cmd/gc: fix eval order in select

Ordinary variable load was assumed to be not worth saving,
but not if one of the function calls later might change
its value.

Fixes #4313.

R=ken2
CC=golang-dev
https://golang.org/cl/6997047

src/cmd/gc/order.c

@@ -276,11 +276,11 @@ orderstmt(Node *n, NodeList **out)
 			case OSELRECV2:
 				orderexprinplace(&r->left);
 				orderexprinplace(&r->ntest);
-				orderexpr(&r->right->left, out);
+				orderexpr(&r->right->left, &l->n->ninit);
 				break;
 			case OSEND:
-				orderexpr(&r->left, out);
+				orderexpr(&r->left, &l->n->ninit);
-				orderexpr(&r->right, out);
+				orderexpr(&r->right, &l->n->ninit);
 				break;
 			}
 		}

src/cmd/gc/select.c

@@ -297,15 +297,15 @@ walkselect(Node *sel)
 		setlineno(cas);
 		n = cas->left;
 		r = nod(OIF, N, N);
-		r->nbody = cas->ninit;
+		r->ninit = cas->ninit;
 		cas->ninit = nil;
 		if(n != nil) {
-			r->nbody = concat(r->nbody, n->ninit);
+			r->ninit = concat(r->ninit, n->ninit);
 			n->ninit = nil;
 		}
 		if(n == nil) {
 			// selectdefault(sel *byte);
-			r->ntest = mkcall("selectdefault", types[TBOOL], &init, var);
+			r->ntest = mkcall("selectdefault", types[TBOOL], &r->ninit, var);
 		} else {
 			switch(n->op) {
 			default:
@@ -313,25 +313,25 @@ walkselect(Node *sel)
 	
 			case OSEND:
 				// selectsend(sel *byte, hchan *chan any, elem *any) (selected bool);
-				n->left = safeexpr(n->left, &r->ninit);
+				n->left = localexpr(safeexpr(n->left, &r->ninit), n->left->type, &r->ninit);
 				n->right = localexpr(n->right, n->left->type->type, &r->ninit);
 				n->right = nod(OADDR, n->right, N);
 				n->right->etype = 1;  // pointer does not escape
 				typecheck(&n->right, Erv);
 				r->ntest = mkcall1(chanfn("selectsend", 2, n->left->type), types[TBOOL],
-					&init, var, n->left, n->right);
+					&r->ninit, var, n->left, n->right);
 				break;
 
 			case OSELRECV:
 				// selectrecv(sel *byte, hchan *chan any, elem *any) (selected bool);
 				r->ntest = mkcall1(chanfn("selectrecv", 2, n->right->left->type), types[TBOOL],
-					&init, var, n->right->left, n->left);
+					&r->ninit, var, n->right->left, n->left);
 				break;
 
 			case OSELRECV2:
 				// selectrecv2(sel *byte, hchan *chan any, elem *any, received *bool) (selected bool);
 				r->ntest = mkcall1(chanfn("selectrecv2", 2, n->right->left->type), types[TBOOL],
-					&init, var, n->right->left, n->left, n->ntest);
+					&r->ninit, var, n->right->left, n->left, n->ntest);
 				break;
 			}
 		}

src/cmd/gc/subr.c

@@ -2040,11 +2040,13 @@ cheapexpr(Node *n, NodeList **init)
 
 /*
  * return n in a local variable of type t if it is not already.
+ * the value is guaranteed not to change except by direct
+ * assignment to it.
  */
 Node*
 localexpr(Node *n, Type *t, NodeList **init)
 {
-	if(n->op == ONAME &&
+	if(n->op == ONAME && !n->addrtaken &&
 		(n->class == PAUTO || n->class == PPARAM || n->class == PPARAMOUT) &&
 		convertop(n->type, t, nil) == OCONVNOP)
 		return n;

test/fixedbugs/issue4313.go

@@ -0,0 +1,28 @@
+// run
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Order of operations in select.
+
+package main
+
+func main() {
+	c := make(chan int, 1)
+	x := 0
+	select {
+	case c <- x: // should see x = 0, not x = 42 (after makec)
+	case <-makec(&x): // should be evaluated only after c and x on previous line
+	}
+	y := <-c
+	if y != 0 {
+		panic(y)
+	}
+}
+
+func makec(px *int) chan bool {
+	if false { for {} }
+	*px = 42
+	return make(chan bool, 0)
+}