Re: [gentoo-user] New computer and Gentoo

[Pandu Poluan <pandu@poluan.info>]

On Thu, Jul 21, 2011 at 16:21, Florian Philipp <lists@binarywings.net> wrote:
>
> Am 21.07.2011 10:57, schrieb Pandu Poluan:
> > -original message-
> > Subject: Re: [gentoo-user] New computer and Gentoo
> > From: Bill Kenworthy <billk@iinet.net.au>
> > Date: 2011-07-21 12:54
> >
> >> On Thu, 2011-07-21 at 06:26 +0100, Mick wrote:
> [...]
> >> Ive just stumbled on something weird with march=native:
> >>
> >> At some point I had march=prescott on a core2 E4600 running 32bit -
> >> worked well.  Changed to march=native and did some upgrades with a few
> >> odd things like asterisk segfaulting in a glibc library afterwards, and
> >> some things not building.  Then to add confusion, I changed to an
> >> pentium Duo E6600 (flies!) and added another stick of ram.  More odd
> >> things happening such as reiserfs oopsing on shutdown.
> >>
> >> Last night the penny dropped and I looked the new processor up and
> >> changed to march=core2 and have mostly corrected (recompiled) the
> >> damage.
> >>
> >> So not sure about march=native now as it is only what was built with
> >> native thats been problematic.  With 20-20 hindsight it was perhaps
> >> predictable ...
> >
> > IMO you're not supposed to compile part of the system with -march=<something>
> > and the rest with -march=native. The instructions (and optimizations)
> > emitted by -march=native might not be compatible with your previous
> > -march.
>
> I'd like to see a reference for this claim. -march=native doesn't do
> more than set -march=core2 and some other optimizations for cache size
> etc. This should be no more troublesome than mixing code compiled with
> different specific -march settings. When you look at binary
> distributions (and especially precompiled packages from the developer
> instead of the distribution), this is pretty much normal.
>
> The compiler is not allowed to change the external interfaces of
> functions for optimization purposes (see [1]). Besides this, I can only
> think of alignment problems ([2]) but even this should be handled
> correctly by the compiler. Everything else is a compiler bug that should
> be reported.
>
> [1] http://en.wikipedia.org/wiki/Calling_convention
> [2] http://en.wikipedia.org/wiki/Data_alignment
>

Hmmm... You do have a point. So, gcc's "-march" option *should*
maintain ABI compatibility.

There *are* other switches that can impact compatibility [1]. What I
can't be sure of, whether the "-march" option (improperly) activates
one or more of those switches when set to "-march=native"

[1] http://gcc.gnu.org/onlinedocs/gcc/i386-and-x86_002d64-Options.html

Rgds,
--
Pandu E Poluan
~ IT Optimizer ~

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