[Octave-bug-tracker] [bug #42716] Transpose multiply not detected

[Ceral Paquet]

URL:
  <http://savannah.gnu.org/bugs/?42716>

                 Summary: Transpose multiply not detected
                 Project: GNU Octave
            Submitted by: octavebugs
            Submitted on: Wed 09 Jul 2014 10:57:02 AM GMT
                Category: None
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: None
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 3.8.1
        Operating System: GNU/Linux

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Details:

I was reading this page 

  http://www.walkingrandomly.com/?p=4912

regarding the ability of MATLAB to detect when the result of a matrix multiply
is symmetric so it can speed up the calculation. In the comments that Octave
too has this ability.

These are the results from the blog (MATLAB):

  >> a=rand(4000);
  >> b=rand(4000);
  >> tic;a*a';toc 
  Elapsed time is 0.887524 seconds.
  >> tic;a*b;toc  
  Elapsed time is 1.473208 seconds.
  >> tic;b*b';toc
  Elapsed time is 0.966085 seconds.

However in Octave 3.8.1, I find all the multiplies take the same time, as if
the full calculation is done for each.

  >> a=rand(4000);
  >> b=rand(4000);
  >> tic;a*b;toc
  Elapsed time is 2.18606 seconds.
  >> tic;a*a';toc
  Elapsed time is 2.19176 seconds.
  >> tic;b*b';toc
  Elapsed time is 2.18921 seconds.
  >> tic;a'*a;toc
  Elapsed time is 2.21384 seconds.





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