🚀 go-pugleaf

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.