FORTRAN Optimization

[Reinhard Doelz]

One could try variuos things to spped up F77. One main drawback 
is the fixed memory allocation. Within molecular dynamics routines
I found out that much time is lost by swapping the memory. If 
one uses arrays one should keep them as small as possible. I.e., 
if you store integers in a real array it takes much more space.
But even the dimensions of large arrays could influence the spped
in an unexplained way. I recognized that it results in higher 
spped if you set large dimensions for an array more frequently used: it 
is processed faster than with the (smaller) dimensions needed. 
The indices are essential: x(3,4) is stored as X11 X21 X31 X21 X22 X32 etc.
(Sorry if this is trivial). It is possible the other way round as well.
The IRIS will do it, however, kill itself by swapping. You could 
also try to use multiple assignments to memory, instead or reassigning 
variables, with EQUIVALENCES (dangerous but sppeds things up a lot, 
and saves space, too.) We fool around a bit with structures, but 
that is more for convenience than improving speed. another possibility
to make things go faster is to use /dev/mem, but I think MIPS F77 
does not support the mem calls of C. Another useful thing is to think 
about trigonometric functions. If they're used without challenging 
accuracy too much (e.g., we need horrible ammounts of sines and cosines 
for the normals in lighting models), it is worth to set up a table 
once containing the values in, lets say, 180 steps and to skip the 
time-consuming sin(x) by simply pointing to the table. 

Maybe this helps a little, 

Reinhard