[Python-Dev] -O2 faster than -O3?

[Neal Norwitz]

On Nov 30, 2007 7:16 PM, Brett Cannon <brett at python.org> wrote:
> On Nov 30, 2007 12:02 PM, Neil Toronto <ntoronto at cs.byu.edu> wrote:
> > On both of my systems, using -O2 reduces execution time in pystone by 9%
> > and in pybench by 8%. It's function inlining: "-O3
> > -fno-inline-functions" works just as well as "-O2". Removing "-g" has
> > little effect on the result.
> >
> > Systems:
> >   - AMD Athlon 64 X2 Dual Core 4600+, 512 KB cache (desktop)
> >   - Intel T2300 Dual Core 1.66GHz, 512 KB cache (laptop)
> >
> > Both are Ubuntu 7.04, GCC 4.1.2.
> >
> > Does anybody else see this?
> >
> > It may be GCC being stupid (which has happened before) or not enough
> > cache on my systems (definitely possible). If it's not one of those, I'd
> > say it's because CPython core functions are already very large, and
> > almost everything that ought to be inlined is already a macro.
> >
>
> That's quite possible.  Previous benchmarks by AMK have shown that
> perhaps -0m (or whatever the flag is to optimize for size) sometimes
> is the best solution.  It has always been believed that the eval loop
> is already large and manages to hit some cache sweet spot.

The flag is -Os.  I suspect you will do better to limit the size of
inlining rather disabling it completely.  The option is
-finline-limit=number.  I don't know the default value or what you
should try.  I would be interested to hear more results though.

n