:
```
b(unsigned int):
bsr eax, edi
ret
c(unsigned int):
bsr ecx, edi
xor ecx, 31
mov al, 32
sub al, cl
ret
```
GCC gives good output (probably by being less clever about normalization), as does Clang if I add an extra optimization barrier. https://godbolt.org/z/3ansdnMxa
![]()
_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
