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Re: two fpu compared

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On 15.2.2015 г. 16:33, glen herrmannsfeldt wrote:
> Dimiter_Popoff <dp@tgi-sci.com> wrote:
>
> (snip on validating floating point calculations, where someone 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?
>
> (snip, then I wrote)
>
>>> Are you sure the calculation should be done in floating point?
>>> Without more details, I suspect it could be done in fixed point,
>>> the results would not vary depending on implementations, and
>>> everyone would be happy.
>
>> Sometimes the choice to do something using an FPU is not because things
>> cannot be done using the integer unit but just because one needs the
>> horsepowers the FPU adds to the chip. Was the case with me on my
>> netMCA-3 design - the FPU did make things a bit more complicated but
>> using it was the only way to do what I did. Apart from the sheer
>> ops per clock cycle it (being 64 bit of course, 32 bit FPU-s are
>> pretty much useless) gave me the dynamic range I needed (32 bits
>> integers would not have sufficed and multiprecision would have
>> slowed things down beyond any practical usability).
>
> Many science and engineering problems have a large dynamic
> range, and require results with a relative error (uncertainty).
>
> Lengths can be from nanometers to gigameter, times from picoseconds
> to gigayears, and masses from eV to the mass of large stars.
> One can measure the atomic spacing in a crystal lattice or the
> distance between planets with a relative uncertainty of about
> one part in a million. Floating point is great for this.

Yes, this is the primary purpose of FP. I just gave an example how
it can be practical to use an FPU for some other reason, not the
primary one (i.e. one can make use of the horsepowers it provides
in parallel to the integer unit even though the calculations can
be done quite well in integers (the first time I implemented that
same conversion algorithm of mine was on a TI 5420 DSP, using integers).


>
> There are a fair number of problems in mathematics where one
> wants a modulo (remainder) operations with this property.

I did not realize you were talking integer divide, I though you
were referring to FP division hence me being puzzled. FP (IEEE FP
at least) maintains the mantissa as an absolute value and the sign
is not a part of it, then there is no remainder etc :-).

Dimiter

------------------------------------------------------
Dimiter Popoff, TGI             http://www.tgi-sci.com
------------------------------------------------------
http://www.flickr.com/photos/didi_tgi/

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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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