bug196

R=ken
OCL=35905
CL=35905

src/cmd/gc/go.h

@@ -847,6 +847,7 @@ Node*	staticname(Type*);
 int	iscomposite(Type*);
 Node*	callnew(Type*);
 Node*	saferef(Node*, NodeList**);
+Node*	safeval(Node*, NodeList**);
 int	is64(Type*);
 int	noconv(Type*, Type*);
 NodeList*	list1(Node*);

src/cmd/gc/subr.c

@@ -2346,7 +2346,7 @@ staticname(Type *t)
 }
 
 /*
- * return side effect-free n, appending side effects to init.
+ * return side effect-free, assignable n, appending side effects to init.
  */
 Node*
 saferef(Node *n, NodeList **init)
@@ -2387,6 +2387,33 @@ saferef(Node *n, NodeList **init)
 	return N;
 }
 
+/*
+ * return side effect-free n, appending side effects to init.
+ */
+Node*
+safeval(Node *n, NodeList **init)
+{
+	Node *l;
+	Node *r;
+	Node *a;
+
+	// is this a local variable or a dot of a local variable?
+	for(l=n; l->op == ODOT; l=l->left)
+		if(l->left->type != T && isptr[l->left->type->etype])
+			goto copy;
+	if(l->op == ONAME && (l->class == PAUTO || l->class == PPARAM))
+		return n;
+
+copy:
+	l = nod(OXXX, N, N);
+	tempname(l, n->type);
+	a = nod(OAS, l, n);
+	typecheck(&a, Etop);
+	walkexpr(&a, init);
+	*init = list(*init, a);
+	return l;
+}
+
 void
 setmaxarg(Type *t)
 {

src/cmd/gc/walk.c

@@ -1592,7 +1592,6 @@ Node*
 mapop(Node *n, NodeList **init)
 {
 	Node *r, *a;
-	Type *t;
 
 	r = n;
 	switch(n->op) {
@@ -1601,16 +1600,10 @@ mapop(Node *n, NodeList **init)
 	case OASOP:
 		// rewrite map[index] op= right
 		// into tmpi := index; map[tmpi] = map[tmpi] op right
-		// TODO(rsc): does this double-evaluate map?
 
-		t = n->left->left->type;
-		a = nod(OXXX, N, N);
-		tempname(a, t->down);			// tmpi
-		r = nod(OAS, a, n->left->right);	// tmpi := index
-		n->left->right = a;			// m[tmpi]
-		typecheck(&r, Etop);
-		walkexpr(&r, init);
-		*init = list(*init, r);
+		// make it ok to double-evaluate map[tmpi]
+		n->left->left = safeval(n->left->left, init);
+		n->left->right = safeval(n->left->right, init);
 
 		a = nod(OXXX, N, N);
 		*a = *n->left;		// copy of map[tmpi]

test/golden.out

@@ -146,8 +146,3 @@ BUG: should fail
 
 =========== bugs/bug193.go
 BUG: errchk: bugs/bug193.go:14: missing expected error: 'shift'
-
-=========== bugs/bug196.go
-too many calls: 5
-panic PC=xxx
-BUG: bug196