https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785
--- Comment #15 from dhowells at redhat dot com <dhowells at redhat dot com> ---
(In reply to Jakub Jelinek from comment #14)
> (In reply to dhowe...@redhat.com from comment #13)
> ...
> Ugh, no. Why not just x && (x & -x) == x ? __builtin_ctz (x) : -1
> (or ctzl or ctzll depending on what the type is)?
Comparing the kernel's ilog2() to your suggestion:
int kernel_ilog2(unsigned long x)
{
return ilog2(x);
}
int jakub_ilog2(unsigned long x)
{
return x && (x & -x) == x ? __builtin_ctz (x) : -1;
}
compiled with -Os for x86_64 gives:
0000000000000000 <kernel_ilog2>:
0: 83 c8 ff or $0xffffffff,%eax
3: 48 0f bd c7 bsr %rdi,%rax
7: c3 retq
0000000000000008 <jakub_ilog2>:
8: 83 c8 ff or $0xffffffff,%eax
b: 48 85 ff test %rdi,%rdi
e: 74 17 je 27 <jakub_ilog2+0x1f>
10: 48 89 fa mov %rdi,%rdx
13: 0f bc c7 bsf %edi,%eax
16: 48 f7 da neg %rdx
19: 48 21 fa and %rdi,%rdx
1c: 48 39 d7 cmp %rdx,%rdi
1f: ba ff ff ff ff mov $0xffffffff,%edx
24: 0f 45 c2 cmovne %edx,%eax
27: c3 retq
Note that in the kernel variant, the initial OR instruction is superfluous, but
is required by fls() and fls64() which x86_64 is using behind the scenes. Not
all arches, however, use fls() and fls64() to implement ilog2().
