On 2012-10-11 3:12 PM, Anthony Jones wrote:
On 11/10/12 19:33, Mike Hommey wrote:
That being said, PGO on Linux is between 5 and 20% improvement on our
various talos tests. That's with the version of gcc we currently use,
which is 4.5. I'd expect 4.7 to do a better job even, especially if we
added lto to the equation (and since we are now building on x86-64
machines, we wouldn't have to worry about memory usage ; link time could
be a problem, though).
Perhaps the problems can be resolved or ameliorated by bumping the
minimum version of GCC that we support for PGO.
Link-time optimization is described as an experimental new feature in
the GCC 4.5.0 release notes[1]. The 4.6.0 release notes[2] say that it
has now "stabilized to the point of being usable", and the 4.7.0 release
notes[3] describe it as still further improved both in reliability and
code quality. If we're trying to use the 4.5 LTO, I'm not at all
surprised to hear it's causing more trouble than it's worth.
PGO is not the same thing as LTO, of course, but GCC's PGO was kind of
an unloved stepchild until they got serious about LTO, so that, too, is
likely to be much improved in 4.7.
zw
[1] http://gcc.gnu.org/gcc-4.5/changes.html
[2] http://gcc.gnu.org/gcc-4.6/changes.html
[3] http://gcc.gnu.org/gcc-4.7/changes.html
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