From: Chris Moller <cmoller@redhat.com>
To: archer@sourceware.org
Subject: Re: vtable?
Date: Thu, 14 Jan 2010 20:30:00 -0000 [thread overview]
Message-ID: <4B4F7EC1.2000901@redhat.com> (raw)
In-Reply-To: <m38wc0fnvw.fsf@fleche.redhat.com>
On 01/14/10 15:10, Tom Tromey wrote:
> Chris> One of my guesses of the moment is that under the circumstances of the
> Chris> bug, the "vtable" is either being built wrong or is somehow being
> Chris> corrupted--does anyone have a clue where the "vtable" is built?
>
> From what I understand, gdb doesn't actually use the dwarf stuff to
> build the class' vtable. There is a bug report or two in gcc bugzilla
> about this; I gather that gcc doesn't emit all the needed info.
>
> So, rather than rely on the dwarf, gdb encodes knowledge of the ABI.
>
okay, i'll poke around in there.
> Chris> Another guess is that in gnuv3_baseclass_offset, gnuv3_get_vtable is
> Chris> being called with a bad type argument, but I haven't looked into that
> Chris> yet.
>
> I don't know about this code, but one interesting thing about the bug is
> that casts to the base types seem to return the right answers:
>
> (gdb) p e
> $1 = (E *) 0x804a008
> (gdb) p (D*)e
> $2 = (D *) 0x804a00c
> (gdb) p (C*)e
> $3 = (C *) 0x804a010
>
> This makes me wonder whether the bug is actually in the value printing
> code -- because computing the base offsets seems to work in at least one
> case.
>
> That's just something I would look at though, don't give it too much
> credence. I don't know much about this code.
>
hmmm... but that doesn't explain why faking the offset--forcing
gnuv3_baseclass_offset to return an 8 iff the type name == "D"--makes
things work. on yet the other hand, using a slightly more complex test
case, which i just did, where struct A actually has something in it:
struct A {int k; A() {k=13;};};
struct B : virtual A {};
you get
$1 = {<C> = {v = 13}, _vptr.D = 0x804870c}
where the 13 in 'v = 13' looks like it might have come from struct A,
suggesting that you're right--the printing is screwed up.
thx,
chris
> Tom
>
next prev parent reply other threads:[~2010-01-14 20:30 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-01-14 18:01 vtable? Chris Moller
2010-01-14 20:11 ` vtable? Tom Tromey
2010-01-14 20:30 ` Chris Moller [this message]
2010-01-19 16:15 ` vtable? Chris Moller
2010-01-19 22:33 ` vtable? Tom Tromey
2010-01-20 8:44 ` vtable? Chris Moller
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