7029458: G1: Add newly-reclaimed regions to the beginning of the region free list, not the end

Summary: What the synopsis says.
Reviewed-by: jwilhelm, iveresov, johnc

src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

@@ -3879,7 +3879,7 @@ void G1CollectedHeap::release_gc_alloc_regions(bool totally) {
       if (r->is_empty()) {
         // We didn't actually allocate anything in it; let's just put
         // it back on the free list.
-        _free_list.add_as_tail(r);
+        _free_list.add_as_head(r);
       } else if (_retain_gc_alloc_region[ap] && !totally) {
         // retain it so that we can use it at the beginning of the next GC
         _retained_gc_alloc_regions[ap] = r;
@@ -5013,7 +5013,7 @@ void G1CollectedHeap::free_region(HeapRegion* hr,
 
   *pre_used += hr->used();
   hr->hr_clear(par, true /* clear_space */);
-  free_list->add_as_tail(hr);
+  free_list->add_as_head(hr);
 }
 
 void G1CollectedHeap::free_humongous_region(HeapRegion* hr,
@@ -5065,7 +5065,7 @@ void G1CollectedHeap::update_sets_after_freeing_regions(size_t pre_used,
   }
   if (free_list != NULL && !free_list->is_empty()) {
     MutexLockerEx x(FreeList_lock, Mutex::_no_safepoint_check_flag);
-    _free_list.add_as_tail(free_list);
+    _free_list.add_as_head(free_list);
   }
   if (humongous_proxy_set != NULL && !humongous_proxy_set->is_empty()) {
     MutexLockerEx x(OldSets_lock, Mutex::_no_safepoint_check_flag);

src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp

@@ -1061,7 +1061,7 @@ public:
   }
 
   void append_secondary_free_list() {
-    _free_list.add_as_tail(&_secondary_free_list);
+    _free_list.add_as_head(&_secondary_free_list);
   }
 
   void append_secondary_free_list_if_not_empty_with_lock() {

src/share/vm/gc_implementation/g1/heapRegionSet.cpp

@@ -261,6 +261,45 @@ void HeapRegionLinkedList::fill_in_ext_msg_extra(hrs_ext_msg* msg) {
   msg->append(" hd: "PTR_FORMAT" tl: "PTR_FORMAT, head(), tail());
 }
 
+void HeapRegionLinkedList::add_as_head(HeapRegionLinkedList* from_list) {
+  hrs_assert_mt_safety_ok(this);
+  hrs_assert_mt_safety_ok(from_list);
+
+  verify_optional();
+  from_list->verify_optional();
+
+  if (from_list->is_empty()) return;
+
+#ifdef ASSERT
+  HeapRegionLinkedListIterator iter(from_list);
+  while (iter.more_available()) {
+    HeapRegion* hr = iter.get_next();
+    // In set_containing_set() we check that we either set the value
+    // from NULL to non-NULL or vice versa to catch bugs. So, we have
+    // to NULL it first before setting it to the value.
+    hr->set_containing_set(NULL);
+    hr->set_containing_set(this);
+  }
+#endif // ASSERT
+
+  if (_head != NULL) {
+    assert(length() >  0 && _tail != NULL, hrs_ext_msg(this, "invariant"));
+    from_list->_tail->set_next(_head);
+  } else {
+    assert(length() == 0 && _head == NULL, hrs_ext_msg(this, "invariant"));
+    _tail = from_list->_tail;
+  }
+  _head = from_list->_head;
+
+  _length           += from_list->length();
+  _region_num       += from_list->region_num();
+  _total_used_bytes += from_list->total_used_bytes();
+  from_list->clear();
+
+  verify_optional();
+  from_list->verify_optional();
+}
+
 void HeapRegionLinkedList::add_as_tail(HeapRegionLinkedList* from_list) {
   hrs_assert_mt_safety_ok(this);
   hrs_assert_mt_safety_ok(from_list);

src/share/vm/gc_implementation/g1/heapRegionSet.hpp

@@ -277,6 +277,10 @@ protected:
   }
 
 public:
+  // It adds hr to the list as the new head. The region should not be
+  // a member of another set.
+  inline void add_as_head(HeapRegion* hr);
+
   // It adds hr to the list as the new tail. The region should not be
   // a member of another set.
   inline void add_as_tail(HeapRegion* hr);
@@ -288,6 +292,11 @@ public:
   // Convenience method.
   inline HeapRegion* remove_head_or_null();
 
+  // It moves the regions from from_list to this list and empties
+  // from_list. The new regions will appear in the same order as they
+  // were in from_list and be linked in the beginning of this list.
+  void add_as_head(HeapRegionLinkedList* from_list);
+
   // It moves the regions from from_list to this list and empties
   // from_list. The new regions will appear in the same order as they
   // were in from_list and be linked in the end of this list.

src/share/vm/gc_implementation/g1/heapRegionSet.inline.hpp

@@ -110,6 +110,23 @@ inline void HeapRegionSet::remove_with_proxy(HeapRegion* hr,
 
 //////////////////// HeapRegionLinkedList ////////////////////
 
+inline void HeapRegionLinkedList::add_as_head(HeapRegion* hr) {
+  hrs_assert_mt_safety_ok(this);
+  assert((length() == 0 && _head == NULL && _tail == NULL) ||
+         (length() >  0 && _head != NULL && _tail != NULL),
+         hrs_ext_msg(this, "invariant"));
+  // add_internal() will verify the region.
+  add_internal(hr);
+
+  // Now link the region.
+  if (_head != NULL) {
+    hr->set_next(_head);
+  } else {
+    _tail = hr;
+  }
+  _head = hr;
+}
+
 inline void HeapRegionLinkedList::add_as_tail(HeapRegion* hr) {
   hrs_assert_mt_safety_ok(this);
   assert((length() == 0 && _head == NULL && _tail == NULL) ||