Messages in this thread | | | Date | Thu, 1 May 2003 09:02:13 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [RFC][PATCH] Faster generic_fls |
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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
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