on 11.09.2009 at 4:39 Topher Cooper wrote :
You might find some sections of Sean Aaron Anderson's "Bit-Twiddling Hacks" of interest, though perhaps not of practical use. Check out: Compute the minimum (min) or maximum (max) of two integers without branching <http://graphics.stanford.edu/%7Eseander/bithacks.html#IntegerMinOrMax> Conditionally set or clear bits without branching <http://graphics.stanford.edu/%7Eseander/bithacks.html#ConditionalSetOrClearBitsWithoutBranching> Conditionally negate a value without branching <http://graphics.stanford.edu/%7Eseander/bithacks.html#ConditionalNegate> i'll check it out
Its worth noting that the better optimization your compiler has the better the loop in your test case will do on your branchless method. But a good optimizing compiler would probably avoid the branch in the first place. To use the technique to its fullest you or the optimizer should translate your loop into something like (untested): type a[2] = {BIG_VALUE, ANY_VALUE}; type* vp[2] = {--&v1[0], --&v2[0]}; // I know, but I'm expressing some possibly low level optimization. for (size_t i = 0; i<n; ++i) { a[1] = *vp[*++vp[1]<*++vp[0]]; a[0] = a[a[1]<a[0]]; } i thought about it but didn't try although
A good optimizing compiler should be able to do most or all of this from the inlined method, though the "relaxation" of the transfer of the move to "c[]" into an indirection as vp is pretty sophisticated. Binding the local a and the temporary containing the min of the v's to the vector they would otherwise be transfered to is routine. Of course, if this were code you really wanted to optimize, rather than a test loop, you would improve locality by processing each vector separately by making c[] a reference my intention was just to heavily load min() implementation alternatively one can compute 'res[i] = min(v1[i], v2[i])' in a loop the timing would be very similar
If you are going to benchmark over several platforms, please include the technique described here: http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax
on 11.09.2009 at 13:10 Thorsten Ottosen wrote : thanks for the link by the way msvc abs() does just that thing like in this paper -- Pavel