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Re: two fpu compared

From Nobody <nobody@nowhere.invalid>
Newsgroups comp.arch.embedded
Subject Re: two fpu compared
Date 2015-02-23 00:13 +0000
Organization A noiseless patient Spider
Message-ID <pan.2015.02.23.00.13.19.260000@nowhere.invalid> (permalink)
References <ck36l8Fak5nU1@mid.individual.net> <mbht2f$v21$1@dont-email.me> <ck6f5eF66ovU1@mid.individual.net> <mbl4rt$77q$1@dont-email.me>

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On Fri, 13 Feb 2015 16:23:15 +0100, David Brown wrote:

> What this all boils down to is that the client wants to be sure that the
> code is "correct" - but has an unreasonable and restrictive definition of
> "correct".

Or maybe they just want to be sure that they have a *precise*
specification in case they ever need to source perfectly-compatible parts,
or perform simulations or analysis whose results aren't rendered
meaningless by "minor" differences between implementations.

There are situations where you need different implementations to be able
to perform identical calculations on identical inputs and obtain
*identical* results, down to the least-significant bit. In such
cases, whether the results are "correct" is often less relevant than
whether they're consistent.

Key things to avoid are unsafe optimisations (-ffast-math etc,
i.e. anything which allows the compiler to pretend that floats
observe the same rules as reals), extended intermediate precision (use
-ffloat-store and/or -fexcess-precision=standard), fused multiply-add (use
-ffp-contract=off and/or -mno-fused-madd), and transcendental functions
(these invariably have to be implemented in software or avoided altogether).

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two fpu compared al.basili@gmail.com (alb) - 2015-02-12 08:30 +0000
  Re: two fpu compared David Brown <david.brown@hesbynett.no> - 2015-02-12 10:51 +0100
    Re: two fpu compared al.basili@gmail.com (alb) - 2015-02-13 14:14 +0000
      Re: two fpu compared David Brown <david.brown@hesbynett.no> - 2015-02-13 16:23 +0100
        Re: two fpu compared Colin Paul de Gloucester <not_a_real_email@address.com> - 2015-02-13 17:37 +0100
          Re: two fpu compared al.basili@gmail.com (alb) - 2015-02-16 08:01 +0000
            Re: two fpu compared Colin Paul de Gloucester <not_a_real_email@address.com> - 2015-02-16 11:43 +0100
              Re: two fpu compared al.basili@gmail.com (alb) - 2015-02-17 10:35 +0000
                Re: two fpu compared David Brown <david.brown@hesbynett.no> - 2015-02-17 12:18 +0100
                Re: two fpu compared George Neuner <gneuner2@comcast.net> - 2015-02-17 09:26 -0500
                Re: two fpu compared David Brown <david.brown@hesbynett.no> - 2015-02-17 16:47 +0100
                Re: two fpu compared Dimiter_Popoff <dp@tgi-sci.com> - 2015-02-17 20:54 +0200
        Re: two fpu compared al.basili@gmail.com (alb) - 2015-02-16 07:58 +0000
          Re: two fpu compared David Brown <david.brown@hesbynett.no> - 2015-02-16 09:53 +0100
            Re: two fpu compared al.basili@gmail.com (alb) - 2015-02-16 09:47 +0000
              Re: two fpu compared Richard Damon <Richard@Damon-Family.org> - 2015-02-16 08:27 -0500
        Re: two fpu compared Nobody <nobody@nowhere.invalid> - 2015-02-23 00:13 +0000
      Re: two fpu compared George Neuner <gneuner2@comcast.net> - 2015-02-13 18:43 -0500
      Re: two fpu compared glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-14 15:30 +0000
        Re: two fpu compared Dimiter_Popoff <dp@tgi-sci.com> - 2015-02-14 23:19 +0200
          Re: two fpu compared Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2015-02-14 23:28 +0100
          Re: two fpu compared glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-15 14:33 +0000
            Re: two fpu compared Dimiter_Popoff <dp@tgi-sci.com> - 2015-02-17 20:53 +0200
              Re: two fpu compared glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-18 01:55 +0000
                Re: two fpu compared Dimiter_Popoff <dp@tgi-sci.com> - 2015-02-18 17:54 +0200
                Re: two fpu compared glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-18 16:52 +0000
                Re: two fpu compared Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2015-02-18 18:12 +0100
                Re: two fpu compared Robert Wessel <robertwessel2@yahoo.com> - 2015-02-18 18:02 -0600
                Re: two fpu compared Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2015-02-19 09:42 +0200
                Re: two fpu compared Robert Wessel <robertwessel2@yahoo.com> - 2015-02-19 11:04 -0600
                Re: two fpu compared Reinhardt Behm <rbehm@hushmail.com> - 2015-02-19 23:26 +0800
                Re: two fpu compared Dimiter_Popoff <dp@tgi-sci.com> - 2015-02-18 19:18 +0200
                Re: two fpu compared Dimiter_Popoff <dp@tgi-sci.com> - 2015-02-19 02:28 +0200
                Re: two fpu compared glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-19 16:29 +0000
                Re: two fpu compared Dimiter_Popoff <dp@tgi-sci.com> - 2015-02-21 16:07 +0200
  Re: two fpu compared glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-02-12 11:26 +0000
  Re: two fpu compared "Boudewijn Dijkstra" <sp4mtr4p.boudewijn@indes.com> - 2015-02-12 12:29 +0100
  Re: two fpu compared Dennis <dennis@none.none> - 2015-02-12 09:45 -0600

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