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[Gcl-devel] Re: [Maxima] Re: m68k gcl/maxima: Minor Floating point error

From: Camm Maguire
Subject: [Gcl-devel] Re: [Maxima] Re: m68k gcl/maxima: Minor Floating point errors
Date: 20 Sep 2002 11:25:49 -0400

Greetings, and thanks for pointing this out.  Actually, I maintain a
few numerical packages requiring this flag on m68k to pass the
regression tests in the package (gcl/maxima, atlas, blas, lapack,...).
I suppose the sacrifice of performance is the correct approach.
Anyone actually using the 68k with a different opinion will likely
sway my mind in the other direction :).  This would of course require
sidestepping integrity tests in this case, as I won't have the time to
maintain dual sets of expected results.

Take care,

Steve Haflich <address@hidden> writes:

>    From: Camm Maguire <address@hidden>
>    >        -ffloat-store
>    >               Do  not  store floating point variables in registers.  
> This pre-
>    >               vents undesirable excess precision on machines such as the 
> 68000
>    >               where  the floating registers (of the 68881) keep more 
> precision
>    >               than a double is supposed to have.
>    > 
>    >               For most programs, the excess precision does only  good,  
> but  a
>    >               few  programs  rely  on  the precise definition of IEEE 
> floating
>    >               point.  Use `-ffloat-store' for such programs.
>    That did it -- thanks!  As Richard Fateman observed, its quite
>    surprising the magnitude of the difference was so large.  Apparently
>    m68k has the more accurate answer, right?  In any case, I'm adding
>    -ffloat-store to gcl's compile flags on m68k to avoid testing
>    problems. 
> But don't be misled -- the effect is not only precision.  The 68881/2
> were admirably fast and capable chips for their day, but had a serious
> bottleneck loading and storing floats.  You may find that performance
> is greatly reduced in some circumstances.
> Of course, anyone still using decade-old processor chips probably
> doesn't consider speed the most important issue.

Camm Maguire                                            address@hidden
"The earth is but one country, and mankind its citizens."  --  Baha'u'llah

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