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

[Bryce McKinlay <mckinlay at redhat dot com>]

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.

Bryce