Re: [Boost-build] Changing GCC flags for <optimization>speed

From: Jos Hickson (jos.hickson_at_[hidden])
Date: 2007-07-13 10:25:15

As an addendum to the Gentoo quote I gave in a previous email I found
more along the lines of "don't use -O3" on the official Gentoo CFLAGS
guide (http://www.gentoo.org/doc/en/gcc-optimization.xml) which
suggests there are particular problems with gcc 4.x with erroneous
builds and slower runtimes.  So, I contacted the author of that page
to find out what this was based on.  His response was that it was
based on "years of experience" of users having problems especially in
the transition from gcc 3.x. to 4.x and using -O3.

Jos

On 11/07/07, Chris Fairles <chris.fairles_at_[hidden]> wrote:
> Just so we're clear, here's the descriptions of the extra
> optimizations -O3 turns on.
>
> -funswitch-loops
>     Move branches with loop invariant conditions out of the loop, with
> duplicates of the loop on both branches (modified according to result
> of the condition).
>
> So, for example:
>
> for i = 0 ... 100
>   if (a > 0)  f(i);
>   else g(i);
> end for
>
> can become:
>
> if (a > 0)
>   for i = 0..100
>     f(i)
>   end for
> else
>   for i = 0..100
>     g(i)
>   end for
> end if
>
> It doubles code which can be an issue for embedded platforms but
> hopefully they're using -Os anyway. I can't conjure up a scenario
> where this might cause code to run slower.
>
> -finline-functions
>     Integrate all simple functions into their callers. The compiler
> heuristically decides which functions are simple enough to be worth
> integrating in this way. If all calls to a given function are
> integrated, and the function is declared static, then the function is
> normally not output as assembler code in its own right.
>
> Dependant on the hueristics of the compiler but inlining is not always
> best for some functions. I think this has the greatest potential for
> not being "safe" and/or slowing down code as opposed to making it
> faster. See:
> http://www.new-brunswick.net/workshop/c++/faq/inline-functions.html#faq-9.3
>
> I
>
> -fgcse-after-reload
>     When -fgcse-after-reload is enabled, a redundant load elimination
> pass is performed after reload. The purpose of this pass is to cleanup
> redundant spilling.
>
> Not clear what issues can arise with this one.
>
> I as well opt for -O2 99% of the time and use a workaround (cxxflags)
> to force -O2.
>
> Chris
>
> On 7/11/07, Jos Hickson <jos.hickson_at_[hidden]> wrote:
> > I've trawled the GCC mailing list and found the following from 01/2007:
> >
> > <quote>
> > Andreas Bogk <andreas_at_[hidden]> writes:
> >
> > > Then maybe it shouldn't be the default in autoconf.  But wasn't -O3 the
> > > set of optimizations considered potentially unsafe?
> >
> > No.  -O3 is a set of optimizations which are useful for many program
> > but which will cause a substantial number of programs to run slower.
> > -O2 is a set of optimizations which we believe will make (almost) all
> > programs run faster.  -O1 also makes (almost) all programs run faster.
> > The difference between -O1 and -O2 is that it takes longer to run the
> > compiler with -O2.
> >
> > gcc never enables unsafe optimizations except by explicit request via
> > -f options (e.g., -ffast-math), where "unsafe" is defined as
> > "violating the language standard."
> > </quote>
> >
> > This is from http://gcc.gnu.org/ml/gcc/2007-01/msg01095.html.  So, I
> > am mistaken in my belief and apologies for that.  However, here we
> > have had problems with the inlining turned on by O3 even in a very
> > simple program that I don't have to hand right now though this
> > particular problem at least may well be a GCC bug, we're not sure yet.
> >
> > However, it is interesting to note that the author of the response
> > above says that -O3 "will cause a substantial number of programs to
> > run slower."  How "official" this statement is, I don't know.
> >
> > Regards,
> >
> > Jos
> >
> >
> >
> >
> >
> >
> > On 11/07/07, Dean Michael Berris <mikhailberis_at_[hidden]> wrote:
> > > On 7/11/07, Jos Hickson <jos.hickson_at_[hidden]> wrote:
> > > > [snip]
> > > > >
> > > > > > One thing I was wonder was why -O3 is the default as that is usually
> > > > > > considered a bit risky?
> > > > >
> > > > > Is it? I'm not aware of that.
> > > > >
> > > >
> > > > To quote from the Gentoo Linux "Safe CFLAGS" page
> > > > (http://gentoo-wiki.com/Safe_Cflags) as an example:
> > > >
> > > > <quote>
> > > > Note that -O2 is regarded as safer than "-O3", and "-O3" can often be
> > > > a counter-productive attempt at optimization.
> > > > </quote>
> > > >
> > >
> > > Any particular reason why this would be the case? How is "safety"
> > > defined in this manner? Does conservative optimization mean safer?
> > >
> > > --
> > > Dean Michael C. Berris
> > > http://cplusplus-soup.blogspot.com/
> > > mikhailberis AT gmail DOT com
> > > [phone]
> > > _______________________________________________
> > > Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost-build
> > >
> > _______________________________________________
> > Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost-build
> >
> _______________________________________________
> Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost-build
>