Re: [gentoo-amd64] gcc 4.1 + CFLAGS

[Vladimir G. Ivanovic]

On Fri, 2006-06-09 at 20:50 +0200, Hemmann, Volker Armin wrote:

> > If -ffast-math is filtered or stripped out, there is no harm in leaving
> > it in CFLAGS, right?
>
> no, because a lot of other flags are filtered too!

What??? Whether only -ffast-math is filtered or all flags are filtered,
-ffast-math still filtered. I don't see where the harm lies.

> Some ebuilds filter ALL flags, only because some ricers had to use ffast-math
> and other 'controversial' flags.
>
> And some day, you will hit a package, that will built a damaged binary doing
> the wrong stuff, that does not filter out, because nobody knew about it. And
> then you are f*.

>
> >
> > But, on my system, only 14 packages filter out -ffast-math: gnubg
> > postgresql pgcluster libpq zoom mpfr gmp octave openoffice gsl goffice
> > rrdtool xv gimp. None strip it out. So, the huge majority of the
> > packages on my system are compiled with -ffast-math, unless I've made a
> > mistake in grepping for "fast-math" in ebuilds that contain
> > "filter-flags".
>
> and look for 'strip-flags'

There are only small number of ebuilds that strip out all flags.
Certainly less than 30.

>
>
> >         http://gcc.gnu.org/ml/gcc/2004-03/msg01511.html
> >         For me, it's very simple: If -ffast-math leads to answers that are
> > less accurate (in the verification-against-observation-sense) or unphysical
> > (against theoretical limit analysis), then I'll inspect my Fortran code and
> > repair the formulation (either a single expression or the layout of loop
> > code) that is responsible for the mayhem.
> >
> >         Under no circumstances I will give up using -ffast-math.
> >
> >
> > http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/320
> >35.pdf -ffast-math is recommended by AMD.
>
> FOR SPECIAL APPS. AND THIS DOCUMENT IS MEANT FOR PROGRAMMERS! PEOPLE WHO KNOW
> THEIR CODE!

I didn't see anywhere that the AMD document is for special applications
only. Where does it say that?

What it does say is that applications the rely on exact implementation
of IEEE rules or specifications for floating-point behavior shouldn't
use -ffast-math.

>
> >
> > These opinions seem to contradict your advice. Could you be more
> > specific about why -ffast-math should not be used?
>
> no, they don't contradict me. You did not understand them.
>
> Nowhere does AMD tells endusers to build their complete system with that flag.

Chapter 1 Introduction, Section 1.1 Audience says:

        This document is intended for ISVs and end-users of the AMD
        Athlon 64 processor-based platforms... 

It's clear that you think using -ffast-math is dangerous. I don't, so I
plan on continuing to use it. So far, the evidence from my system is in
my favor. (I did run across two more instances, just today, where, in
addition, to busybox, ebuilds fail with -ffast-math: cdparanoia and
pcmciautils.)

What would make me change my mind is some evidence, or some authority
who could convincingly explain why the experiences of those who do use
-ffast-math is anomalous. SHOUTING OR MERELY ASSERTING THAT IT'S A
PROBLEM is not really convincing. 

How about continuing this discussion offline?

--- Vladimir

-- 
Vladimir G. Ivanovic <vgivanovic@×××××××.net>
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