This change turns the HashTable that JVMTI uses for object tagging into a regular Hotspot hashtable - the one in hashtable.hpp with resizing and rehashing. Instead of pointing directly to oops so that GC has to walk the table to follow oops and then to rehash the table, this table points to WeakHandle. GC walks the backing OopStorages concurrently.
The hash function for the table is a hash of the lower 32 bits of the address. A flag is set during GC (gc_notification if in a safepoint, and through a call to JvmtiTagMap::needs_processing()) so that the table is rehashed at the next use. The gc_notification mechanism of weak oop processing is used to notify Jvmti to post ObjectFree events. In concurrent GCs there can be a window of time between weak oop marking where the oop is unmarked, so dead (the phantom load in peek returns NULL) but the gc_notification hasn't been done yet. In this window, a heap walk or GetObjectsWithTags call would not find an object before the ObjectFree event is posted. This is dealt with in two ways: 1. In the Heap walk, there's an unconditional table walk to post events if events are needed to post. 2. For GetObjectWithTags, if a dead oop is found in the table and posting is required, we use the VM thread to post the event. Event posting cannot be done in a JavaThread because the posting needs to be done while holding the table lock, so that the JvmtiEnv state doesn't change before posting is done. ObjectFree callbacks are limited in what they can do as per the JVMTI Specification. The allowed callbacks to the VM already have code to allow NonJava threads. To avoid rehashing, I also tried to use object->identity_hash() but this breaks because entries can be added to the table during heapwalk, where the objects use marking. The starting markWord is saved and restored. Adding a hashcode during this operation makes restoring the former markWord (locked, inflated, etc) too complicated. Plus we don't want all these objects to have hashcodes because locking operations after tagging would have to always use inflated locks. Much of this change is to remove serial weak oop processing for the weakProcessor, ZGC and Shenandoah. The GCs have been stress tested with jvmti code. It has also been tested with tier1-6. Thank you to Stefan, Erik and Kim for their help with this change. ------------- Commit messages: - 8212879: Make JVMTI TagMap table not hash on oop address Changes: https://git.openjdk.java.net/jdk/pull/967/files Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=967&range=00 Issue: https://bugs.openjdk.java.net/browse/JDK-8212879 Stats: 1737 lines in 41 files changed: 631 ins; 990 del; 116 mod Patch: https://git.openjdk.java.net/jdk/pull/967.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/967/head:pull/967 PR: https://git.openjdk.java.net/jdk/pull/967
