8008079: G1: Add nextObject routine to CMBitMapRO and replace nextWord

Summary: Update the task local finger to the start of the next object when marking aborts, in order to avoid the redundant scanning of all 0's when the marking task restarts, if otherwise updating to the next word. In addition, reuse the routine nextObject() in routine iterate().
Reviewed-by: johnc, ysr
Contributed-by: Ntamao <tao.mao@oracle.com>

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

@@ -4111,7 +4111,7 @@ void CMTask::do_marking_step(double time_target_ms,
         // bitmap knows by how much we need to move it as it knows its
         // granularity).
         assert(_finger < _region_limit, "invariant");
-        HeapWord* new_finger = _nextMarkBitMap->nextWord(_finger);
+        HeapWord* new_finger = _nextMarkBitMap->nextObject(_finger);
         // Check if bitmap iteration was aborted while scanning the last object
         if (new_finger >= _region_limit) {
           giveup_current_region();

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

@@ -97,7 +97,6 @@ class CMBitMapRO VALUE_OBJ_CLASS_SPEC {
                                        HeapWord* limit = NULL) const;
 
   // conversion utilities
-  // XXX Fix these so that offsets are size_t's...
   HeapWord* offsetToHeapWord(size_t offset) const {
     return _bmStartWord + (offset << _shifter);
   }
@@ -105,8 +104,13 @@ class CMBitMapRO VALUE_OBJ_CLASS_SPEC {
     return pointer_delta(addr, _bmStartWord) >> _shifter;
   }
   int heapWordDiffToOffsetDiff(size_t diff) const;
-  HeapWord* nextWord(HeapWord* addr) {
-    return offsetToHeapWord(heapWordToOffset(addr) + 1);
+
+  // The argument addr should be the start address of a valid object
+  HeapWord* nextObject(HeapWord* addr) {
+    oop obj = (oop) addr;
+    HeapWord* res =  addr + obj->size();
+    assert(offsetToHeapWord(heapWordToOffset(res)) == res, "sanity");
+    return res;
   }
 
   // debugging

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

 /*
- * Copyright (c) 2001, 2012, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2013, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -252,12 +252,10 @@ inline bool CMBitMapRO::iterate(BitMapClosure* cl, MemRegion mr) {
 
     start_offset = _bm.get_next_one_offset(start_offset, end_offset);
     while (start_offset < end_offset) {
-      HeapWord* obj_addr = offsetToHeapWord(start_offset);
-      oop obj = (oop) obj_addr;
       if (!cl->do_bit(start_offset)) {
         return false;
       }
-      HeapWord* next_addr = MIN2(obj_addr + obj->size(), end_addr);
+      HeapWord* next_addr = MIN2(nextObject(offsetToHeapWord(start_offset)), end_addr);
       BitMap::idx_t next_offset = heapWordToOffset(next_addr);
       start_offset = _bm.get_next_one_offset(next_offset, end_offset);
     }