cmd/gc: fix parallel assignment in range

for expr1, expr2 = range slice
was assigning to expr1 and expr2 in sequence
instead of in parallel.  Now it assigns in parallel,
as it should.  This matters for things like
for i, x[i] = range slice.

Fixes #3464.

R=ken2
CC=golang-dev
https://golang.org/cl/6252048

src/cmd/gc/range.c

@@ -152,9 +152,14 @@ walkrange(Node *n)
 		n->ntest = nod(OLT, hv1, hn);
 		n->nincr = nod(OASOP, hv1, nodintconst(1));
 		n->nincr->etype = OADD;
-		body = list1(nod(OAS, v1, hv1));
-		if(v2) {
-			body = list(body, nod(OAS, v2, nod(OIND, hp, N)));
+		if(v2 == N)
+			body = list1(nod(OAS, v1, hv1));
+		else {
+			a = nod(OAS2, N, N);
+			a->list = list(list1(v1), v2);
+			a->rlist = list(list1(hv1), nod(OIND, hp, N));
+			body = list1(a);
+
 			tmp = nod(OADD, hp, nodintconst(t->type->width));
 			tmp->type = hp->type;
 			tmp->typecheck = 1;

src/cmd/gc/subr.c

@@ -1950,6 +1950,12 @@ safeexpr(Node *n, NodeList **init)
 	if(n == N)
 		return N;
 
+	if(n->ninit) {
+		walkstmtlist(n->ninit);
+		*init = concat(*init, n->ninit);
+		n->ninit = nil;
+	}
+
 	switch(n->op) {
 	case ONAME:
 	case OLITERAL:

test/range.go

@@ -58,6 +58,17 @@ func testslice() {
 		println("wrong sum ranging over makeslice")
 		panic("fail")
 	}
+	
+	x := []int{10, 20}
+	y := []int{99}
+	i := 1
+	for i, x[i] = range y {
+		break
+	}
+	if i != 0 || x[0] != 10 || x[1] != 99 {
+		println("wrong parallel assignment", i, x[0], x[1])
+		panic("fail")
+	}
 }
 
 func testslice1() {