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To: gentoo-dev@g.o
From: George Prowse <cokehabit@...>
Subject: Re: [RFC] CFLAGS paragraph for the GWN
Date: Sun, 01 Oct 2006 10:37:55 +1000
Lionel Bouton wrote:
> 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 ---
> <section>
> <title>CFLAGS</title>
> <body>
>
> <p>
> 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.
> </p>
> <p>
> 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 :
> <ul>
> <li>nss_ldap stopped working with -ffast-math</li>
> <li>...</li>
> </ul>
> </p>
> <p>
> 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.
> </p>
> </body>
> </section>
> --- 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.
>
> Cheers,
>
> Lionel.
>   
I agree in principle because it would stop people using stupid CFLAGS. 
It should have an information section and a "use this CFLAG and dont ask 
us for help" section:

<p>
Good Compiler Flag
</p>
<p>
-floop-optimize
Enables safe loop optimisation and is enabled in most -O$
</p>
<p>
Bad Compiler flag
</p>
<p>
Sets |-fno-math-errno|, |-funsafe-math-optimizations|,
|-fno-trapping-math|, |-ffinite-math-only|, |-fno-rounding-math| and 
|-fno-signaling-nans|

Used to speed up math functions but causes major b0rkage because it can 
result in incorrect output for programs which depend on an exact 
implementation of IEEE or ISO rules/specifications for math functions.

Use this and dont bother asking for help.
</p>
-- 
gentoo-dev@g.o mailing list


References:
[RFC] CFLAGS paragraph for the GWN
-- Lionel Bouton
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