Binary Compatibility: debug info for compiled Java programs

Wed Jun 9 13:38:00 GMT 2004

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.