produce a warning if a mask produced by complementing a 32-bit value is subsequently
zero-extended to 64-bits, which is unlikely to do what was intended.

Reference: /n/sources/patch/applied/6c-checkmask
Date: Thu Feb 28 18:53:26 CET 2013
Signed-off-by: charles.forsyth@gmail.com

--- /sys/src/cmd/6c/cgen.c	Thu Feb 28 18:52:20 2013
+++ /sys/src/cmd/6c/cgen.c	Thu Feb 28 18:52:13 2013
@@ -2,6 +2,7 @@
 
 /* ,x/^(print|prtree)\(/i/\/\/ */
 int castup(Type*, Type*);
+void checkmask(Node*, Node*);
 
 void
 cgen(Node *n, Node *nn)
@@ -257,6 +258,8 @@
 				break;
 			}
 		}
+		if(n->op == OAND)
+			checkmask(n, r);
 		if(r->addable >= INDEXED && !hardconst(r)) {
 			regalloc(&nod, l, nn);
 			cgen(l, &nod);
@@ -521,6 +524,8 @@
 			goto asbitop;
 		if(typefd[l->type->etype] || typefd[r->type->etype])
 			goto asfop;
+		if(o == OASAND)
+			checkmask(n, r);
 		if(l->complex >= r->complex) {
 			if(hardleft)
 				reglcgen(&nod, l, Z);
@@ -1861,6 +1866,22 @@
 		return ft == TULONG || ft == TUINT || ft == TUSHORT;
 	}
 	return 0;
+}
+
+/*
+ * vl &= ~ul or vl & ~ul
+ * create a ul mask with top bits zero, which is usually wrong
+ */
+void
+checkmask(Node *n, Node *r)
+{
+	Node *rl;
+
+	if((n->op == OAND || n->op == OASAND) &&
+	   r->op == OCAST &&
+	   (rl = r->left)->op == OCOM &&
+	   typesuv[n->type->etype] && typeu[rl->type->etype] && typechl[rl->type->etype])
+		warn(n, "32-bit mask zero-extended to 64 bits");
 }
 
 void