# HG changeset patch
# User rkennke
# Date 1489685926 -3600
#      Thu Mar 16 18:38:46 2017 +0100
# Node ID 33a7de594e20524dd9d6973b449bca5c84c19f55
# Parent  b35e273953d4a3bcbab187203fcd4f44232ab7c2
Partial GC: Only evacuating threads push oop to work queue.

diff --git a/src/share/vm/gc/shenandoah/shenandoahBarrierSet.cpp b/src/share/vm/gc/shenandoah/shenandoahBarrierSet.cpp
--- a/src/share/vm/gc/shenandoah/shenandoahBarrierSet.cpp
+++ b/src/share/vm/gc/shenandoah/shenandoahBarrierSet.cpp
@@ -398,7 +398,8 @@
   assert(_heap->in_collection_set(src), "only evacuate objects in collection set");
   assert(! _heap->heap_region_containing(src)->is_humongous(), "never evacuate humongous objects");
   // TODO: Consider passing thread from caller.
-  oop dst = _heap->evacuate_object(src, Thread::current());
+  bool evac;
+  oop dst = _heap->evacuate_object(src, Thread::current(), evac);
 
   log_develop_trace(gc, compaction)("src = "PTR_FORMAT" dst = "PTR_FORMAT" src-2 = "PTR_FORMAT,
                                     p2i(src), p2i(dst), p2i(((HeapWord*) src) - 2));
diff --git a/src/share/vm/gc/shenandoah/shenandoahHeap.cpp b/src/share/vm/gc/shenandoah/shenandoahHeap.cpp
--- a/src/share/vm/gc/shenandoah/shenandoahHeap.cpp
+++ b/src/share/vm/gc/shenandoah/shenandoahHeap.cpp
@@ -796,7 +796,8 @@
 
     assert(_heap->is_marked_complete(p), "expect only marked objects");
     if (oopDesc::unsafe_equals(p, ShenandoahBarrierSet::resolve_oop_static_not_null(p))) {
-      _heap->evacuate_object(p, _thread);
+      bool evac;
+      _heap->evacuate_object(p, _thread, evac);
     }
   }
 };
@@ -1230,7 +1231,8 @@
                _heap->is_marked_complete(obj), _heap->is_marked_complete(ShenandoahBarrierSet::resolve_oop_static_not_null(obj)));
         oop resolved = ShenandoahBarrierSet::resolve_oop_static_not_null(obj);
         if (oopDesc::unsafe_equals(resolved, obj)) {
-          resolved = _heap->evacuate_object(obj, _thread);
+          bool evac;
+          resolved = _heap->evacuate_object(obj, _thread, evac);
         }
         oopDesc::encode_store_heap_oop(p, resolved);
       }
diff --git a/src/share/vm/gc/shenandoah/shenandoahHeap.hpp b/src/share/vm/gc/shenandoah/shenandoahHeap.hpp
--- a/src/share/vm/gc/shenandoah/shenandoahHeap.hpp
+++ b/src/share/vm/gc/shenandoah/shenandoahHeap.hpp
@@ -337,7 +337,11 @@
   void set_complete_top_at_mark_start(HeapWord* region_base, HeapWord* addr);
   HeapWord* complete_top_at_mark_start(HeapWord* region_base);
 
-  inline oop  evacuate_object(oop src, Thread* thread);
+  // Evacuates object src. Returns the evacuated object if this thread
+  // succeeded, otherwise rolls back the evacuation and returns the
+  // evacuated object by the competing thread. 'succeeded' is an out
+  // param and set to true if this thread succeeded, otherwise to false.
+  inline oop  evacuate_object(oop src, Thread* thread, bool& evacuated);
   inline bool cancelled_concgc() const;
   inline void set_cancelled_concgc(bool v);
   inline bool try_cancel_concgc() const;
diff --git a/src/share/vm/gc/shenandoah/shenandoahHeap.inline.hpp b/src/share/vm/gc/shenandoah/shenandoahHeap.inline.hpp
--- a/src/share/vm/gc/shenandoah/shenandoahHeap.inline.hpp
+++ b/src/share/vm/gc/shenandoah/shenandoahHeap.inline.hpp
@@ -276,9 +276,11 @@
   log_develop_trace(gc, compaction)("copy object from "PTR_FORMAT" to: "PTR_FORMAT, p2i((HeapWord*) p), p2i(copy));
 }
 
-inline oop ShenandoahHeap::evacuate_object(oop p, Thread* thread) {
+inline oop ShenandoahHeap::evacuate_object(oop p, Thread* thread, bool& evacuated) {
   size_t required;
 
+  evacuated = false;
+
 #ifdef ASSERT
   ShenandoahHeapRegion* hr = NULL;
   if (ShenandoahVerifyReadsToFromSpace) {
@@ -341,6 +343,7 @@
 
   oop return_val;
   if (oopDesc::unsafe_equals(result, p)) {
+    evacuated = true;
     return_val = copy_val;
 
     log_develop_trace(gc, compaction)("Copy of "PTR_FORMAT" to "PTR_FORMAT" succeeded \n",
diff --git a/src/share/vm/gc/shenandoah/shenandoahPartialGC.cpp b/src/share/vm/gc/shenandoah/shenandoahPartialGC.cpp
--- a/src/share/vm/gc/shenandoah/shenandoahPartialGC.cpp
+++ b/src/share/vm/gc/shenandoah/shenandoahPartialGC.cpp
@@ -279,21 +279,19 @@
     if (_heap->in_collection_set(obj)) {
       oop forw = ShenandoahBarrierSet::resolve_oop_static_not_null(obj);
       if (oopDesc::unsafe_equals(obj, forw)) {
-        forw = _heap->evacuate_object(obj, thread);
+        bool evacuated = false;
+        forw = _heap->evacuate_object(obj, thread, evacuated);
+
+        // Only the thread that succeeded evacuating this object pushes it to its work queue.
+        if (evacuated) {
+          assert(forw->is_oop(), "sanity");
+          bool succeeded = queue->push(SCMTask(forw));
+          assert(succeeded, "must succeed to push to task queue");
+        }
       }
       assert(! oopDesc::unsafe_equals(obj, forw) || _heap->cancelled_concgc(), "must be evacuated");
-      // TODO: Only the thread that succeeds *evacuating* the object should need to
-      // update the matrix and push the evacuated object to its queue. This would also
-      // enable to only have one CAS (the one in evacuate_object()) and use simple
-      // store for updating the ref.
-      oop oldval = _heap->atomic_compare_exchange_oop(forw, p, obj);
-      if (oopDesc::unsafe_equals(obj, oldval)) {
-        assert(forw->is_oop(), "sanity");
-        bool succeeded = queue->push(SCMTask(forw));
-        assert(succeeded, "must succeed to push to task queue");
-      } else {
-        assert(oopDesc::unsafe_equals(oldval, forw), "other thread must have punched the same forwarded oop");
-      }
+      // Update reference.
+      oopDesc::encode_store_heap_oop_not_null(p, forw);
       obj = forw; // For matrix update below.
     }
     // TODO: Make this templated