7040068: CMS: Possibly unsafe initialization of BlockOffsetArrayUseUnallocatedBlock

Summary: BlockOffsetArrayUseUnallocatedBlock was intended to be turned off as part of BUG 6948538 but a code line in collectorPolicy.cpp actually kept it turned on.
Reviewed-by: jwilhelm, ysr

src/share/vm/memory/collectorPolicy.cpp

@@ -265,8 +265,6 @@ void TwoGenerationCollectorPolicy::initialize_flags() {
   MaxHeapSize = align_size_up(MaxHeapSize, max_alignment());
 
   always_do_update_barrier = UseConcMarkSweepGC;
-  BlockOffsetArrayUseUnallocatedBlock =
-      BlockOffsetArrayUseUnallocatedBlock || ParallelGCThreads > 0;
 
   // Check validity of heap flags
   assert(OldSize     % min_alignment() == 0, "old space alignment");

src/share/vm/runtime/globals.hpp

@@ -1824,7 +1824,7 @@ class CommandLineFlags {
   develop(bool, VerifyBlockOffsetArray, false,                              \
           "Do (expensive!) block offset array verification")                \
                                                                             \
-  product(bool, BlockOffsetArrayUseUnallocatedBlock, false,                 \
+  diagnostic(bool, BlockOffsetArrayUseUnallocatedBlock, false,              \
           "Maintain _unallocated_block in BlockOffsetArray"                 \
           " (currently applicable only to CMS collector)")                  \
                                                                             \