loop optimized in 4.7.2, not in 4.8.1?

[Oleg Endo]

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