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Re: RFA: Re: Funky code in gnuv2_virtual_fn_field
- To: Jim Blandy <jimb at zwingli dot cygnus dot com>
- Subject: Re: RFA: Re: Funky code in gnuv2_virtual_fn_field
- From: Daniel Berlin <dan at cgsoftware dot com>
- Date: 24 May 2001 00:23:25 -0400
- Cc: Daniel Berlin <dan at cgsoftware dot com>, gdb at sources dot redhat dot com, gdb-patches at sources dot redhat dot com
- References: <20010520160159.3484E5E9DB@zwingli.cygnus.com><87u22e19kz.fsf@dynamic-addr-83-177.resnet.rochester.edu><npzoc513tk.fsf_-_@zwingli.cygnus.com>
Jim Blandy <jimb@zwingli.cygnus.com> writes:
> Daniel Berlin <dan@cgsoftware.com> writes:
> > Jim Blandy <jimb@zwingli.cygnus.com> writes:
> >
> > > I'm looking at lines 112--118 in gnu-v2-abi.c:
> > >
> > > if (TYPE_TARGET_TYPE (context) != type1)
> > > {
> > > value_ptr tmp = value_cast (context, value_addr (arg1));
> > > VALUE_POINTED_TO_OFFSET (tmp) = 0;
> > > arg1 = value_ind (tmp);
> > > type1 = check_typedef (VALUE_TYPE (arg1));
> > > }
> > >
> > > This looks fishy to me. If we smash the POINTED_TO_OFFSET without
> > > smashing the ENCLOSING_TYPE in a corresponding manner, and then we
> > > indirect through that pointer, don't we get a value whose
> > > ENCLOSING_TYPE is set, but whose address points to the embedded
> > > object, and not the enclosing object?
> >
> > Yup.
> > However, although it's not documented anywhere, value_cast
> > approriately smashes the enclosing type.
>
> That's what I was afraid of. (I *hate* it when GDB does something
> with a `struct value' that isn't really legal, but just happens to be
> okay because we know internal details about where that `struct value'
> came from...)
This is actually why i stopped using value_ptrs when doing the
location expression evaluation. Even the value operations and the
read_var_value routine seem to "know too much" about struct value, and
don't just do what they should.
>
> > IMHO, in any case, we shouldn't be needing to set the
> > pointed_to_offset here. If we have to, value_cast is doing something
> > wrong, or not enough of the right thing.
> > This is because all we are trying to do is a simple cast, which is what
> > value_cast is supposed to do for us. If we have to start mucking
> > around with it's results to get a correct value, then it's not doing
> > it's job right, or completely.
>
> Great. So how about this patch?
>
Approved.
> 2001-05-22 Jim Blandy <jimb@redhat.com>
>
> * gnu-v2-abi.c (gnuv2_virtual_fn_field): There's no need to clear
> VALUE_POINTED_TO_OFFSET here; if value_cast doesn't return a
> useful value, then we should fix that instead.
>
> Index: gdb/gnu-v2-abi.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/gnu-v2-abi.c,v
> retrieving revision 1.2
> diff -c -r1.2 gnu-v2-abi.c
> *** gdb/gnu-v2-abi.c 2001/05/12 04:01:16 1.2
> --- gdb/gnu-v2-abi.c 2001/05/22 21:14:35
> ***************
> *** 111,117 ****
> if (TYPE_TARGET_TYPE (context) != type1)
> {
> value_ptr tmp = value_cast (context, value_addr (arg1));
> - VALUE_POINTED_TO_OFFSET (tmp) = 0;
> arg1 = value_ind (tmp);
> type1 = check_typedef (VALUE_TYPE (arg1));
> }
> --- 111,116 ----
--
""-Steven Wright