https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81759
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Daniel Fruzynski from comment #2)
> Looks that __builtin_ffs does not check if input value is nonzero at all.
> Assembler code for following code also has unnecessary instructions:
>
> [code]
> unsigned int test(unsigned int n)
> {
> if (n == 0)
> __builtin_unreachable();
> return __builtin_ffs(n) - 1;
> }
> [/code]
The above is now handled since GCC 11.
