diff --git a/C/absmi.c b/C/absmi.c
index a4ec65a98..bf2c4780d 100644
--- a/C/absmi.c
+++ b/C/absmi.c
@@ -6329,7 +6329,6 @@ Yap_absmi(int inp)
 	setregs();
 	CACHED_A1() = ARG1;
 	PREG = ap->CodeOfPred;
-	WPP = NULL;
 	WRITE_UNLOCK(ap->PRWLock);
       }
       JMPNext();
diff --git a/C/cdmgr.c b/C/cdmgr.c
index 9225b37c4..f91bef1cc 100644
--- a/C/cdmgr.c
+++ b/C/cdmgr.c
@@ -1053,7 +1053,9 @@ addclause(Term t, yamop *cp, int mode, int mod)
   }
   Yap_PutValue(AtomAbol, TermNil);
   WRITE_LOCK(p->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
   WPP = p;
+#endif
   pflags = p->PredFlags;
   /* we are redefining a prolog module predicate */
   if (p->ModuleOfPred == 0 && mod != 0) {
@@ -1073,7 +1075,9 @@ addclause(Term t, yamop *cp, int mode, int mod)
     not_was_reconsulted(p, t, TRUE);
   /* always check if we have a valid error first */
   if (Yap_ErrorMessage && Yap_Error_TYPE == PERMISSION_ERROR_MODIFY_STATIC_PROCEDURE) {
+#if defined(YAPOR) || defined(THREADS)
     WPP = NULL;
+#endif
     WRITE_UNLOCK(p->PRWLock);
     return TermNil;
   }
@@ -1127,7 +1131,9 @@ addclause(Term t, yamop *cp, int mode, int mod)
       p->OpcodeOfPred = ((yamop *)(p->CodeOfPred))->opc;
     }
   }
+#if defined(YAPOR) || defined(THREADS)
   WPP = NULL;
+#endif
   WRITE_UNLOCK(p->PRWLock);
   if (pflags & LogUpdatePredFlag) {
     return MkDBRefTerm((DBRef)ClauseCodeToLogUpdClause(cp));
@@ -1608,15 +1614,21 @@ p_purge_clauses(void)
   } else
     return (FALSE);
   WRITE_LOCK(pred->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
   WPP = pred;
+#endif
   if (pred->PredFlags & StandardPredFlag) {
+#if defined(YAPOR) || defined(THREADS)
     WPP = NULL;
+#endif
     WRITE_UNLOCK(pred->PRWLock);
     Yap_Error(PERMISSION_ERROR_MODIFY_STATIC_PROCEDURE, t, "assert/1");
     return (FALSE);
   }
   purge_clauses(pred);
+#if defined(YAPOR) || defined(THREADS)
   WPP = NULL;
+#endif
   WRITE_UNLOCK(pred->PRWLock);
   return (TRUE);
 }
@@ -1714,9 +1726,12 @@ p_rmspy(void)
     WRITE_UNLOCK(pred->PRWLock);
     return (FALSE);
   }
+#if THREADS
   if (!(pred->PredFlags & ThreadLocalPredFlag)) {
     pred->OpcodeOfPred = Yap_opcode(_thread_local);
-  } else if (!(pred->PredFlags & DynamicPredFlag)) {
+  } else
+#endif
+    if (!(pred->PredFlags & DynamicPredFlag)) {
     pred->CodeOfPred = pred->cs.p_code.TrueCodeOfPred;
     pred->OpcodeOfPred = ((yamop *)(pred->CodeOfPred))->opc;
   } else if (pred->OpcodeOfPred == Yap_opcode(_spy_or_trymark)) {
@@ -3066,7 +3081,9 @@ fetch_next_lu_clause(PredEntry *pe, yamop *i_code, Term th, Term tb, Term tr, ya
 
