https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445
--- Comment #45 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> > __attribute__ ((noinline))
> > int
> > my_fls64 (__u64 x)
> > {
> > asm volatile ("movl $-1, %eax");
> > return (__builtin_clzll (x) ^ 63) + 1;
> > }
>
> Aha, bsr is not doing anything if parameter is 0, so pattern is correct
> (just the instruction is undefined for 0 which makes sense).
> But with that pattern GCC can't synthetize the code sequence above :)
The docs explicitly say that bsf/bsr DEST is undefined if ZF is set (operand is
zero), so relying on it preserving the previous value of the register is quite
dangerous.
