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From: Ken Garlington <kennieg@flash.net>
Subject: Re: 'size attribute inheritance
Date: 1997/08/11
Message-ID: <33EFCB66.285D@flash.net>#1/1
X-Deja-AN: 263644389
References: <33ECF679.4B5D@lmco.com> <dewar.871238340@merv>
Organization: Flashnet Communications, http://www.flash.net
Reply-To: Ken.Garlington@computer.org
Newsgroups: comp.lang.ada
Date: 1997-08-11T00:00:00+00:00
List-Id: <comp.lang.ada>


Robert Dewar wrote:
> 
> <<     When running this code, we get a bus alignment error.  We tracked
> it
>      down to the fact that the compiler does NOT really allocate 16 bits
>      for Month with subtype Month_Type.  We assumed that since>>
> 
> This is definitely a compiler bug, there is no way that legal declarations
> that are accepted by the compiler can lead to bus errors.

This seems like an odd statement. Maybe I'm not understanding the
problem,
but I thought the real issue was code like the following:

  package A_Test is

    Word : constant := 4;

    type Small_Value is range 0 .. 15;

    type A_Record is record
      Component_1 : Integer;
      Component_2 : Small_Value;
    end record;

    for A_Record use record
      Component_1 at 0*Word range 0 .. 31;
      Component_2 at 1*Word range 0 .. 31;
    end record;

    My_Record : A_Record := (3, 15);

  end A_Test;

For this code, will My_Record.Component_2 be stored in
bits 0 .. 3 of word 1, bits 28 .. 31, or elsewhere?
I know in Ada83 that this sometimes ended up on one
side, and something like

  type B_Array is array(0 .. 3) of Boolean;
  pragma Pack(B_Array);

sometimes ended up on the other side.

Is there something in the Ada83/Ada RMs that discuss this?
I couldn't find it.

> 
> However, the treatment of the size attribute seems to be quite in accord
> with the RM. Ada 95 effectively introduced pretty serious incompatibilites
> in the handling of 'Size by requiring an interpretaytion that was legal
> in Ada 83 (and happened by no mere coincidence to be the one used by
> the Intermetrics compiler :-) but was not typical of Ada 83 compilers.
> 
> So a lot of old code breaks but the break should consist of rejections
> at compile time: NOT bus errors!

Yeah, I don't think 'Size is really the issue here. It's the behavior
when a record representation clause allocates more space to a component
that it needs. Unless there's something in the RM that controls this,
I could see where a compiler might do something different than you might
expect. For example, storing it in an "unconventional" manner might
allow for fewer shift/mask instructions.

> 
> GNAT has introduced new attributes Value_Size and Object_Size wjhich can
> be applied to subtypes as well as first named types, to provide complete
> control over sizes, and allow old code to be easily ported no matter what
> conventions the compiler on which it was compiled was using.
> 
> Our default conventions are as close as we are allowed to the VADS compiler,
> but we are not allowed to be 100% compatible. In any case the bus error
> is a serious bug. From the description, it does not sound like you are
> using GNAT, and certainly the current version of GNAT does what you
> expect with the declarations you gave.