Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!mcnc!uvaarpa!murdoch!astsun.astro.Virginia.EDU!gl8f
From: gl8f@astsun.astro.Virginia.EDU (Greg Lindahl)
Newsgroups: comp.lang.misc
Subject: Re: C's sins of commission
Message-ID: <1990Nov8.024049.9731@murdoch.acc.Virginia.EDU>
Date: 8 Nov 90 02:40:49 GMT
References: <3673@stl.stc.co.uk> <1990Nov2.052815.22188@murdoch.acc.Virginia.EDU> <2440:Nov607:32:5890@kramden.acf.nyu.edu>
Sender: news@murdoch.acc.Virginia.EDU
Organization: Department of Astronomy, University of Virginia
Lines: 38

In article <2440:Nov607:32:5890@kramden.acf.nyu.edu> brnstnd@kramden.acf.nyu.edu (Dan Bernstein) writes:
>> [ I write: ]
>> 	a[i] = b[i] / c[i];
>> 	d[i] = b[i] * c[i];
>  [ Fortran sees separate arrays and loads b[i] and c[i] only once ]
>  [ C sees possible aliasing and loads them twice ]
>
>Hmmm. I would almost automatically optimize the above into
>
>  {
>   register double bi = b[i];
>   register double ci = c[i];
>
>   a[i] = bi / ci;
>   d[i] = bi * ci;
>  }

Great, you've just doubled the # of lines of code. This is known as
"creating a maintenance nightmare".

Now take one of my real loops, with 3 lines of code referencing a
dozen or so array elements, including things like a[i], a[i-2], etc.
How do you pick the names? How do you keep the programmer from making
a mistake when they modify one of the lines?

I don't see why Dan thinks I should use a hard-to-optimize language
for numerical programming, which requires the programmer to do common
sub-expression elimination, when I can use a simple and fast language
instead. Having the compiler do a simple optimization seems much more
reliable than having the programmer optimize and maintain the code
segment, or have a compiler for an unnecessarily compliated language
compile the code segment.

By the way, this code has loops which can be unrolled 3 or 5 times and
keep loaded array values from previous iterations for future
iterations. Writing this out by hand is a nightmare when you have to
permute the temporary register variables. But it's not that difficult
to add such a feature to most Fortran compilers.