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Newsgroups: comp.lang.ada
Subject: Re: Largest size array in Gnat 2005 for the PC?
From: Georg Bauhaus <rm.tsoh+bauhaus@maps.futureapps.de>
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Date: Tue, 30 Oct 2007 20:26:42 +0100
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Date: 2007-10-30T20:26:07+01:00
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On Tue, 2007-10-30 at 02:47 -0700, Ludovic Brenta wrote:
> Pascal Obry wrote:
> > ME a crit :
> >
> > > What is the largest array (in storage units) that you can declare in Gnat
> > > 2005 for the PC?

> In short, the answer is: "it depends" :)

Indeed, it depends;-) I thought that declaring an array *type*
of enormous size is not a problem, since there is no object
yet. It isn't a problem. So maybe using portions of the array might
work (when the program is not actually using physical memory.)
I get unexpected results,though, but they are consistently produced
by two unrelated compilers targeting two (seemingly unrelated)
machines!

This is the only hint I have found in the assembly
listing; '?' is $63, '!' is $33, this is for the lines
assigning Fst and Lst in the second declare block below.
Notice the -1 for the stack offset

        movl    $0, %eax
        movb    $33, -1(%ebp,%eax)    -- ( Fst := '!' )
        movl    $0, %eax
        movb    $63, -1(%ebp,%eax)    -- ( Lst := '?' )


Is there a bug in the following program or maybe I'm just dense?

with Ada.Text_IO;

procedure stk is

	use Ada;

	type Big_Index is range 0 .. 2**50;

	type A is array (Big_Index range <>) of Character;

begin

	-- output is "!?"
	declare
		X: A (Big_Index range 2**40 .. 2**40 + 10_000);
		Fst: Character renames X(X'First);
		Lst: Character renames X(X'Last);
	begin
		Fst := '!';
		Lst := '?';

		Text_IO.Put(Fst);
		Text_IO.Put(Lst);
		Text_IO.New_Line;
	end;

	-- output is "??"
	declare
		X: A (Big_Index range 2**40 .. 2**48);
		Fst: Character renames X(X'First);
		Lst: Character renames X(X'Last);
	begin
		Fst := '!';
		Lst := '?';
		pragma assert(Fst = '!');

		Text_IO.Put(Fst);
		Text_IO.Put(Lst);
		Text_IO.New_Line;
	end;
end stk;