RFC: attribute "unpadded"

[Alexandre Oliva]

On Aug 23, 2002, Mark Mitchell <mark@codesourcery.com> wrote:

> I want to create a new type attribute "unpadded" that would, on
> this code:

>   struct S __attribute__((unpadded)) {
>     int i;
>     char c;
>   };

> define S to have size 5 and alignment 4 on an ILP32 machine.

Instead of attaching this attribute to the type, perhaps it would
bring us less complications to attach it to a struct member, i.e.,
instead of:

>   struct A __attribute__((unpadded)) { void *vptr; char c; };
>   struct B { struct A __base; char c2; };

you'd write:

>   struct A { void *vptr; char c; };
>   struct B { struct A __base __attribute__((unpadded)); char c2; };

this wouldn't require any change in the typing rules, since the type
of struct A would be unchanged.  The attribute would only be used to
affect the lay out of struct B.  I have no idea whether this would
make the implementation of this feature much harder, though, since it
would have to peek into struct A to see how much padding it had, and
cancel that for the layout of struct B.  But it sounds like a less
intrusive change in the type system.

-- 
Alexandre Oliva   Enjoy Guarana', see http://www.ic.unicamp.br/~oliva/
Red Hat GCC Developer                 aoliva@{redhat.com, gcc.gnu.org}
CS PhD student at IC-Unicamp        oliva@{lsd.ic.unicamp.br, gnu.org}
Free Software Evangelist                Professional serial bug killer