harmony-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
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 14:46:24 GMT
On 6/28/07, Gregory Shimansky <gshimansky@apache.org> wrote:
> Gregory Shimansky wrote:
> > Xiao-Feng Li wrote:
> >> 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.
> >
> > If it is guaranteed that object allocation routine also sets VT pointer
> > correctly, then I can prepare a patch (I don't think it should be
> > committed before we unfreeze the code) to skip the first
> > ManagedObject::get_constant_header_size() bytes when copying memory in
> > object_clone() and not reset any information in object header relying GC
> > on the allocation routine.
>
> BTW, it is necessary that gc_cc also initialize the object header, VT
> and obj_info should contain correct information after object is allocated.

Yes, this is actually the default behavior of object allocation. For
example, gc_alloc_fast API will return a well-formed object to jitted
code without any vt or obj_info manipulation by the jitted code.

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).
> >>>
> >>> --
> >>> This message is automatically generated by JIRA.
> >>> -
> >>> You can reply to this email to add a comment to the issue online.
> >>>
> >>>
> >>
> >>
> >
> >
>
>
> --
> Gregory
>
>


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

Mime
View raw message