--- a/src/cpu/sparc/vm/vm_version_sparc.cpp	Fri Nov 22 15:26:37 2013 +0100
+++ b/src/cpu/sparc/vm/vm_version_sparc.cpp	Mon Dec 02 11:12:32 2013 +0100
@@ -89,6 +89,27 @@
   _supports_cx8 = has_v9();
   _supports_atomic_getset4 = true; // swap instruction
 
+  // There are Fujitsu Sparc64 CPUs which support blk_init as well so
+  // we have to take this check out of the 'is_niagara()' block below.
+  if (has_blk_init()) {
+    // 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);
+    }
+    // Issue a stern warning if the user has explicitly set
+    // UseMemSetInBOT (it is known to cause issues), but allow
+    // use for experimentation and debugging.
+    if (UseConcMarkSweepGC || UseG1GC) {
+      if (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!");
+      }
+    }
+  }
+
   if (is_niagara()) {
     // Indirect branch is the same cost as direct
     if (FLAG_IS_DEFAULT(UseInlineCaches)) {
@@ -98,12 +119,6 @@
     if (FLAG_IS_DEFAULT(OptoLoopAlignment)) {
       FLAG_SET_DEFAULT(OptoLoopAlignment, 4);
     }
-    // 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
     // 32-bit oops don't make sense for the 64-bit VM on sparc
     // since the 32-bit VM has the same registers and smaller objects.