thread-safe exceptions (first cut)

[Mike Stump]

> To: egcs@cygnus.com
> From: Jason Merrill <jason@cygnus.com>
> Date: 12 Sep 1997 15:07:44 -0700

> I agree.  The __get_eh_context function would return a pointer to a struct
> which includes a pointer to any language-specific information.

Since the frontend manages the structure, it's allocation and so on,
it can just make it larger, and directly include the extra stuff at
the end...  I think we need the extra overhead of a pointer.

> BTW, I'm not sure __eh_pc is actually useful; you can find the throw
> point from __throw's return address.

This is nice in theory, but differs from the code in practice.  The
code of course could be changed to remove the assumption it has, but I
don't see that as pressing as the current todo list for EH.