Hi, On Sunday 06 November 2005 16:20, Mattias Engdegård wrote: > Robert Dewar <[EMAIL PROTECTED]> writes: > >> Including loop unrolling to -O2 is IMNSHO a bad idea, as loop unrolling > >> increases code size, sometimes a lot. And the distinction between -O2 > >> and -O3 is exactly in the space-for-speed tradeoffs. > > > >That's certainly a valid way of defining the difference (and certainly > >used to be the case in the old days when the principle extra optimization > >was inlining) > > But even -O2 makes several space-for-speed optimisations (multiply by > shifting and adding, align jump targets, etc), so this cannot define > the difference between -O2 and -O3. It is more quantitative in nature: > -O2 only generates bigger code where the payoff in speed is almost > certain, which is not always the case for unrolling/inlining. > > Still, more and more projects seem to be switching to -Os for everything > which could be interpreted as both a consequence of the inevitable > bloating of large programs but also as a dissatisfaction with what -O2/3 > does to the code. Or maybe they are wisening up and actually using the > option that has been there all along.
We used -Os in the past, however I saw major speed regression for C++ with -Os
and 4.0.0. I would need to meassure again with 4.0.x or 4.1 to see if that
changed after the .0 release.
Yours,
--
René Rebe - Rubensstr. 64 - 12157 Berlin (Europe / Germany)
http://www.exactcode.de | http://www.t2-project.org
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