Re: Should representation clauses be complete for each bit?

[Martin <martin@thedowies.com>]

On Jul 20, 3:51 pm, Robert A Duff <bobd...@shell01.TheWorld.com>
wrote:
> okellogg <okell...@users.sourceforge.net> writes:
> > Picking up an age old thread,
> >http://groups.google.com/group/comp.lang.ada/browse_thread/thread/9ab...
>
> Wow.   1998.
>
> > IMHO it would be a real gain if we could explicitly mention the unused
> > bits in the rep spec.
>
> I think the unused bits should be explicitly declared.
>
>
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>
> > Reusing the example from the OP,
>
> >>    type x is record
> >>      one : boolean;
> >>      two : boolean;
> >>      three : boolean;
> >>    end record;
> >>    for x use record
> >>      one   at 0 range 0..0;
> >>      two   at 0 range 3..3;
> >>      three at 0 range 15..15;
> >>    end record;
>
> > What I imagine is something like,
>
> > for x use record
> >    one   at 0 range 0 .. 0;
> >    null   at 0 range 1 .. 2;    // note reserved word "null"
> >    two   at 0 range 3 .. 3;
> >    null   at 0 range 4 .. 14;
> >    three at 0 range 15 .. 15;
> > end record;
> > for x'Size use 16;
>
> > The components with "null" would instruct the compiler to fill the
> > bits with 0.
>
> How about the following?
>
> package P is
>    type Bits2 is mod 2**2;
>    pragma Assert (Bits2'Size = 2);
>    type Bits11 is mod 2**11;
>    pragma Assert (Bits11'Size = 11);
>
>    type T is
>       record
>          One : Boolean;
>          Gap1 : Bits2; -- must be zero
>          Tooth : Boolean;
>          Gap2 : Bits11; -- must be zero
>          Ree : Boolean;
>       end record
>         with Predicate => T.Gap1 = 0 and T.Gap2 = 0;
>
>    for T use
>       record
>          pragma Complete_Representation;
>          One at 0 range 0..0;
>          Gap1 at 0 range 1..2;
>          Tooth at 0 range 3..3;
>          Gap2 at 0 range 4..14;
>          Ree at 0 range 15..15;
>       end record;
> end P;
>
> with Ada.Assertions; use Ada.Assertions;
> procedure P.Main is
>    X : T := (One | Tooth | Ree => True, Gap1 => 0, Gap2 => 0); -- OK
> begin
>    begin
>       X := T'(One | Tooth | Ree => True, Gap1 => 1, Gap2 => 2); -- Wrong.
>    exception
>       when Assertion_Error => null; -- OK
>    end;
>    pragma Assert (X = (One | Tooth | Ree => True, Gap1 => 0, Gap2 => 0));
> end P.Main;
>
> By the way, I find Ada's representation clauses to be at the wrong
> level of abstraction.  Why can't I just write a single line of code
> that means "put all the components in declaration order with no gaps
> in between"?
>
> - Bob

A bit like Telegen2's "pragma Preserve_Layout" (if my memory hasn't
rusted too much)?...

-- Martin