On Fri, Mar 2, 2012 at 3:46 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > This *does* assume that "bsf" is a reasonably fast instruction, which is > not necessarily the case especially on 32-bit x86. So the config option > choice for this might want some tuning even on x86, but it would be lovely > to get comments and have people test it out on older hardware. Ok, so I was thinking about this. I can replace the "bsf" with a multiply, and I just wonder which one is faster. > + /* Get the final path component length */ > + len += __ffs(mask) >> 3; > + > + /* The mask *below* the first high bit set */ > + mask = (mask - 1) & ~mask; > + mask >>= 7; > + hash += a & mask; So instead of the __ffs() on the original mask (to find the first byte with the high bit set), I could use the "mask of bytes" and some math to get the number of bytes set like this (so this goes at the end, *after* we used the mask to mask off the bytes in 'a' - not where the __ffs() is right now): /* Low bits set in each byte we used as a mask */ mask &= ONEBYTES; /* Add up "mask + (mask<<8) + (mask<<16) +... ": same as a multiply */ mask *= ONEBYTES; /* High byte now contains count of bits set */ len += mask >> 8*(sizeof(unsigned long)-1); which I find intriguing because it just continues with the whole "bitmask tricks" thing and even happens to re-use one of the bitmasks we already had. On machines with slow bit scanning (and a good multiplier), that might be faster. Sadly, it's a multiply with a big constant. Yes, we could make the constant smaller by not counting the highest byte: it is never set, so we could use "ONEBYTES>>8" and shift right by 8*sizeof(unsigned long)-2) instead, but it's still not as cheap as just doing adds and masks. I can't come up with anything really cheap to calculate "number of bytes set". But the above may be cheaper than the bsf on some older 32-bit machines that have horrible bit scanning performance (eg Atom or P4). An integer multiply tends to be around four cycles, the bsf performance is all over the map (2-17 cycles latency). Linus -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html