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From: hp@vmars.tuwien.ac.at (Peter Holzer)
Newsgroups: comp.lang.fortran,comp.lang.c
Subject: Re: Fortran vs. C for numerical work
Keywords: dependence analysis, C, Fortran, register variables
Message-ID: <2173@tuvie>
Date: 26 Nov 90 16:33:05 GMT
References: <BGLENDEN.90Nov21003342@mandrill.cv.nrao.edu> <21884@orstcs.CS.ORST.EDU> <1990Nov21.220816.15220@rice.edu>
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paco@rice.edu (Paul Havlak) writes:

>But watch out if you use C in its full generality.  All but the simplest
>pointers
>will confuse a compiler and reduce its ability to optimize. 

Even simple pointer can confuse a compiler on brain-dead
architectures.
I had a little program that did something with two arrays

item * a, * b;
int i;
/* init a, b */
for (i = 0; i < N; i ++) {
	a [i] = f (a [i], b [i]);
}

After I optimized it to:

item * a, * b, * pa, * pb;
/* init a, b */
for (pa = a + N, pb = b + N;
     pa >= a; 	/* I know that is not portable, but it works
	     	 * with this compiler if sizeof (item) <= 8
		 */
     pa --, pb --) {
	* pa = f (* pa, * pb);
}

it ran 80% slower, because on this architecture (80286) far
pointers don't fit into registers whereas the integer i does,
and indexing is relatively fast. 

To make things more complicated: On the same computer program 2
is faster than program 1 if both are compiled with near
pointers.

Morale: Write your code readable. Usually the compiler knows
more about the architecture of the target computer than the
programmer (especially if the program has to be compiled on lots
of different computers) and can therefore optimize better.

--
|    _  | Peter J. Holzer                       | Think of it   |
| |_|_) | Technical University Vienna           | as evolution  |
| | |   | Dept. for Real-Time Systems           | in action!    |
| __/   | hp@vmars.tuwien.ac.at                 |     Tony Rand |