RE: [PATCH 1/2][PR fortran/26373][PR fortran/22497] gdb/fortran: add support for accessing fields of extended types

["Kempke, Nils-Christian" <nils-christian.kempke@intel.com>]

Kindly pinging for thoughts.

Cheers,
Nils

> -----Original Message-----
> From: Gdb-patches <gdb-patches-bounces+nils-
> christian.kempke=intel.com@sourceware.org> On Behalf Of Kempke, Nils-
> Christian via Gdb-patches
> Sent: Monday, January 17, 2022 11:28 AM
> To: Tom Tromey <tom@tromey.com>; Nils-Christian Kempke via Gdb-
> patches <gdb-patches@sourceware.org>
> Subject: RE: [PATCH 1/2][PR fortran/26373][PR fortran/22497] gdb/fortran:
> add support for accessing fields of extended types
> 
> Hi Tom,
> 
> > -----Original Message-----
> > From: Tom Tromey <tom@tromey.com>
> > Sent: Friday, January 14, 2022 9:06 PM
> > To: Nils-Christian Kempke via Gdb-patches <gdb-
> patches@sourceware.org>
> > Cc: Kempke, Nils-Christian <nils-christian.kempke@intel.com>; Bernhard
> > Heckel <bernhard.heckel@intel.com>
> > Subject: Re: [PATCH 1/2][PR fortran/26373][PR fortran/22497] gdb/fortran:
> > add support for accessing fields of extended types
> >
> > >>>>> ">" == Nils-Christian Kempke via Gdb-patches <gdb-
> > patches@sourceware.org> writes:
> >
> > >> From: Bernhard Heckel <bernhard.heckel@intel.com>
> > >> Fortran 2003 supports type extension.  This patch allows access
> > >> to inherited members by using their fully qualified name as described
> > >> in the Fortran standard.
> >
> > Thanks for the patch.
> >
> > >> In doing so the patch also fixes a bug in GDB when trying to access the
> > >> members of a base class in a derived class via the derived class' base
> > >> class member.
> >
> > >> @@ -2374,6 +2374,12 @@ value_struct_elt (struct value **argp,
> > >>        if (v)
> > >>  	return v;
> >
> > >> +      /* Fortran: If it is not a field it is the type name of an inherited
> > >> +	 structure.  */
> > >> +      v = search_struct_field (name, *argp, t, 1);
> > >> +      if (v)
> > >> +	return v;
> >
> > Putting this here makes me wonder if it's possible to construct test
> > cases in other languages that would work without this patch and then
> > fail with it.
> 
> Well, since this patch so far did not care about the language actually used,
> it enables one to do things like
> 
>   (gdb) p derived_obj
>   $1 = {<base> = {...}, <No data fields>}
>   (gdb) p derived_obj.base
>   $2 = {...}
> 
> in all languages, which probably is unexpected.  The above example would be
> C++ and some class "derived" inheriting from "base".  The syntax
> derived_obj.base is no standard C++..
> I am not sure whether one would actually break things since the only leftover
> check in the method is a (according to the comment) C++ specific one for
> member variables and I do not think that one can break C++ with the change.
> 
> >
> > I'm not sure what the options are though.  Doing the work elsewhere, say
> > in a custom Fortran expression operation, might make the Python API
> > harder to work with.  Maybe having value_struct_elt check
> > current_language would work... that's also quite ugly but other value
> > operations are doing that.
> 
> Mh, I am not too sure about the Fortran expression - would that not require
> a Fortran specific version of value_struct_elt?  The methods
> search_struct_method and search_struct_field are local to the valops.c file
> so
> this would be a bigger redesign? I know that Ada has its own value_struct_elt
> but it also implements the two helper methods.
> 
> Checking the language seems ok to me, considering the style of the rest of
> valops.
> 
> On more option that is also cleaner than directly checking the type might be
> to
> generalize the concept of this "member access by base name" a bit and add a
> method - say base_access_by_base_type_name - to language.h allowing or
> disallowing an access to the base type by its type name.  Then one would
> have to
> loop over all base types in valops.c as well.  I am not sure whether there exist
> other
> languages that allow this kind of access though.. If not it might be a bit much?
> 
> Cheers,
> 
> Nils
> 
> >
> > Or maybe my fears are overblown.
> >
> > Any thoughts on this?
> >
> > Tom
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Intel Deutschland GmbH
Registered Address: Am Campeon 10, 85579 Neubiberg, Germany
Tel: +49 89 99 8853-0, www.intel.de <http://www.intel.de>
Managing Directors: Christin Eisenschmid, Sharon Heck, Tiffany Doon Silva  
Chairperson of the Supervisory Board: Nicole Lau
Registered Office: Munich
Commercial Register: Amtsgericht Muenchen HRB 186928