Re: Efficient Matrix?

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Adrian Knoth <adi@drcomp.erfurt.thur.de> writes:

> Robert A Duff <bobduff@shell01.TheWorld.com> wrote:
> 
> > turns it into a binding requirement.  13.1(20) also mentions this.
> 
> ACK.
>
> > So a standards conforming compiler can get away without supporting any
> > representation items at all.  But a compiler that claims to support the
> > SP annex must obey 13.2(9).  
> 
>     9  For a packed array type, if the component subtype's Size is less
>        than or equal to the word size, and Component_Size is not
>        specified for the type, Component_Size should be less than or
>        equal to the Size of the component subtype, rounded up to the
>        nearest factor of the word size.
> 
> How does this _forces_ a compiler to set a packed boolean array's
> Component_Size to 1? As I understand the paragraph it determines
> the compiler's behaviour if the Component_Size of a packed array is
> _not_ defined by the user.

I guess I was talking about that case.  E.g:

    type T is array (Positive range <>) of Boolean;
    pragma Pack(T);

The component subtype is Boolean.

Boolean'Size = 1, by 13.3(49).  Also 13.3(55), combined with the SP
annex thing.

This size (1 bit) is <= word size on every machine.  And it's a factor
of the word size on every machine.  So the compiler must choose
T'Component_Size <= 1.  And it can't very well choose < 1, so it must
choose = 1.

If you specify T'Component_Size, then the above paragraph is saying that
overrides the pragma.  So if you say:

    type T is array (Positive range <>) of Boolean;
    pragma Pack(T);
    for T'Component_Size use 2;

then T'Component_Size = 2.  This is required to be supported by
13.3(73), combined with the SP annex thing, assuming the word size
is an even number of bits (which is quite common ;-)).

> I guess I'll send a bug-report. Unfortunately the gcc-bug-database is
> write-only at the moment, so to say, there seems to be nobody carrying
> much about ProblemReports. 

Unless you're a paying customer, they have no obligation to fix things
soon.  But they do seem to fix things "eventually".

- Bob