Re: Binary Compatibility: debug info for compiled Java programs

[Andrew Haley <aph@redhat.com>]

Daniel Jacobowitz writes:
 > On Wed, Jun 09, 2004 at 02:16:44PM +0100, Andrew Haley wrote:
 > > Daniel Jacobowitz writes:
 > >  > 
 > >  > I'm not familiar with the BC ABI (and missed the talk); could you give
 > >  > me an example of what information you have at compile time and what you
 > >  > generate at runtime?
 > > 
 > > All structures are laid out at runtime.
 > > 
 > > http://people.redhat.com/lockhart/.gcc04/MasterGCC-2side.pdf  Page 169.
 > 
 > So: the list of members, the inheritance tree, et cetera is static at
 > compile time; for their locations you have runtime tables.

No, not exactly.  It's possible to insert a class into the inheritance
chain or add new fields.

 > >  > I suspect that a fourth solution is possible: gcj generating DWARF
 > >  > which describes how to read the metadata.  It may not be very
 > >  > efficient, though, and it will still require a certain amount of
 > >  > playing with GDB.
 > > 
 > > Yeah, that was suggested as well.  I forgot to mention it.  I don't
 > > know if this is possible in DWARF data: don't member offsets have to
 > > be constants?
 > > 
 > > I must admit I like this idea least of all!  Maybe I subconsciously
 > > suppressed the memory.
 > 
 > Could you explain what you don't like about it?

An odd feeling of unease, I suppose.  I imagine the expressions could
become rather complex.

 > Member offsets definitely don't have to be constant.  In fact, from the
 > spec:
 >   For a C++ virtual base, the data member location attribute will
 >   usually consist of a non-trivial location expression.
 > The way C++ handles virtual bases is similar enough in execution that
 > the same thing will work here.  For a member it would look like:
 > 
 >   # Base location of the class is on the stack
 >   DW_OP_addr <otable location>
 >   DW_OP_constu <otable offset>
 >   DW_OP_plus
 >   DW_OP_deref (or DW_OP_deref_size <size of atable entry>)
 >   DW_OP_plus

Okay, that's good.  Would we also be able to cope with not being able
to resolve our superclass till runtime?  It can't just be a pointer to
a symbol, because there may be many identical symbols.

Andrew.