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Hi, I just had an unpleasant experience with -ffast-math and GCC 4.1.1
(it borked my LDAP authentication on several systems which worked with
the same CFLAGS as long as GCC 3.4.6 was used).
There is a lot of material out there about CFLAGS and Gentoo (google
returns 387000 pages) but what's working for someone might not for
another. There are flags that work for a GCC version and most ebuilds
and don't work with another GCC version (my unfortunate experience) or
some ebuilds. Flag combination/architecture/LDFLAGS might be an issue too.
There are already good resources (http://gentoo-wiki.com/CFLAGS_matrix
was mentioned to me by robbat2) but they may not be advertised enough.
I'd like to propose a paragraph to the GWN editor which presents some
gotchas and good references on the subject.
Here's a draft for review. You're welcomed to expand on the subject.
--- Draft BEGIN ---
Being able to tune the CFLAGS is part of one of the core principles of
Gentoo: let the user be in control. Being in control brings both
benefits and problems and CFLAGS tuning is not an exception.
The recent upgrade to gcc-4.1.1 for x86 and amd64 users changed the
landscape. Users that spent some time tuning their CFLAGS with gcc-3.4.6
might find out that an upgrade to gcc-4.1.1 leaves them with an unstable
system. Example of this are :
<li>nss_ldap stopped working with -ffast-math</li>
Users with unsupported CFLAGS (see the <uri
link='http://gentoo-wiki.com/CFLAGS_matrix'>CFLAGS matrix</uri> for
example) might want to return to safe CFLAGS (see <uri
link='http://gentoo-wiki.com/Safe_Cflags'>Safe CFLAGS</uri>) if recent
updates caused them stability problems. On the other hand, more
adventurous users might want to experiment with CFLAGS that didn't work
properly with gcc-3.4.6... As always, the user is in control.
--- Draft END ---
If possible, I'd like to expand the list of 3.4.6 -> 4.1.1 upgrade
problems which are linked to experimental CFLAGS. If you want to expand
the subject to cover other tuning/stability gotchas that recent updates
might have brought into the light, please feel free to do so. As English
is not my native tongue, feel free to spell check too.
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