Re: Changing GCC flags for speed

I've trawled the GCC mailing list and found the following
from 01/2007:

<quote>
Andreas Bogk <andreasandreas.org> 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 <mikhailberisgmail.com> wrote:
> On 7/11/07, Jos Hickson <jos.hicksongmail.com> 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
> +63 928 7291459
> _______________________________________________
> Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost-build
>
_______________________________________________
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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.hicksongmail.com> wrote:
> I've trawled the GCC mailing list and found the
following from 01/2007:
>
> <quote>
> Andreas Bogk <andreasandreas.org> 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 <mikhailberisgmail.com> wrote:
> > On 7/11/07, Jos Hickson <jos.hicksongmail.com> 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
> > +63 928 7291459
> > _______________________________________________
> > Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost-build
> >
> _______________________________________________
> Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost-build
>
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