[Octave-bug-tracker] [bug #53881] [octave forge] (control) impulse response differs greatly on i386 vs x86_64

[Mike Miller]

URL:
  <http://savannah.gnu.org/bugs/?53881>

                 Summary: [octave forge] (control) impulse response differs
greatly on i386 vs x86_64
                 Project: GNU Octave
            Submitted by: mtmiller
            Submitted on: Fri 11 May 2018 01:49:22 PM PDT
                Category: Octave Forge Package
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Inaccurate Result
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: dev
        Operating System: GNU/Linux

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Details:

The impulse response of a simple system is very inaccurate when comparing the
results on X86-32 (i386) to the results on X86-64.

I run the following script on a 32-bit Octave 4.4 with control 3.1.0


pkg load control
sys = tf ([1, 1], [1, 2, 1]);
y = impulse (sys, [0:99] ./ 100).';
save results32.txt


and the following script on a 64-bit Octave 4.4 with control 3.1.0


pkg load control
sys = tf ([1, 1], [1, 2, 1]);
y = impulse (sys, [0:99] ./ 100).';
results32 = load ('results32.txt');
fprintf (stdout, '%4u  %-17.15g  %-17.15g\n', [0:99; results32.y; y]);


and I get the following output. Even at the start of the impulse response the
vectors are accurate to only three decimal places. By the end the accuracy is
down to one decimal place.


   0  1                  1                
   1  0.998999167089151  0.999007621746616
   2  0.997999335844817  0.998016228307829
   3  0.997000505264498  0.997025818706332
   4  0.996002674346696  0.996036391965785
…
 250  0.778541081031692  0.780190040994487
 251  0.777761891495348  0.779415797364297
 252  0.776983481797535  0.778642322076649
 253  0.776205851157766  0.777869614369055
 254  0.775428998796333  0.777097673479787
…
 500  0.606126214853996  0.60869650006698 
 501  0.605519583790042  0.608092442897402
 502  0.604913559862421  0.607488985181024
 503  0.604308142463491  0.60688612632296 
 504  0.60370333098622   0.606283865728916
…
 995  0.369232996221608  0.372355352593404
 996  0.368863455687218  0.371985835238959
 997  0.368494285001156  0.371616684585501
 998  0.368125483793267  0.371247900269124
 999  0.367757051693765  0.370879481926282





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