https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445
--- Comment #35 from Jan Hubicka <hubicka at ucw dot cz> ---
>
> Original asm is:
>
> __attribute__ ((noinline))
> int fls64(__u64 x)
> {
> int bitpos = -1;
> asm("bsrq %1,%q0"
> : "+r" (bitpos)
> : "rm" (x));
> return bitpos + 1;
> }
>
> There seems to be bug in bsr{q} pattern. I can make GCC produce same
> code with:
>
> __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 :)
Honza
