[Dwarf-discuss] Proposal/clarification: "inherited" subrange bounds

[Alexandre Oliva]

On Jul 27, 2024, David Blaikie <dblaikie@gmail.com> wrote:

> If I'm following correctly, Alexandre - you're suggesting a situation where
> an instance of a type, when it appears within a packed struct, may become
> smaller than it would otherwise be?

It's a philosophical question whether it's smaller in a packed struct or
larger otherwise, a bit like whether the Tardis is bigger in the inside
or smaller on the outside ;-D, but fact is that both kinds of uses of
the same type can be common in a unit, so it would be desirable to
represent those types with their sizes once, rather than e.g. override
it at every variable or field.

Right now, we represent two distinct, unrelated types to that end.  ISTM
that it would be better to be able to relate them, using one as the base
type for the other.  Sharing the bounds would be desirable, but it's not
clear that omitting them in the derived type would achieve the desired
result.  That's what this thread is about.

> (usually in C that's not the case -

*nod*, the issue at hand came from Ada.

> If you have a language where "packing" a member might cause it to become
> smaller, not just cause the leading/trailing padding to be omitted -
> perhaps that'd be better modelled, not with a new/distinct type, but with
> the same mechanism as is used for bitfields in C? ie: using bit_size and
> data_bit_offset?

The situation is not very different, but in Ada one can specify the
target size (in bits) for the type (which may require biased
representations, but that's besides the point).  Despite the specified
size, standalone variables and members of unpacked types use full
storage units, unless packing is requested.  See
e.g. https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/testsuite/gnat.dg/bias1.adb

-- 
Alexandre Oliva, happy hacker                    https://FSFLA.org/blogs/lxo/
   Free Software Activist                           GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice but
very few check the facts.  Think Assange & Stallman.  The empires strike back