Re: RFA: static cast from base class to derived class

Daniel Jacobowitz
2002-08-21T12:25:17Z

On Wed, Aug 21, 2002 at 01:00:52AM -0500, Jim Blandy wrote:
>
> Once when my grandfather was a boy, he was riding his bicycle along a
> gravel path, and going pretty fast, when he hit a stone, went flying
> over the handlebars, and landed on his face. The gravel was ground so
> deeply into his face that, even in his forties, he'd still
> occasionally encounter, while shaving, bits of rock that had finally
> worked their way out to the surface.
>
> Here's a patch for a bit of 1998 HP Merge gravel that has finally
> found its way to the surface.
>
> I didn't see any regressions with this patch using either STABS or
> Dwarf 2. I'll put together a regression test for the original bug
> tomorrow.
>
> 2002-08-20 Jim Blandy <jimb@redhat.com>
>
> * valops.c (value_cast): Simplify and correct logic for doing a
> static cast from a pointer to a base class to a pointer to a
> derived class.
>
> Index: gdb/valops.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/valops.c,v
> retrieving revision 1.67
> diff -c -r1.67 valops.c
> *** gdb/valops.c 19 Aug 2002 23:19:53 -0000 1.67
> --- gdb/valops.c 21 Aug 2002 02:42:21 -0000
> ***************
> *** 361,378 ****
> value_zero (t1, not_lval), 0, t1, 1);
> if (v)
> {
> ! struct value *v2 = value_ind (arg2);
> ! VALUE_ADDRESS (v2) -= VALUE_ADDRESS (v)
> ! + VALUE_OFFSET (v);
> !
> ! /* JYG: adjust the new pointer value and
> ! embedded offset. */
> ! v2->aligner.contents[0] -= VALUE_EMBEDDED_OFFSET (v);
> ! VALUE_EMBEDDED_OFFSET (v2) = 0;
> !
> ! v2 = value_addr (v2);
> ! VALUE_TYPE (v2) = type;
> ! return v2;
> }
> }
> }
> --- 361,371 ----
> value_zero (t1, not_lval), 0, t1, 1);
> if (v)
> {
> ! CORE_ADDR addr2 = value_as_address (arg2);
> ! addr2 -= (VALUE_ADDRESS (v)
> ! + VALUE_OFFSET (v)
> ! + VALUE_EMBEDDED_OFFSET (v));
> ! return value_from_pointer (type, addr2);
> }
> }
> }

I've got a question... what does VALUE_ADDRESS mean in this context?
If it means what it normally means (ought to mean?), then taking an
address, subtracting an address, and using it as a pointer doesn't make
a lot of sense.

Answering my own question - it's dereferencing a pointer to a struct at
0. So we'll actually get an address relative to zero, which makes
everything work out. This looks good to me.

--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer