On Sun, 2013-08-11 at 10:42 +0100, Oliver Kullmann wrote: > 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. Using some timing measurements to check whether a loop has been evaluated and eliminated at compile time doesn't sound like a reliable approach. There also doesn't seem to be any connection to whether the loop variable is signed or unsigned. Anyway, I've opened a PR for this: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58122 Let's take further discussions there. Cheers, Oleg
