Re: FYI: Linker & Verifier fixes (really GC mark procedures)

[David Daney <ddaney at avtrex dot com>]

Bryce McKinlay wrote:
> Tom Tromey wrote:
>
> > Tom> I did a little work on the gcjx branch in this area.  But it has
> > Tom> never been tested.  Anyway the idea was that the compiler could emit
> > Tom> real mark descriptors for Class, by special-casing it a bit in
> > Tom> boehm.c.
> >
> > I looked at this today.  We can't scan Class using a bitmap descriptor
> > yet, because its layout is wrong for this.  The descriptor overflows
> > and we have to fall back to procedural scanning.
> >
> > We could still do better than we do today, I think.  For instance we
> > could scan Class conservatively, and make sure to allocate
> > Class-related structures via the GC (using conservative scanning or
> > introducing new mark kinds as desired).
> >   
>
>
> My idea was to rearrange the layout of Class such that all pointers go 
> at the start of the Object, and non-pointers (plus pointers that we 
> never want marked?) go at the end. Then, we should be able to use a 
> length-based descriptor to get precise scanning.
>
> Of course, Class needs to be allocated at runtime before we can make 
> this happen. And, it will break the ABI - perhaps we could come up with 
> some kind of compatibility mode to create a new-Class from the old, 
> however.

Does it really change the ABI in a bad way?  Couldn't we just hack up 
javah to layout the classes the new way.  There is no guarantee of C++ 
ABI stability across versions is there?

And for the BC ABI isn't the class layed out at runtime?

David Daney.