On 10/10/2013 11:38 PM, Richard Yao wrote: > On 10/10/2013 11:29 PM, Xin Li wrote: >> On 10/10/13 20:18, Richard Yao wrote: >>> Thanks for letting us know about this. I have a few comments: >> >>> 1. We could eliminate a branch entirely by doing this: >> >>> mlen = MIN(d_end - dst, mlen); while (--mlen >= 0) *dst++ = *cpy++ >> >> I don't think this eliminates the branching as MIN is usually a macro >> that expands to a > b ? b : a. > > My mistake. I was thinking of generic swap routines. I do think that > using the MIN() macro is more readable though.
On second thought, I was right the first time. It is possible to do this without branching: #define MIN(x, y) ((y) ^ (((x) ^ (y)) & -((x) < (y)))) #define MIN(x, y) ((x) ^ (((x) ^ (y)) & -((x) < (y)))) http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax This makes MIN(d_end - dst, mlen) look inefficient, but a proper optimizing compiler should store the result of d_end - dst in a register to avoid doing the subtraction 3 times.
signature.asc
Description: OpenPGP digital signature
_______________________________________________ developer mailing list [email protected] http://lists.open-zfs.org/mailman/listinfo/developer
