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Re: two fpu compared

From glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups comp.arch.embedded
Subject Re: two fpu compared
Date 2015-02-12 11:26 +0000
Organization Aioe.org NNTP Server
Message-ID <mbi2le$782$1@speranza.aioe.org> (permalink)
References <ck36l8Fak5nU1@mid.individual.net>

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alb <al.basili@gmail.com> wrote:
 
> assuming two differently implemented FPU, both validated against the 
> IEEE754, and limiting operations to the golden 5 (+,-, *, /, sqr), can I 
> be sure that provided the inputs operands are the same the result would 
> be the same bit wise?

The x87, which as well as I know follows the IEEE standard, holds
intermediate results in extended precision. (64 bit significand).
In that case, the results could easily be different from one that
didn't use extra precision. 

For x86-64 code, it is less usual to use x87, and so less likely
that will be a problem. 
 
> BACKGROUND: our dear customer wanted to verify that the control law 
> specified is correctly implemented by means of comparing output with a 
> golden model. Now the golden model is running on a standard PC, with 
> matlab, a pile of mathematical libraries and a processor that is 
> humongous compared to our little embedded unit. I've already warned 
> the customer that bitwise comparison might only be viable if precision 
> wise the two calculations are equally configured (single precision, 
> rounding, etc.). Are there any other elements to consider?

-- glen

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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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