https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118637
Bug ID: 118637
Summary: gcc fails to optimize unsigned division by 2 to shift
right by 1
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: davidfromonline at gmail dot com
Target Milestone: ---
The following code:
```c++
unsigned log2_divide(unsigned x) {
unsigned result = 0;
while (x /= 2) {
++result;
}
return result;
}
unsigned log2_shift(unsigned x) {
unsigned result = 0;
while (x >>= 1) {
++result;
}
return result;
}
```
when compiled with `-O3` generates
```asm
log2_divide(unsigned int):
cmp edi, 1
jbe .L4
shr edi
xor eax, eax
.L3:
mov edx, edi
add eax, 1
shr edi
cmp edx, 1
jne .L3
ret
.L4:
xor eax, eax
ret
log2_shift(unsigned int):
mov eax, edi
shr eax
je .L7
bsr eax, eax
add eax, 1
.L7:
ret
```
I would expect `log2_divide` to generate the same code as `log2_shift`.
See it live: https://godbolt.org/z/fahr3Moe3
