[M3devel] unable to put M3CG.TypeUID into a record

[Rodney M. Bates]

I do like this idea.  It took me a while to catch on to the rule that
a field of packed type not only must occupy exactly the stated bit
count, but also must not have any compiler-added padding ahead of it.
While this is just what is needed for programmer-control of record
layout, it seems that almost everyone keeps missing it and getting
tripped up by it.

After having trouble with record layouts that became illegal after
changes in non-packed fields, I have made it a convention not to
put BITS--FOR in a type declaration, but only use it as an anonymous
type expression for a record field or array element.  The named type in
the type declaration will likely be used in several contexts, with differing
packing contexts.  The packing really needs to be independently thought
about for each record field and array element.

On 10/26/2012 04:31 AM, Jay K wrote:
> This is one reasonable way to fix it:
>
> Index: M3CG.i3
> ===================================================================
> RCS file: /usr/cvs/cm3/m3-sys/m3middle/src/M3CG.i3,v
> retrieving revision 1.10
> diff -u -r1.10 M3CG.i3
> --- M3CG.i36 Sep 2012 02:03:06 -00001.10
> +++ M3CG.i326 Oct 2012 09:30:31 -0000
> @@ -98,9 +98,9 @@
>     Always : Frequency = LAST (Frequency);
>   TYPE
> -  Int32 = [-16_7fffffff-1 .. 16_7fffffff];
> -  TypeUID = BITS 32 FOR Int32;
> +  TypeUID = [-16_7fffffff-1 .. 16_7fffffff];
>     (* a 32-bit unique id (fingerprint) for each type.  *)
> +  assert_TypeUID_fits_in_32bits = BITS 32 FOR TypeUID;
>   TYPE
>     Label = INTEGER;
> Index: M3CG_MultiPass.i3
>
>
>   - Jay
>
>
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 --
> From: jay.krell at cornell.edu
> To: m3devel at elegosoft.com
> Date: Fri, 26 Oct 2012 09:27:36 +0000
> Subject: [M3devel] unable to put M3CG.TypeUID into a record
>
> I still really don't like how M3CG.TypeUID is defined.
>
> Asserting that a type fits in 32bits
> should not preclude putting it into a RECORD.
>
> Currently you cannot put M3CG.TypeUID into a record.
>
> The following does not work:
>
> RECORD whatever = typeid: M3CG.TypeUID; END;
>
> That seems really absurd to me.
>
>   - Jay
>
>
>  > Date: Fri, 26 Oct 2012 11:22:55 +0000
>  > To: m3commit at elegosoft.com
>  > From: jkrell at elego.de
>  > Subject: [M3commit] CVS Update: cm3
>  >
>  > CVSROOT: /usr/cvs
>  > Changes by: jkrell at birch. 12/10/26 11:22:55
>  >
>  > Modified files:
>  > cm3/m3-sys/m3back/src/: M3C.m3
>  >
>  > Log message:
>  > continue to hack around bogus packing of M3CG.TypeUID
>  >