Re: vtable?

[Chris Moller <cmoller at redhat dot 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.
>    

What appears to be happening is that when you try to print the base 
class [p *(D*)e], build_gdb_vtable_type in gnu-v3-abi.c gets called to 
set up a top-level type struct containing, among other stuff, a field 
tagged "vcall_and_vbase_offsets" that, so far as I can tell, contains 
nothing useful--i.e., it's empty.  Later, in gnuv3_baseclass_offset, 
when the vtable is accessed for that field, it still appears to be 
empty.  In fact, it hasn't been touched at all--the numeric values of 
the pointers, and all the other fields, are the same as they where when 
initialised.  If gdb is supposed to be using knowledge of the ABI to 
build that field, it isn't.

So the question is whether the right thing to do is fix the gdb a priori 
information hack, or to fix gcc (if necessary--a DWARF dump of the 
testcase shows a lot of information concerning all of the relevant 
classes, but whether it's right or sufficient I don't know) and fix gdb 
properly to use the DWARF info.

Chris