Hi Anthony, > > On Sat, Aug 10, 2013 at 06:06:26PM -0600, Anthony Foiani wrote: > > > They noticed that their earlier compiler (4.6.3 -O3) successfully > > > reduced the loop, while 4.8.1 didn't. > > > For general enligthenment, could you quickly explain what "reduce > > the loop" means? One can of course just get rid off the loop, > > precomputing sum, > > That's exactly what I meant. Apologies for not knowing the correct > phrasing. > Aha, interesting, that gcc was at some point capable of doing such an optimisation. I have checked here versions 4.7.1 and 4.7.3, and for both the run-times are as shown (so the loop was not eliminated). > > which is, at least for me, an unexpected difference? (I would have > > assumed that, if there is a difference, then Index=UInt should be > > faster.) > > Seeing only a 15% difference, you probably want to run it multiple > times and average out the values. > What I've shown you was already the "typical case" (timing is very stable). > Intuitively, I agree that the unsigned loop variable should be faster, > but I have no idea what's actually going on behind the scenes there. > Could somebody shed light on this? 15% is a large difference. Oliver
