https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106231
Bug ID: 106231
Summary: sign-extension of the result of `__builtin_tzcnt()`
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: lh_mouse at 126 dot com
Target Milestone: ---
Given (godbolt: https://gcc.godbolt.org/z/hqKKW33T7):
```
long long
foo(long long x, unsigned bits)
{
return x + (unsigned) __builtin_ctz(bits);
}
```
GCC output:
```
foo:
bsf eax, esi
cdqe
add rax, rdi
ret
```
Clang output:
```
foo: # @foo
bsf eax, esi
add rax, rdi
ret
```
The CDQE instruction is totally unnecessary, because BSF stores either a
non-negative value or an undefined one into its destination operand.
It's sad that `__builtin_ctz()` yields a signed type; but even we cast it, we
don't get anything better.