   cl = Yap_FollowIndexingCode(pe, i_code, th, tb, tr, NEXTOP(PredLogUpdClause->CodeOfPred,ld), cp_ptr);
   if (cl == NULL) {
+#if defined(YAPOR) || defined(THREADS)
     WPP = NULL;
+#endif
     WRITE_UNLOCK(pe->PRWLock);
     return FALSE;
   }
@@ -3082,7 +3099,9 @@ fetch_next_lu_clause(PredEntry *pe, yamop *i_code, Term th, Term tb, Term tr, ya
     TRAIL_CLREF(cl);	/* So that fail will erase it */
   }
 #endif
+#if defined(YAPOR) || defined(THREADS)
   WPP = NULL;
+#endif
   WRITE_UNLOCK(pe->PRWLock);
   if (cl->ClFlags & FactMask) {
     if (!Yap_unify(tb, MkAtomTerm(AtomTrue)) ||
@@ -3140,7 +3159,9 @@ p_log_update_clause(void)
   if (pe == NULL || EndOfPAEntr(pe))
     return FALSE;
   WRITE_LOCK(pe->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
   WPP = pe;
+#endif
   if(pe->OpcodeOfPred == INDEX_OPCODE) {
     IPred(pe);
   }
@@ -3154,7 +3175,9 @@ p_continue_log_update_clause(void)
   yamop *ipc = (yamop *)IntegerOfTerm(ARG2);
 
   WRITE_LOCK(pe->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
   WPP = pe;
+#endif
   return fetch_next_lu_clause(pe, ipc, Deref(ARG3), ARG4, ARG5, B->cp_ap, FALSE);
 }
 
@@ -3164,7 +3187,9 @@ fetch_next_lu_clause0(PredEntry *pe, yamop *i_code, Term th, Term tb, yamop *cp_
   LogUpdClause *cl;
 
   cl = Yap_FollowIndexingCode(pe, i_code, th, tb, TermNil, NEXTOP(PredLogUpdClause0->CodeOfPred,ld), cp_ptr);
+#if defined(YAPOR) || defined(THREADS)
   WPP = NULL;
+#endif
   WRITE_UNLOCK(pe->PRWLock);
   if (cl == NULL) {
     return FALSE;
@@ -3223,7 +3248,9 @@ p_log_update_clause0(void)
   if (pe == NULL || EndOfPAEntr(pe))
     return FALSE;
   WRITE_LOCK(pe->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
   WPP = pe;
+#endif
   if(pe->OpcodeOfPred == INDEX_OPCODE) {
     IPred(pe);
   }
@@ -3237,7 +3264,9 @@ p_continue_log_update_clause0(void)
   yamop *ipc = (yamop *)IntegerOfTerm(ARG2);
 
   WRITE_LOCK(pe->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
   WPP = pe;
+#endif
   return fetch_next_lu_clause0(pe, ipc, Deref(ARG3), ARG4, B->cp_ap, FALSE);
 }
 
@@ -3329,7 +3358,9 @@ p_nth_clause(void)
   if (pe == NULL || EndOfPAEntr(pe))
     return FALSE;
   WRITE_LOCK(pe->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
   WPP = pe;
+#endif
   if (!(pe->PredFlags & (SourcePredFlag|LogUpdatePredFlag))) {
     WRITE_UNLOCK(pe->PRWLock);
     return FALSE;
diff --git a/C/dbase.c b/C/dbase.c
index 2fce7ed0c..f08a7c8c7 100644
--- a/C/dbase.c
+++ b/C/dbase.c
@@ -1836,9 +1836,13 @@ record_lu(PredEntry *pe, Term t, int position)
   else
     ipc->opc = Yap_opcode(_unify_idb_term);
   WRITE_LOCK(pe->PRWLock);
+#if defined(YAPOR) || defined(THREADS)
   WPP = pe;
+#endif
   Yap_add_logupd_clause(pe, cl, (position == MkFirst ? 2 : 0));
+#if defined(YAPOR) || defined(THREADS)
   WPP = NULL;
+#endif
   WRITE_UNLOCK(pe->PRWLock);
   return cl;
 }
@@ -3848,9 +3852,11 @@ static void
 EraseLogUpdCl(LogUpdClause *clau)
 {
   PredEntry *ap = clau->ClPred;
+#if defined(YAPOR) || defined(THREADS)
   if (WPP != ap) {
     WRITE_LOCK(ap->PRWLock);
   }
+#endif
   /* no need to erase what has been erased */ 
   if (!(clau->ClFlags & ErasedMask)) {
 
