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Re: Loop unrolling-related SPEC regressions?


> On Monday 04 February 2002 11:48, Andreas Jaeger wrote:
> > Paolo Carlini <pcarlini@unitus.it> writes:
> > > Jan Hubicka wrote:
> > 
> > >> THe base/peak flags are not supposed to bring best performance,
> > >> but be good for testing majority of gcc features.
> > >
> > > That's really enlightening Honza! Thanks for the clarification.
> > > We should also remember this when someone compares the SPEC numbers made
> > > available by other compiler producers with those of GCC: my guess is that
> > > this kind of rationale for choosing the PEAK flags it's unfortunately not
> > > so widespread...
> >
> > Didn't I mention it that way?  Feel free to send a patch for my SPEC
> > page to clarify what we're doing...
> Of course, compilers which are sold on the basis of SPEC base performance 
> have different approach to default options than gcc.  One expects the 
> base option set to be the one which is the best single setting conforming to 
> the limit on number of options, to obtain the highest rating.  Thus, a 
> compiler such as Intel's makes a simple option package such as 
> 'icc -xW -Oi-'
> roughly equivalent to
> 'gcc -msse2 -march=pentium4 -Os -funroll-loops -mpreferred-stack-boundary=4 
> -ffast-math'

I am playing with the idea of making -O behaving like -f and allowing -Ospeed
"optimize for maximal speed for common circmuatens" and -Osize.  We can also
invent -O[no]debug "prohibit optimizations that make debugging dificult, like
tail call optimization, frame pointer ellimination, or (currently) register
renaming", or -Odangerous "enable language standard breaking transformations".

Perhaps that can be usefull not only to "fit in" the spec2000 rules, but also
to avoid confusion of users. Many benchmarks published uses far from "sane"
compilation switches.

Honza
> with even the base rating depending on Profile Guided Optimization.
> Of course, one expects the peak rating to be found with a set of options 
> which produces the fastest acceptable result for each test, not necessarily 
> the most aggressive group of optimizations.  In that light, the SPEC 
> disclosures allow one to speculate as to how much trial and error work was 
> needed to obtain the results submitted, and how much more might be needed to 
> achieve equivalent performance on a typical application.
> 
> I thank Andreas and Honza for explaining the difference between what they 
> have done and what some of us may have expected.


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