--- a/hotspot/src/cpu/sparc/vm/vm_version_sparc.cpp	Fri Aug 17 15:41:04 2012 -0700
+++ b/hotspot/src/cpu/sparc/vm/vm_version_sparc.cpp	Tue Aug 21 10:05:57 2012 -0700
@@ -106,10 +106,10 @@
     if (FLAG_IS_DEFAULT(OptoLoopAlignment)) {
       FLAG_SET_DEFAULT(OptoLoopAlignment, 4);
     }
-    // When using CMS, we cannot use memset() in BOT updates because
-    // the sun4v/CMT version in libc_psr uses BIS which exposes
-    // "phantom zeros" to concurrent readers. See 6948537.
-    if (FLAG_IS_DEFAULT(UseMemSetInBOT) && UseConcMarkSweepGC) {
+    // When using CMS or G1, we cannot use memset() in BOT updates
+    // because the sun4v/CMT version in libc_psr uses BIS which
+    // exposes "phantom zeros" to concurrent readers. See 6948537.
+    if (FLAG_IS_DEFAULT(UseMemSetInBOT) && (UseConcMarkSweepGC || UseG1GC)) {
       FLAG_SET_DEFAULT(UseMemSetInBOT, false);
     }
 #ifdef _LP64

--- a/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp	Fri Aug 17 15:41:04 2012 -0700
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp	Tue Aug 21 10:05:57 2012 -0700
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2011, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2012, 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
@@ -159,14 +159,30 @@
            "right address out of range");
     assert(left  < right, "Heap addresses out of order");
     size_t num_cards = pointer_delta(right, left) >> LogN_words;
-    memset(&_offset_array[index_for(left)], offset, num_cards);
+    if (UseMemSetInBOT) {
+      memset(&_offset_array[index_for(left)], offset, num_cards);
+    } else {
+      size_t i = index_for(left);
+      const size_t end = i + num_cards;
+      for (; i < end; i++) {
+        _offset_array[i] = offset;
+      }
+    }
   }
 
   void set_offset_array(size_t left, size_t right, u_char offset) {
     assert(right < _vs.committed_size(), "right address out of range");
-    assert(left  <= right, "indexes out of order");
+    assert(left <= right, "indexes out of order");
     size_t num_cards = right - left + 1;
-    memset(&_offset_array[left], offset, num_cards);
+    if (UseMemSetInBOT) {
+      memset(&_offset_array[left], offset, num_cards);
+    } else {
+      size_t i = left;
+      const size_t end = i + num_cards;
+      for (; i < end; i++) {
+        _offset_array[i] = offset;
+      }
+    }
   }
 
   void check_offset_array(size_t index, HeapWord* high, HeapWord* low) const {

--- a/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Fri Aug 17 15:41:04 2012 -0700
+++ b/hotspot/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Tue Aug 21 10:05:57 2012 -0700
@@ -1934,6 +1934,14 @@
   clear_cset_start_regions();
 
   guarantee(_task_queues != NULL, "task_queues allocation failure.");
+#ifdef SPARC
+  // Issue a stern warning, but allow use for experimentation and debugging.
+  if (VM_Version::is_sun4v() && UseMemSetInBOT) {
+    assert(!FLAG_IS_DEFAULT(UseMemSetInBOT), "Error");
+    warning("Experimental flag -XX:+UseMemSetInBOT is known to cause instability"
+            " on sun4v; please understand that you are using at your own risk!");
+  }
+#endif
 }
 
 jint G1CollectedHeap::initialize() {