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From "Xiao-Feng Li" <xiaofeng...@gmail.com>
Subject Re: [jira] Commented: (HARMONY-4282) [drlvm][vmcore] obj_info agreement between VM and GC
Date Thu, 28 Jun 2007 13:04:16 GMT
Yes, exactly. The newly allocated object may have some bits set in
obj_info by GC (only possible for those bits owned by GC). The
allocation routine can guarantee the rest bits of obj_info are clear.
This situation happens, e.g., when GC sets a property flag in obj_info
so as to avoid the re-computation of the property every time using it.
 One example property flag is whether the object has reference fields;
the other example is whether the object is bigger than 4KB; yet
another example is, in concurrent GC, the newly allocated object is
marked black in obj_info.

Basically we want to set up a contract between GC and VM that, object
allocation will take care of the object header initialization.

Thanks,
xiaofeng

On 6/27/07, Gregory Shimansky (JIRA) <jira@apache.org> wrote:
>
>     [ https://issues.apache.org/jira/browse/HARMONY-4282?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12508516
]
>
> Gregory Shimansky commented on HARMONY-4282:
> --------------------------------------------
>
> There cannot be an object without VT, it can be always copied IMHO. But I think that
object allocation already takes care about setting correct pointer to object's VT, so copying
it in clone may be redundant.
>
> The obj_info field is reset in the GC disabled condition, so no GC is possible at this
moment. Do you mean that some of the last 10 bits may be set in the newly allocated object
and VM clears them? What is the value of obj_info after allocation is done? Are the higher
22 bits clear? If yes, then clearing them is not required.
>
> > [drlvm][vmcore] obj_info agreement between VM and GC
> > ----------------------------------------------------
> >
> >                 Key: HARMONY-4282
> >                 URL: https://issues.apache.org/jira/browse/HARMONY-4282
> >             Project: Harmony
> >          Issue Type: Bug
> >          Components: DRLVM
> >         Environment: All
> >            Reporter: Li-Gang Wang
> >            Assignee: Gregory Shimansky
> >
> > I found some issue with obj_info initialization protocol between VM and GC.
> > Since there is a partitioning of the obj_info bits between VM and GC (GC takes the
last 10 bits), VM should not touch the last 10 bits without strong argument. But in following
piece of code, VM assumes the obj_info is empty after the object is allocated. This code makes
some GC design impossible.
> > jobject object_clone(JNIEnv *jenv, jobject jobj)
> > {
> >     ......
> >   result = (ManagedObject*)vm_new_vector_using_vtable_and_thread_pointer(length,
vt->clss->get_allocation_handle(), vm_get_gc_thread_local());    // call gc_alloc
> >     ......
> >     memcpy(result, h->object, size);   // copy the old object
> >     result->set_obj_info(0);                  // obj_info is reset here
> >     ......
> > }
> > This code should be changed to remove the obj_info reset statement, and the memcpy
should copy only the object fields part, excluding the object header (vt and obj_info).
>
> --
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>
>


-- 
http://xiao-feng.blogspot.com

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