Binary Compatibility: debug info for compiled Java programs

Andrew Haley
Thu Jun 10 16:44:00 GMT 2004

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?


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