Re: Portability questions (Arrays)

31 May 1997

      Peter Gerwinski wrote:
...
...
(Is it generally allowed to cast a pointer into an integer,
   in the first place?)
I just found out that on the DEC Alpha a pointer has 8 bytes, while
an integer has 4.
Good that I didn't use this "feature" yet.

I'm briefly describing the situation I have. It's about speed optimization.
I have the following (simplified):

type t=array[0..max] of integer;

function f(var a:t):integer;
var
  i:integer;
begin
  i:=0;
  {some loop}
    if {...} then
      begin
        a[i]:=...;
        inc(i)
      end;
  {...}
  f:=i
end.

I thought about optimizing it with a pointer like this:

function f(var a:t):integer;
var
  p:^integer;
begin
  p:=@a[0];
  {some loop}
    if {...} then
      begin
        p^:=...;
        inc(p) {is this ok, BTW?}
      end;
  {...}
  f:="offset of p in a"
end.

So "f:=(integer(p)-integer(@a)) div sizeof(a[0])" doesn't seem to work then.
Or are other means of pointer arithmetic available -- or completely other
ways to do it?

I know that all of this is not very "Pascalish", but this routine requires
the utmost speed possible, so I tried to elimitate the need for an array
lookup, and to save one register (only p instead of i and the address of a)
-- this really does make a difference since the rest of the loops uses some
more variables. I hope I don't have to convert the routine into C to get
the best speed.
-- 
Frank Heckenbach, Erlangen, Germany
heckenb@mi.uni-erlangen.de
Turbo Pascal:   http://www.mi.uni-erlangen.de/~heckenb/programs.htm
Internet links: http://www.mi.uni-erlangen.de/~heckenb/links.htm

Frank Heckenbach

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