gcc-4.5-RH in F14

[Jakub Jelinek]

On Thu, Jul 08, 2010 at 11:31:09AM -0400, Brandon Lozza wrote:
> A mass rebuild would be recommended as the new compiler will produce faster
> code. I believe everything will benefit and it's worth looking into. For
> example I noticed a significant difference on the OpenSUSE distro when GCC
> was upgraded and they repackaged their software with it in their development
> version 11.3. Anecdotal for sure but everything seemed faster than the build
> before that change. Phoronix has also done some benchmarks with the Ubuntu
> distro to determine that GCC 4.5 produces faster code (in some areas).

And slower in other areas, see e.g.
http://gcc.gnu.org/ml/gcc/2010-06/msg00715.html
or
http://gcc.gnu.org/gcc-4.5/changes.html
(part talking about -fexcess-precision=* default).
See http://vmakarov.fedorapeople.org/spec/ for SPEC numbers on x86_64 and
x86 for 4.4 and 4.5, in some scores 4.5 is better, in others worse, but
nothing major in either direction, except for LTO, but as I said earlier,
LTO isn't really usable in 4.5.

Generally, much better speedup can be achieved by using PGO
(-fprofile-generate, run on some testsuite, -fprofile-use).
GCC itself is built that way for several years, but it would be useful if
other performance sensitive packages were built that way too, assuming they
have some testsuite which resembles common use.

E.g. bash can be easily trained on some configure or some other
large shell scripts, similarly for python, perl, ...
The speedups from this can go up to say 30% or so.

	Jakub