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From: bruce@seismo.gps.caltech.edu (Bruce Worden)
Newsgroups: comp.lang.c
Subject: Re: p[1] vs. *(p+1)
Message-ID: <1990Aug31.231033.19235@laguna.ccsf.caltech.edu>
Date: 31 Aug 90 23:10:33 GMT
References: <26D952F5.4E44@tct.uucp> <1745@io.UUCP> <1990Aug31.190103.15043@laguna.ccsf.caltech.edu> <29605:Aug3120:51:0290@kramden.acf.nyu.edu>
Sender: bruce@seismo.gps.caltech.edu (Bruce Worden)
Organization: California Institute of Technology, CA
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In article <29605:Aug3120:51:0290@kramden.acf.nyu.edu> brnstnd@kramden.acf.nyu.edu (Dan Bernstein) writes:

>Huh? p[1] means exactly the same thing as *(p+1). At least two compilers
>here (neither pcc-based) do that translation as the very first step.
>There's no ``optimization'' involved.

It is really an extremely small point.  As far as I know, nothing specifies 
that the code generated by the two statements has to be identical, only that 
the results are the same.  One could imagine a compiler that did one less
efficiently than the other.

>>    *p1++ = *a1++ * *b1++;  
>>     p2[i] = a2[i] * b2[i]; 
>Note that on vector machines the latter is much faster in a loop.

My point exactly.  Except that the example you give isn't necessarily
true.  On our Convex both will vectorize, and the run time is the same.
--------------------------------------------------------------------------
C. Bruce Worden                            bruce@seismo.gps.caltech.edu
252-21 Seismological Laboratory, Caltech, Pasadena, CA 91125