Only emit %write_barrier primitive for THREADED_RTS

includes/Cmm.h

 /* -----------------------------------------------------------------------------
  *
- * (c) The University of Glasgow 2004-2012
+ * (c) The University of Glasgow 2004-2013
  *
  * This file is included at the top of all .cmm source files (and
  * *only* .cmm files).  It defines a collection of useful macros for
@@ -583,6 +583,12 @@
 #define OVERWRITING_CLOSURE(c) /* nothing */
 #endif
 
+#ifdef THREADED_RTS
+#define prim_write_barrier prim %write_barrier()
+#else
+#define prim_write_barrier /* nothing */
+#endif
+
 /* -----------------------------------------------------------------------------
    Ticky macros 
    -------------------------------------------------------------------------- */

includes/rts/storage/SMPClosureOps.h

 /* ----------------------------------------------------------------------------
  *
- * (c) The GHC Team, 2005
+ * (c) The GHC Team, 2005-2013
  *
  * Macros for THREADED_RTS support
  *
@@ -12,7 +12,7 @@
 #ifdef CMINUSMINUS
 
 #define unlockClosure(ptr,info)                 \
-    prim %write_barrier();                      \
+    prim_write_barrier;                         \
     StgHeader_info(ptr) = info;    
 
 #else

rts/Updates.h

 /* -----------------------------------------------------------------------------
  *
- * (c) The GHC Team, 1998-2004
+ * (c) The GHC Team, 1998-2013
  *
  * Performing updates.
  *
@@ -46,7 +46,7 @@
 								\
     OVERWRITING_CLOSURE(p1);                                    \
     StgInd_indirectee(p1) = p2;                                 \
-    prim %write_barrier();                                      \
+    prim_write_barrier;                                         \
     SET_INFO(p1, stg_BLACKHOLE_info);                           \
     LDV_RECORD_CREATE(p1);                                      \
     bd = Bdescr(p1);						\