[sh/jdk11u]: JVMTI related backports

[Aleksey Shipilev]

On 2/28/20 10:37 PM, Zhengyu Gu wrote:
> I would like to backport these two JVMTI heap walk related backports:
> 
> 8237396: JvmtiTagMap::weak_oops_do() should not trigger barriers
> 8237632: Shenandoah: accept NULL fwdptr to cooperate with JVMTI and JFR
> 
> The backports clear JVMTI related test failures.
> 
> Webrev: 
> http://cr.openjdk.java.net/~zgu/shenandoah/jvmti_sh_jdk11u/webrev.00/

If we want to cherry-pick 8237396 ahead of 11u, we have to protect it with UseShenandoahGC. I think
like this:

   inline oop object_raw() {
     if (UseShenandoahGC) {
       return RawAccess<>::oop_load(object_addr());
     }
     return object_peek();
   }

...also the synopsis should reflect what is going on:
 Cherry-pick: 8237396: JvmtiTagMap::weak_oops_do() should not trigger barriers

Otherwise looks fine

-- 
Thanks,
-Aleksey