abort in ZForwarding

[Stuart Monteith]

Hello,
   I'm currently getting the following with -XX:+ZVerifyForwarding:

#  Internal Error
(/home/stumon01/repos/jdk/src/hotspot/share/gc/z/zForwarding.cpp:72),
pid=13355, tid=13359
#  guarantee(entry.from_index() != other.from_index()) failed: Duplicate from

Current thread (0x00007fd65c04f3a0):  GCTaskThread "ZWorker#2" [stack:
0x00007fd661542000,0x00007fd661642000] [id=13359]

Stack: [0x00007fd661542000,0x00007fd661642000],
sp=0x00007fd661640c30,  free space=1019k
Native frames: (J=compiled Java code, A=aot compiled Java code,
j=interpreted, Vv=VM code, C=native code)
V  [libjvm.so+0x13729b2]  ZForwarding::verify() const+0x1c6
V  [libjvm.so+0x13948c2]  ZRelocate::work(ZRelocationSetParallelIterator*)+0x7a
V  [libjvm.so+0x139557d]  ZRelocateTask::work()+0x27
V  [libjvm.so+0x139f55c]  ZTask::GangTask::work(unsigned int)+0x38
V  [libjvm.so+0x135353b]  GangWorker::run_task(WorkData)+0xab
V  [libjvm.so+0x13535f3]  GangWorker::loop()+0x37
V  [libjvm.so+0x135327a]  AbstractGangWorker::run()+0x3e
V  [libjvm.so+0x125ce2b]  Thread::call_run()+0x195
V  [libjvm.so+0xfb9fae]  thread_native_entry(Thread*)+0x1ee

I had assumed that this was because of something I'd some wrong on
AArch64, but I find it also occurs on x86.

ZForwarding::verify() is diligent enough to skip over empty entries in
the outer loop. However, this is not done for the inner loop.

This resolves the issue:

diff -r 6eb8c555644a src/hotspot/share/gc/z/zForwarding.cpp
--- a/src/hotspot/share/gc/z/zForwarding.cpp    Mon Apr 08 09:44:49 2019 +0100
+++ b/src/hotspot/share/gc/z/zForwarding.cpp    Mon Apr 08 17:30:18 2019 +0100
@@ -69,6 +69,11 @@
     // Check for duplicates
     for (ZForwardingCursor j = i + 1; j < _entries.length(); j++) {
       const ZForwardingEntry other = at(&j);
+      if (!other.populated()) {
+         // Skip empty entries
+         continue;
+       }
+
       guarantee(entry.from_index() != other.from_index(), "Duplicate from");
       guarantee(entry.to_offset() != other.to_offset(), "Duplicate to");
     }

Have you already encountered this, shall I create a bug+  patchset ?

BR,
    Stuart