Re: [gentoo-amd64] gcc 4.1 + CFLAGS

["Vladimir G. Ivanovic" <vgivanovic@comcast.net>]

On Fri, 2006-06-09 at 15:36 +0200, Hemmann, Volker Armin wrote:

> > Haven't noticed any problems on my laptop or a workstation.
> >
> 
> because they were filtered out.
> 
> DO NEVER USE FFAST-MATH!
> 
> Apps that can use it safely, usually set it in their makefile.
> 
> Everything else is prone to break.
> 
> And that is the reason, that almost all ebuilds have 'filter-flags' 
> and 'strip-flags'.
> 
> Again: NEVER USE IT.
> 
> And: NEVER TELL ANYBODY TO USE IT.

If -ffast-math is filtered or stripped out, there is no harm in leaving
it in CFLAGS, right?

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".

Here are 4 other opinions about using -ffast-math.

        http://forums.gentoo.org/viewtopic-t-10535.html
        --fast-math can and does cause errors, but as I understand it
        (can't remember where I read this) the errors it can create are
        only single-byte errors. IOW, your answer to a FP
        calculation /might/ be one least-signifigant digit off. 
        
        Unless you're doing important scientific calculations, you're
        probably never going to notice /or/ care. I would expect that
        any linux packages that would have a problem with this (ALSA, or
        GSL and the like) probably tell you not to use it if they don't
        like it.
        
        http://gcc.gnu.org/ml/gcc/2004-03/msg01459.html
        I've heard it argued that people who are serious about floating
        point don't use -ffast-math.  I consider myself serious, and
        make a very nice  living from selling software to solve
        finite-difference Poison-Boltzmann electrostatic calculations on
        regular grids, and molecular minimizations using quasi-newtonian
        numerical optimizers.  Toon does numerical weather forecasting,
        and he seems happy with -ffast-math.  Laurent performs large
        scale Monte-Carlo simulations, and he also seems happy with it.
        
        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/32035.pdf
        -ffast-math is recommended by AMD.
        
These opinions seem to contradict your advice. Could you be more
specific about why -ffast-math should not be used? 

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