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iterations are s l o w

From: Francesco Potorti`
Subject: iterations are s l o w
Date: Fri, 14 Jul 1995 10:56 +0100 (MET)

I got a tenfold or some increase on a simple instructions loop by
modifying it to use matrix ranges.  While I expected a speed increase,
I have the feeling that the increase is too big, and something is
wrong in the for iteration.

In the following, FBM is a 2^n+1 long column vector.  The second code
does the same thing more or less 10 times quicker on an alpha (I can
time it exactly if it is important) when n is 12.  With greater n, the
difference is greater yet (this is obvious, but it is too much, in my

Is there a reason or is it a sort of "performance bug"?

##for i = 1:n
##  d = 2^(n-i);
##  scale = scale * 2^-H;
##  for j = d+1:2*d:N-d+1
##    FBM(j) = 0.5 * (FBM(j-d) + FBM(j+d)) + FBM(j)*scale;
##  endfor

  for i = 1:n
    d = 2^(n-i);
    scale = scale * 2^-H;
    range = [d+1:2*d:N-d+1];
    FBM(range) = 0.5 * (FBM(range-d) + FBM(range+d)) + FBM(range)*scale;

         Francesco Potorti`     | address@hidden (Internet)
         researcher at          | 39369::pot (DECnet)
         CNUCE-CNR, Pisa, Italy | +39-50-593203 (voice) 904052 (fax)

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