https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124453
Bug ID: 124453
Summary: suboptimal read of 64-bit bitfields
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: lh_mouse at 126 dot com
Target Milestone: ---
https://gcc.godbolt.org/z/9fPMqT77f:
```c
typedef __UINT64_TYPE__ uint64_t;
struct Value
{
uint64_t a : 1;
uint64_t b : 4;
uint64_t c : 59;
};
int
ctz_b(const struct Value* p)
{
struct Value val = *p;
if(val.b == 0)
return -1;
return __builtin_ctz(val.b);
}
```
GCC 15:
```
get_b:
mov rax, QWORD PTR [rdi]
test al, 30
je .L3
shr rax # this just needs to be `shr eax`
and eax, 15
rep bsf eax, eax
ret
.L3:
mov eax, -1
ret
```
GCC 16:
```
"get_b":
mov rdx, QWORD PTR [rdi]
mov rax, rdx # should be `mov eax, edx`
shr rax # likewise `shr eax`
and eax, 15
rep bsf eax, eax
and edx, 30
mov edx, -1
cmove eax, edx
ret
```
