Re: efficient function parameter copying?

[Michael Pronath]

address@hidden said:
>  Octave values are implemented using reference-counted objects.  The
> data is not duplicated unless it is necessary.  So, for something like

>   function y = f (x)
>     y = x;
>   endfunction

>   w = f (z);

> only one copy of z should actually exist (but the reference count will
> be incremented and decremented as necessary).

> For something like

>   function y = f (x)
>     y = x;
>     y(some_index) = some_value;  # or some other way of modifying y
>   endfunction

> however, a copy of x will be made when y is modified. 

yes, that's what I thought, too.  But in your example definitions, it's
function y=f(x), while I meant function x=f(x).  And you call it with
w=f(z), while I meant z=f(z).  This is an important difference, as in
my case, the formal input and output parameters are the same plus the
actual input and output parameters are the same.  Of course, my function
performs changes on x, else it would be without any effect.

If octave uses reference-counted objects without any further optimization,
the complete matrix will be copied for every call to f unnecessarily.

Take for an example

  X=eye(500,500);
  for i=1:1000
     X=iterate(X);
  endfor

and

  function M = iterate(M)
     M(1,1) = M(1,1)+3*M(2,2);
     M(2,2) = -M(1,1);         %% could do something useful here as well
  endfunction

Is there a need to copy the 500x500 matrix 2000 times?  Certainly not,
and I wanted to know if octave can detect this and avoid the copying.
If not, it is better to write

   global X;
   X=eye(500,500);
   for i=1:1000
      iterate();
   endfor

and

   function iterate()
     global X;
     X(1,1) = X(1,1)+3*X(2,2);
     X(2,2) = -X(1,1);         %% could do something useful here as well
   endfunction


which is not quite what I want to do.

This optimization could be simply done by taking the names of the
actual parameters and the formal parameters of a function call and
searching for "matching pairs".  This could make a true "call by
reference" possible.

 Michael




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