--- old/src/cpu/x86/vm/vm_version_x86.cpp	2017-06-08 08:26:38.431732860 -0700
+++ new/src/cpu/x86/vm/vm_version_x86.cpp	2017-06-08 08:26:38.301721099 -0700
@@ -1291,27 +1291,37 @@
   }
 #endif // COMPILER2
 
-  if( AllocatePrefetchInstr == 3 && !supports_3dnow_prefetch() ) AllocatePrefetchInstr=0;
-  if( !supports_sse() && supports_3dnow_prefetch() ) AllocatePrefetchInstr = 3;
+  if (FLAG_IS_DEFAULT(AllocatePrefetchInstr)) {
+    if (AllocatePrefetchInstr == 3 && !supports_3dnow_prefetch()) {
+      FLAG_SET_DEFAULT(AllocatePrefetchInstr, 0);
+    } else if (!supports_sse() && supports_3dnow_prefetch()) {
+      FLAG_SET_DEFAULT(AllocatePrefetchInstr, 3);
+    }
+  }
 
   // Allocation prefetch settings
   intx cache_line_size = prefetch_data_size();
-  if( cache_line_size > AllocatePrefetchStepSize )
-    AllocatePrefetchStepSize = cache_line_size;
+  if (FLAG_IS_DEFAULT(AllocatePrefetchDistance) &&
+      (cache_line_size > AllocatePrefetchStepSize)) {
+    FLAG_SET_DEFAULT(AllocatePrefetchStepSize, cache_line_size);
+  }
 
   AllocatePrefetchDistance = allocate_prefetch_distance();
   AllocatePrefetchStyle    = allocate_prefetch_style();
 
   if (is_intel() && cpu_family() == 6 && supports_sse3()) {
-    if (AllocatePrefetchStyle == 2) { // watermark prefetching on Core
+    if (FLAG_IS_DEFAULT(AllocatePrefetchDistance) &&
+        (AllocatePrefetchStyle == 2)) { // watermark prefetching on Core
 #ifdef _LP64
-      AllocatePrefetchDistance = 384;
+      FLAG_SET_DEFAULT(AllocatePrefetchDistance, 384);
 #else
-      AllocatePrefetchDistance = 320;
+      FLAG_SET_DEFAULT(AllocatePrefetchDistance, 320);
 #endif
     }
     if (supports_sse4_2() && supports_ht()) { // Nehalem based cpus
-      AllocatePrefetchDistance = 192;
+      if (FLAG_IS_DEFAULT(AllocatePrefetchDistance)) {
+        FLAG_SET_DEFAULT(AllocatePrefetchDistance, 192);
+      }
       if (FLAG_IS_DEFAULT(AllocatePrefetchLines)) {
         FLAG_SET_DEFAULT(AllocatePrefetchLines, 4);
       }