Re: Why not 03 ?

[James Cloos <cloos@jhcloos.com>]

>>>>> "CP" == Charles Plessy <plessy@debian.org> writes:

CP> Perhaps we can stop overriding this option ?  For a lot of scientific
CP> packages, -O3 is chosen by the upstream author, and I always feel bad
CP> that if we make the programs slower by overriding it to -O2, it will
CP> reflect poorly on Debian as a distribution for scientific works.

On my gentoo box, *everything* was slower with O3.

I ended up with this list, which enables the parts of O3 which do not
unroll too much, and therefore do not bloat the text sections like O3:

  -O2 -fgcse-after-reload -ftree-partial-pre -ftree-vectorize
  -fpredictive-commoning -fvect-cost-model -frename-registers
  -floop-interchange -floop-strip-mine -floop-block

The last three (-floop-*) apply only when the graphite extensions are
compiled into gcc (as I do).  They are no-ops otherwise.

Empirically, on x86_64 at least, that is optimal.

-JimC
--
James Cloos <cloos@jhcloos.com>         OpenPGP: 0x997A9F17ED7DAEA6