--- /sys/src/cmd/cc/cc.h
+++ /sys/src/cmd/cc/cc.h
@@ -665,6 +665,7 @@ void	dclfunct(Type*, Sym*);
  * sub.c
  */
 void	arith(Node*, int);
+int	castucom(Node*);
 int	deadheads(Node*);
 Type*	dotsearch(Sym*, Type*, Node*, long*);
 long	dotoffset(Type*, Type*, Node*);
--- /sys/src/cmd/cc/com.c
+++ /sys/src/cmd/cc/com.c
@@ -1055,6 +1055,8 @@ loop:
 		break;
 
 	case OCAST:
+		if(castucom(n))
+			warn(n, "32-bit unsigned complement zero-extended to 64 bits");
 		ccom(l);
 		if(l->op == OCONST) {
 			evconst(n);
--- /sys/src/cmd/cc/sub.c
+++ /sys/src/cmd/cc/sub.c
@@ -2030,3 +2030,21 @@ mixedasop(Type *l, Type *r)
 {
 	return !typefd[l->etype] && typefd[r->etype];
 }
+
+
+/*
+ * (uvlong)~ul creates a ul mask with top bits zero, which is usually wrong
+ * an explicit cast to ulong after ~ suppresses the diagnostic
+ */
+int
+castucom(Node *r)
+{
+	Node *rl;
+
+	if(r->op == OCAST &&
+	   (rl = r->left)->op == OCOM &&
+	   (r->type->etype == TVLONG || r->type->etype == TUVLONG) &&
+	   typeu[rl->type->etype] && typechl[rl->type->etype])
+		return 1;
+	return 0;
+}