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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Subject: Re: Efficient Matrix?
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Date: Fri, 27 Dec 2002 16:58:14 GMT
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Date: 2002-12-27T16:58:14+00:00
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Adrian Knoth <adi@drcomp.erfurt.thur.de> writes:

> Alvery Grazebrook <nospam@grazebrook.demon.co.uk> wrote:
> 
> >>> type MATRIX is array (POSITIVE range <>, POSITIVE range <>) of BOOLEAN;
> >>> type POINTER_MATRIX is access all MATRIX;
> >>> A : POINTER_MATRIX := new MATRIX(1..10000,1..5000);
> >>
> >>> ideas to optimize this.
> >>   pragma Pack (MATRIX);
> 
> > As you are probably aware, your matrix will take up 5e7 bits, or just
> > under 10 Mbytes of storage, assuming the compiler is efficient in the
> > way it packs the data. 
> 
> I'm not sure whether the compiler packs it at all. It is heap-allocated
> memory (not stack) and it is not a bounded array (the type MATRIX).
> 
> That is why I cancelled my posting you're referring to.

Compilers are required to pack tightly in this case (one bit per Boolean
component).  I.e., Matrix'Component_Size will be 1 if there's a pragma
Pack.  See RM-13.2(9).

It's got nothing to do with the fact that the thing is heap allocated,
nor the fact that the bounds are not known at compile time.
(If the compiler used a different representation for a heap allocated
array than for a stack allocated array, it would have to be continually
translating back and forth.  So compilers don't do that.)

- Bob