6920090: G1: Disable ReduceInitialCardMarks at least until 6920109 is fixed

Summary: G1 now answers "no" to the query can_elide_initializing_store_barrier() in the product build. A debug flag allows alternate behaviour in debug builds.
Reviewed-by: iveresov, tonyp

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

@@ -1004,7 +1004,12 @@ public:
   // storage in the heap comes from a young region or not.
   // See ReduceInitialCardMarks.
   virtual bool can_elide_tlab_store_barriers() const {
-    return true;
+    // 6920090: Temporarily disabled, because of lingering
+    // instabilities related to RICM with G1. In the
+    // interim, the option ReduceInitialCardMarksForG1
+    // below is left solely as a debugging device at least
+    // until 6920109 fixes the instabilities.
+    return ReduceInitialCardMarksForG1;
   }
 
   virtual bool card_mark_must_follow_store() const {
@@ -1026,6 +1031,8 @@ public:
   // However, non-generational G1 (-XX:-G1Gen) appears to have
   // bit-rotted so was not tested below.
   virtual bool can_elide_initializing_store_barrier(oop new_obj) {
+    // Re 6920090, 6920109 above.
+    assert(ReduceInitialCardMarksForG1, "Else cannot be here");
     assert(G1Gen || !is_in_young(new_obj),
            "Non-generational G1 should never return true below");
     return is_in_young(new_obj);

src/share/vm/gc_implementation/g1/g1_globals.hpp

@@ -291,6 +291,10 @@
           "a particular entry exceeds this value.")                         \
                                                                             \
   develop(bool, G1VerifyCTCleanup, false,                                   \
-          "Verify card table cleanup.")
+          "Verify card table cleanup.")                                     \
+                                                                            \
+  develop(bool, ReduceInitialCardMarksForG1, false,                         \
+          "When ReduceInitialCardMarks is true, this flag setting "         \
+          " controls whether G1 allows the RICM optimization")
 
 G1_FLAGS(DECLARE_DEVELOPER_FLAG, DECLARE_PD_DEVELOPER_FLAG, DECLARE_PRODUCT_FLAG, DECLARE_PD_PRODUCT_FLAG, DECLARE_DIAGNOSTIC_FLAG, DECLARE_EXPERIMENTAL_FLAG, DECLARE_NOTPRODUCT_FLAG, DECLARE_MANAGEABLE_FLAG, DECLARE_PRODUCT_RW_FLAG)

src/share/vm/gc_interface/collectedHeap.cpp

@@ -66,7 +66,8 @@ void CollectedHeap::pre_initialize() {
   // Used for ReduceInitialCardMarks (when COMPILER2 is used);
   // otherwise remains unused.
 #ifdef COMPLER2
-  _defer_initial_card_mark = ReduceInitialCardMarks && (DeferInitialCardMark || card_mark_must_follow_store());
+  _defer_initial_card_mark =    ReduceInitialCardMarks && can_elide_tlab_store_barriers()
+                             && (DeferInitialCardMark || card_mark_must_follow_store());
 #else
   assert(_defer_initial_card_mark == false, "Who would set it?");
 #endif