Re: Binary Compatibility: debug info for compiled Java programs

Andrew Haley
2004-06-10T16:44:40Z

Daniel Jacobowitz writes:
> On Wed, Jun 09, 2004 at 04:25:01PM -0600, Tom Tromey wrote:
> > However, this is only part of the picture. The other part is that the
> > Java runtime environment can differ from the compile-time environment.
> > In particular, a given class can be loaded into a running virtual
> > machine any number of times, via different ClassLoaders. And since
> > all references used by a class are symbolic, and since a ClassLoader
> > mediates the name->class lookup, it follows that each separately
> > loaded instance of such a class can have different superclasses.
> >
> > I.e., we load class Derived, which inherits from class Base, twice.
> > We load it once via ClassLoader A and once via ClassLoader B. Then we
> > can end up with different versions of Base, that might have different
> > properties. E.g., B's Base might have extra private fields. (Exactly
> > what changes are valid is what is described in that chapter of the
> > JLS.)
> >
> >
> > In our BC ABI, we add a new level of indirection. So, a field lookup
> > isn't just *(object+offset), but instead *(object+otable[index]),
> > where the otable ("offset table") is computed at class initialization
> > time.
>
> I still don't see how this mechanism implements the above principle,
> i.e. converting symbolic field names to offsets; is the otable
> associated with the object doing the access or with the objet defining
> the class?

The object doing the access. Every class has an otable that is fixed
up with the field offsets of every field that it accesses.

> > Generating Dwarf that redirects through the otable, like the code
> > itself does, is tempting. But is it possible? I don't see how
> > something like 'print object' would work -- you would have to look
> > more closely at all the reflection data to discover all the fields in
> > a given class.
>
> Generating Dwarf that redirects through a particular otable is easy.
> Generating information to describe the sort of symbolic changes to
> inheritance and fields, on the other hand, is not.

That's kinda what I expected. Is it hard to generate DWARF to
describe class layout?

Andrew.