Re: Helping out GDB

["Martin v. Loewis" <martin at loewis dot home dot cs dot tu-berlin dot de>]

> As the C++ maintainer for GDB, I run into a lot of fun problems
> where g++ isn't providing info we need.

While I strongly sympathize with the need to support gdb as good as
possible, I can't see some of the problems you have.

> First and formost, for the old ABI, i have no way to detect what
> kind of vtable thunks the platform is using, or lack thereof.

Why is that? If the symbols start with __vt_, it's vtable thunks; if
they start with __vt., it's old-style vtables. Is that difficult to
tell apart?

> I have no way to tell whether something was compiled with the new or
> old abi.

Sure you do. Mangled symbols of the new ABI all start with _Z (except
for the extern "C" ones, of course); there won't be any __vt symbols.
If that is not enough identification, I could also give you a list of
functions that will be there.

> That's just a bad hack because gcc isn't telling us what we need to
> know.

If the approaches above don't work, please elaborate what problems
they cause.

> It would be nice if GCC, like a few other compilers, provided a
> callback so that I could say "when you get an exception in the
> program, call this routine", and it would call the callback with a
> few key parameters that i could use to tell what was thrown/caught
> and where.

I'm not sure I understand this request. First, are you aware of
__unwinding_cleanup? One problem with debugging exceptions is that it
is ok for an exception to occur as long as it is going to be caught,
so in a debugger, I'd like to break only if an exception is thrown
that won't be caught; this is what __unwinding_cleanup is for.

I can't see what the callback routine would achieve. For C++ alone,
there are different ways to initiate stack unwinding, specifically
throw <value>; and throw;. They require different entries into the
run-time system, and likely would also require different callbacks.
For example, __cp_push_exception indeed has all the information you
want.

> A callback would also remove the problem of their being so many
> different implementations of exception handling, since GDB would no
> longer have to care about how it was implemented.

Can you propose a specific ABI that you want to use. I have a hard
time guessing what the requirements should be.

Regards,
Martin