Re: Performance optimization (allocation inside a for loop)

[Rob Mahurin]

On Apr 1, 2009, at 7:18 PM, r wrote:
> BTW, low "for"
> loop performance is commonly blamed on the lack of a JIT compiler, but
> I wouldn't be surprised if at least some of the test cases were
> actually suffering from issues like memory allocation. After all, JIT
> compilers don't typically speed up evaluation by more than a constant
> factor.

Right, but it's a big factor.  Compare your test2

> > > function retval = test2(n)
> > >  retval = zeros(1, n);
> > >  for n = [1:n]
> > >    retval(n) = n;
> > >  endfor
> > > endfunction
> > >
> > > has a complexity of O(n):
> > >
> > > octave:29> tic;test2(10000);toc
> > > Elapsed time is 0.16 seconds.
> > > octave:30> tic;test2(100000);toc
> > > Elapsed time is 1.9 seconds.


to the equivalent

octave:29> tic; n = 1e5; retval = 1:n; toc
Elapsed time is 0.000756025 seconds.
octave:30> tic; n = 1e5; retval = (1:n)(1:n); toc
Elapsed time is 0.00757694 seconds.
octave:31> tic; n = 1e5; retval = [1:n]; toc
Elapsed time is 0.0125589 seconds.

(These apparently take different paths through Octave's lazy-copying  
logic.)

Rob

--
Rob Mahurin
Department of Physics and Astronomy
University of Tennessee                 865 207 2594
Knoxville, TN 37996                     address@hidden