Basic frontend question about layout

Basic frontend question about layout

[Jerry Quinn]

Hi, folks,

I'm having trouble seeing how layout is specified at the GENERIC level
for RECORD_TYPEs.  The docs and comments in tree.def say that you cannot
rely on the order of fields of the type.  In stor-layout.c,
layout_types() seems to do the obvious thing, taking the fields in
order, but the docs make it sound like there is no way to be sure what
you'll get.

Theoretically this would mean that you couldn't even reliably link a
structure in two separate compilation units, which is bogus.

Could someone please clear up my confusion?

Thanks,
Jerry Quinn

Re: Basic frontend question about layout

[Andrew Pinski]

On Tue, Jun 23, 2009 at 8:48 PM, Jerry Quinn<jlquinn@optonline.net> wrote:
> Hi, folks,
>
> I'm having trouble seeing how layout is specified at the GENERIC level
> for RECORD_TYPEs.  The docs and comments in tree.def say that you cannot
> rely on the order of fields of the type.  In stor-layout.c,
> layout_types() seems to do the obvious thing, taking the fields in
> order, but the docs make it sound like there is no way to be sure what
> you'll get.
> Could someone please clear up my confusion?

The confusion here is that layout_types is separate from the rest of
the middle-end and the front-end could do the layout themselves and
the front-end calls layout_type if it does not do the layout itself.

So for an example, Ada layouts the records themselves and some times
has a different order of the fields than the layouted offsets.

Thanks,
Andrew Pinski

Re: Basic frontend question about layout

[Jerry Quinn]

On Tue, 2009-06-23 at 20:52 -0700, Andrew Pinski wrote:
> On Tue, Jun 23, 2009 at 8:48 PM, Jerry Quinn<jlquinn@optonline.net> wrote:
> > Hi, folks,
> >
> > I'm having trouble seeing how layout is specified at the GENERIC level
> > for RECORD_TYPEs.  The docs and comments in tree.def say that you cannot
> > rely on the order of fields of the type.  In stor-layout.c,
> > layout_types() seems to do the obvious thing, taking the fields in
> > order, but the docs make it sound like there is no way to be sure what
> > you'll get.
> > Could someone please clear up my confusion?
> 
> The confusion here is that layout_types is separate from the rest of
> the middle-end and the front-end could do the layout themselves and
> the front-end calls layout_type if it does not do the layout itself.

As I look at the code, it seems like a front end doesn't actually need
to call layout_type at all.  Is that correct?  If so, is the layout
C-compatible, assuming the field types being used are individually
compatible?

I'm gleaning this from a mix of code, comments, and gccint texinfo docs.
Would it make sense to update them to indicate that layout is in order
as long as the rest of the language-specific front end doesn't choose to
lay things out differently?

Thanks,
Jerry

Re: Basic frontend question about layout

[Eric Botcazou]

> So for an example, Ada layouts the records themselves and some times
> has a different order of the fields than the layouted offsets.

The Ada compiler uses stor-layout.c by default, like all the other compilers.
It does its own layout only when it has nothing to do, i.e. when the user has 
specified the layout with a representation clause; and, in this case, it does 
sort the fields by increasing offsets (see components_to_record) because the 
middle-end expects the canonical order in various circumstances.

-- 
Eric Botcazou