From mboxrd@z Thu Jan  1 00:00:00 1970
From: Daniel Berlin <dan@cgsoftware.com>
To: Daniel Jacobowitz <drow@mvista.com>
Cc: Benjamin Kosnik <bkoz@redhat.com>, Eli Zaretskii <eliz@is.elta.co.il>, <jason@redhat.com>, <libstdc++@gcc.gnu.org>, <gdb@sources.redhat.com>
Subject: Re: c++ debugging hosed
Date: Mon, 26 Nov 2001 16:53:00 -0000
Message-ID: <Pine.LNX.4.33.0111261947250.20651-100000@www.cgsoftware.com>
References: <20011126183836.A9099@nevyn.them.org>
X-SW-Source: 2001-11/msg00268.html
Message-ID: <20011126165300.LwQHvOUNmO3lqfY_p1L82Bzb3J3zcPoWk1-tcN1RXSI@z>

On Mon, 26 Nov 2001, Daniel Jacobowitz wrote:

> On Sun, Nov 25, 2001 at 03:29:08PM -0500, Daniel Berlin wrote:
> >
> >
> > On Sun, 25 Nov 2001, Benjamin Kosnik wrote:
> >
> > >
> > > > No, it's relevant to STABS, too.
> > > > It's due to not properly searching virtual baseclasses.
> > > > I might still have the changes to do that somewhere, if someone wants
> > > > them.
> > >
> > > This has been broken for some time now. I'd appreciate it if it got
> > > fixed, so that C++ could be debugged properly.
> > I'm sure a lot of people would.
> > Unfortunately, it appears I don't have the code i had worked up to
> > abstract out the base class offset finding anymore.
> > What's happening is that GDB wants to know the baseclass offset, and
> > the routine it uses is tuned to either gnu-v2 or hp aCC (it checks
> > for something only hp's debug readers fill in, and if it doesn't find
> > it, it assumes gnu-v2), and the gnu-v2 code either returns the wrong
> > value (weird errors occur), or -1, which gives you "virtual baseclass
> > botch"/attempts to deref int values as pointers.
> >
> > baseclass_offset needs to be abstracted into the cp abi model, and all
> > existing uses need to use it.
>
> Could you clarify what symptom this fix you're discussing is supposed
> to solve?  I thought it was the one I was working on, but
> baseclass_offset actually does nothing ABI specific at all.

I'm also quite confused how you can say this after looking at it:
baseclass_offset:
...

      /* First look for the virtual baseclass pointer
         in the fields.  */
      for (i = n_baseclasses; i < len; i++)
        {
          if (vb_match (type, i, basetype))
            {
              CORE_ADDR addr
              = unpack_pointer (TYPE_FIELD_TYPE (type, i),
                                valaddr + (TYPE_FIELD_BITPOS (type, i) /
8));

              return addr - (LONGEST) address;
            }
        }

Then looking above it at vb_match.

It's, in fact, the most abi specific stuff still left for C++.


It happens to basically work for non-virtual classes, but it couldn't
possibly work for virtual baseclasses (IE even if you pulled some magic in gcc to make
it still output the things vb_match was looking for, it wouldn't work
anyway.).


baseclass offset finding is a fundamental piece of C++ support in gdb.
It's required for printing things, calling methods, etc.


--Dan