@@ -3897,9 +3903,11 @@ EraseLogUpdCl(LogUpdClause *clau)
     clau->ClRefCount--;
   }
   complete_lu_erase(clau);
+#if defined(YAPOR) || defined(THREADS)
   if (WPP != ap) {
     WRITE_UNLOCK(ap->PRWLock);
   }
+#endif
 }
 
 static void
@@ -3974,9 +3982,11 @@ PrepareToEraseLogUpdClause(LogUpdClause *clau, DBRef dbr)
   if (clau->ClFlags & ErasedMask)
     return;
   clau->ClFlags |= ErasedMask;
+#if defined(YAPOR) || defined(THREADS)
   if (WPP != p) {
     WRITE_LOCK(p->PRWLock);
   }
+#endif
   if (p->cs.p_code.FirstClause != cl) {
     /* we are not the first clause... */
     yamop *prev_code_p = (yamop *)(dbr->Prev->Code);
@@ -4026,9 +4036,11 @@ PrepareToEraseLogUpdClause(LogUpdClause *clau, DBRef dbr)
       p->CodeOfPred = (yamop *)(&(p->OpcodeOfPred)); 
     }
   }
+#if defined(YAPOR) || defined(THREADS)
   if (WPP != p) {
     WRITE_UNLOCK(p->PRWLock);
   }
+#endif
 }
 
 static void 
@@ -4905,6 +4917,7 @@ Yap_ReleaseTermFromDB(DBTerm *ref)
 static Int 
 p_install_thread_local(void)
 {				/* '$is_dynamic'(+P)	 */
+#if THREADS
   PredEntry      *pe;
   Term            t = Deref(ARG1);
   Term            t2 = Deref(ARG2);
@@ -4941,6 +4954,7 @@ p_install_thread_local(void)
   pe->OpcodeOfPred = Yap_opcode(_thread_local);
   pe->CodeOfPred = (yamop *)&pe->OpcodeOfPred;
   WRITE_UNLOCK(pe->PRWLock);
+#endif
   return TRUE;
 }
 
diff --git a/C/index.c b/C/index.c
index 357e0b232..f1b2c56af 100644
--- a/C/index.c
+++ b/C/index.c
@@ -6556,12 +6556,12 @@ Yap_FollowIndexingCode(PredEntry *ap, yamop *ipc, Term t1, Term tb, Term tr, yam
     case _unlock_lu:
       ipc = NEXTOP(ipc,e);
       break;
-    case _thread_local:
 #if THREADS
+    case _thread_local:
       ap = Yap_GetThreadPred(ap);
       ipc = ap->CodeOfPred;
-#endif
       break;
+#endif
     case _index_pred:
     case _spy_pred:
       Yap_IPred(ap);
diff --git a/C/threads.c b/C/threads.c
index eff774832..0b130fddc 100644
--- a/C/threads.c
+++ b/C/threads.c
@@ -18,14 +18,6 @@
 static char     SccsId[] = "%W% %G%";
 #endif
 
-#if THREADS
-
-/*
- * This file includes the definition of threads in Yap. Threads
- * are supposed to be compatible with the SWI-Prolog thread package.
- *
- */
-
 #include "Yap.h"
 #include "Yatom.h"
 #include "Heap.h"
@@ -36,6 +28,14 @@ static char     SccsId[] = "%W% %G%";
 #include <string.h>
 #endif
 
+#if THREADS
+
+/*
+ * This file includes the definition of threads in Yap. Threads
+ * are supposed to be compatible with the SWI-Prolog thread package.
+ *
+ */
+
 static int
 allocate_new_tid(void)
 {
@@ -402,7 +402,6 @@ void Yap_InitThreadPreds(void)
 
 #else
 
-
 static Int 
 p_no_threads(void)
 {				/* '$thread_signal'(+P)	 */
@@ -411,7 +410,7 @@ p_no_threads(void)
 
 void Yap_InitThreadPreds(void)
 {
-  Yap_InitCPred("$no_threads", 0, p_create_thread, 0);
+  Yap_InitCPred("$no_threads", 0, p_no_threads, 0);
 }