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From: jac@gandalf..llnl.gov (James Crotinger)
Newsgroups: comp.lang.c++
Subject: Re: parameter[][]
Message-ID: <65983@lll-winken.LLNL.GOV>
Date: 6 Aug 90 22:09:55 GMT
References: <1545@m1.cs.man.ac.uk> <26B9BF5A.7291@orion.oac.uci.edu> <37961@ucbvax.BERKELEY.EDU>
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In article <37961@ucbvax.BERKELEY.EDU> jbuck@galileo.berkeley.edu (Joe Buck) writes:
>
>
>double **mat_alloc (int nrows, int ncols) {
>	double **t = new double* [nrows];
>	for (int i = 0; i < ncols; i++)
>		t[i] = new double [ncols];
>}
>
  There are at least a couple of problems with this approach. The
first is that, as written, the inter-row spacing is not constant. This
may hurt your performance on some architectures when you try to do a
loop over the first index. (This is particularly true of vectorizing
architectures but can also give a performance hit on systems with
interleaved memory access).  In fact optimization in general is
greatly hindered by the use of a double ** to represent a simple
square or rectanglar array since the optimizer no longer has any way
of knowing that it is, indeed, dealing with such a simple layout of
the data.

>Joe Buck
>jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck     


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  James A. Crotinger     Lawrence Livermore National Laboratory      //
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