Re: [RFC][PATCH] Faster generic_fls

[Linus Torvalds]

On Thu, 1 May 2003, Willy Tarreau wrote:
>
>  BTW, has someone benchmarked BSF/BSR on x86 ? It should be
> faster, it it's also possible that a poor microcode implements it with a one
> bit/cycle algo, which will result in one instruction not being as fast as your
> code.

I think the original i386 did it with a one-bit-per-cycle algorithm,
anything since should be fine. In particular, on a P4 where I just tested,
the bsf seems to be 4 cycles over the whole input set (actually, my whole
loop was 4 cycles per iteration, so 4 cycles is worst-case. I'm assuming
the rest could have been done in parallell).

		Linus