[Octave-bug-tracker] [bug #48686] Copy of columns of the matrix

[John W. Eaton]

Follow-up Comment #5, bug #48686 (project octave):

I think this bug report is related to bug #42118.

When you do


a=rand(100000000,1);
tic;b=a(1:2);toc


the variable B is a reference to a slice of A.  It is only when you do


a(1:2) = [1 2]';


that a copy is made.  At this point, however, Octave doesn't know that it
could just copy the two elements referenced by B and decrement the reference
count for A.  So instead, it copies all the data of A and then modifies the
copy.  This is indeed wasteful.  It doesn't happen when working with rows of a
matrix because Octave uses column-major storage so you can't create a slice
using a row.

Maybe some smarter slice management could fix this problem so that Octave
could know about the slice B when looking at A.  But I'm not sure it's worth
the extra complexity.  Or, as I think we've discussed in the other bug report,
we could make copies instead of slices when the slice size is small (for some
definition of "small").

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