Binary Compatibility: debug info for compiled Java programs

Daniel Jacobowitz
Thu Jun 10 16:54:00 GMT 2004

On Thu, Jun 10, 2004 at 05:42:20PM +0100, Andrew Haley wrote:
> Daniel Jacobowitz writes:
>  > 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.

Thanks, that's what I was missing.

>  > > 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?

That depends.  A decent-sized chunk of dwarf2out is devoted to this; on
the other hand, you've got a much simpler problem to solve since you're
only dealing with Java aggregates.  The hardest part would be
describing the types of fields with appropriate cross-referencing.

I'm not sure how to do this without nasty runtime overhead; it might be
easier to keep the logic in GDB.  I haven't given it much thought,

Daniel Jacobowitz

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