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From "Gregory Shimansky (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HARMONY-4282) [drlvm][vmcore] obj_info agreement between VM and GC
Date Wed, 27 Jun 2007 11:39:26 GMT

    [ 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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