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From: gl8f@astsun9.astro.Virginia.EDU (Greg Lindahl)
Newsgroups: comp.lang.misc
Subject: Re: C's sins of commission
Message-ID: <1990Nov2.052815.22188@murdoch.acc.Virginia.EDU>
Date: 2 Nov 90 05:28:15 GMT
References: <1990Oct26.155937.29185@maths.nott.ac.uk> <PCG.90Oct30201101@teachk.cs.aber.ac.uk> <3673@stl.stc.co.uk>
Sender: news@murdoch.acc.Virginia.EDU
Organization: Department of Astronomy, University of Virginia
Lines: 19

In article <3673@stl.stc.co.uk> "Tom Thomson" <tom@nw.stl.stc.co.uk> writes:

>Jim's arrays and indices down them are all very well, but they certainly
>do not solve the aliasing issue he claims they solve; if I write 
>  a[i] := a[j] the compiler does not in general know whether i and j
>have different values, ie whether i and j are distinct.

Er, actually the example I use from my code looks like this:

	a[i] = b[i] / c[i];
	d[i] = b[i] * c[i];

Fortran only loads b[i] and c[i] once.

C, if you confuse the issue by using pointers, will generally load
b[i] and c[i] twice. If the locations of b or c are passed in, no C
compiler that I know of will optimize it. You can talk about the
general case, and you can debate the definition of a pointer, but
don't make my code run 30% slower.