cmd/compile: Ensure left-to-right assignment

Add temporaries to reorder the assignment for OAS2XXX nodes.
This makes orderstmt(), rewrite
  a, b, c = ...
as
  tmp1, tmp2, tmp3 = ...
  a, b, c = tmp1, tmp2, tmp3
and
  a, ok = ...
as
  t1, t2 = ...
  a  = t1
  ok = t2

Fixes #13433.

Change-Id: Id0f5956e3a254d0a6f4b89b5f7b0e055b1f0e21f
Reviewed-on: https://go-review.googlesource.com/34713
Run-TryBot: Dhananjay Nakrani <dhananjayn@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/order.go

@@ -508,7 +508,7 @@ func orderstmt(n *Node, order *Order) {
 		orderexprlist(n.List, order)
 		orderexprlist(n.Rlist, order)
 		switch n.Op {
-		case OAS2, OAS2DOTTYPE:
+		case OAS2:
 			ordermapassign(n, order)
 		default:
 			order.out = append(order.out, n)
@@ -554,7 +554,7 @@ func orderstmt(n *Node, order *Order) {
 			r.Right.Op = OARRAYBYTESTRTMP
 		}
 		ordermapkeytemp(r, order)
-		ordermapassign(n, order)
+		orderokas2(n, order)
 		cleantemp(t, order)
 
 	// Special: avoid copy of func call n->rlist->n.
@@ -563,7 +563,7 @@ func orderstmt(n *Node, order *Order) {
 
 		orderexprlist(n.List, order)
 		ordercall(n.Rlist.First(), order)
-		ordermapassign(n, order)
+		orderas2(n, order)
 		cleantemp(t, order)
 
 	// Special: use temporary variables to hold result,
@@ -574,31 +574,7 @@ func orderstmt(n *Node, order *Order) {
 
 		orderexprlist(n.List, order)
 		n.Rlist.First().Left = orderexpr(n.Rlist.First().Left, order, nil) // i in i.(T)
-
-		var tmp1, tmp2 *Node
-		if !isblank(n.List.First()) {
-			typ := n.Rlist.First().Type
-			tmp1 = ordertemp(typ, order, haspointers(typ))
-		}
-		if !isblank(n.List.Second()) && !n.List.Second().Type.IsBoolean() {
-			tmp2 = ordertemp(Types[TBOOL], order, false)
-		}
-
-		order.out = append(order.out, n)
-
-		if tmp1 != nil {
-			r := nod(OAS, n.List.First(), tmp1)
-			r = typecheck(r, Etop)
-			ordermapassign(r, order)
-			n.List.SetIndex(0, tmp1)
-		}
-		if tmp2 != nil {
-			r := okas(n.List.Second(), tmp2)
-			r = typecheck(r, Etop)
-			ordermapassign(r, order)
-			n.List.SetIndex(1, tmp2)
-		}
-
+		orderokas2(n, order)
 		cleantemp(t, order)
 
 	// Special: use temporary variables to hold result,
@@ -1229,3 +1205,68 @@ func okas(ok, val *Node) *Node {
 	}
 	return nod(OAS, ok, val)
 }
+
+// orderas2 orders OAS2XXXX nodes. It creates temporaries to ensure left-to-right assignment.
+// The caller should order the right-hand side of the assignment before calling orderas2.
+// It rewrites,
+// 	a, b, a = ...
+// as
+//	tmp1, tmp2, tmp3 = ...
+// 	a, b, a = tmp1, tmp2, tmp3
+// This is necessary to ensure left to right assignment order.
+func orderas2(n *Node, order *Order) {
+	tmplist := []*Node{}
+	left := []*Node{}
+	for _, l := range n.List.Slice() {
+		if !isblank(l) {
+			tmp := ordertemp(l.Type, order, haspointers(l.Type))
+			tmplist = append(tmplist, tmp)
+			left = append(left, l)
+		}
+	}
+
+	order.out = append(order.out, n)
+
+	as := nod(OAS2, nil, nil)
+	as.List.Set(left)
+	as.Rlist.Set(tmplist)
+	as = typecheck(as, Etop)
+	orderstmt(as, order)
+
+	ti := 0
+	for ni, l := range n.List.Slice() {
+		if !isblank(l) {
+			n.List.SetIndex(ni, tmplist[ti])
+			ti++
+		}
+	}
+}
+
+// orderokas2 orders OAS2 with ok.
+// Just like orderas2(), this also adds temporaries to ensure left-to-right assignment.
+func orderokas2(n *Node, order *Order) {
+	var tmp1, tmp2 *Node
+	if !isblank(n.List.First()) {
+		typ := n.Rlist.First().Type
+		tmp1 = ordertemp(typ, order, haspointers(typ))
+	}
+
+	if !isblank(n.List.Second()) {
+		tmp2 = ordertemp(Types[TBOOL], order, false)
+	}
+
+	order.out = append(order.out, n)
+
+	if tmp1 != nil {
+		r := nod(OAS, n.List.First(), tmp1)
+		r = typecheck(r, Etop)
+		ordermapassign(r, order)
+		n.List.SetIndex(0, tmp1)
+	}
+	if tmp2 != nil {
+		r := okas(n.List.Second(), tmp2)
+		r = typecheck(r, Etop)
+		ordermapassign(r, order)
+		n.List.SetIndex(1, tmp2)
+	}
+}

test/reorder.go

@@ -19,6 +19,7 @@ func main() {
 	p6()
 	p7()
 	p8()
+	p9()
 }
 
 var gx []int
@@ -112,3 +113,39 @@ func p8() {
 		panic(m[0])
 	}
 }
+
+// Issue #13433: Left-to-right assignment of OAS2XXX nodes.
+func p9() {
+	var x bool
+
+	// OAS2FUNC
+	x, x = fn()
+	checkOAS2XXX(x, "x, x = fn()")
+
+	// OAS2RECV
+	var c = make(chan bool, 10)
+	c <- false
+	x, x = <-c
+	checkOAS2XXX(x, "x, x <-c")
+
+	// OAS2MAPR
+	var m = map[int]bool{0: false}
+	x, x = m[0]
+	checkOAS2XXX(x, "x, x = m[0]")
+
+	// OAS2DOTTYPE
+	var i interface{} = false
+	x, x = i.(bool)
+	checkOAS2XXX(x, "x, x = i.(bool)")
+}
+
+//go:noinline
+func fn() (bool, bool) { return false, true }
+
+// checks the order of OAS2XXX.
+func checkOAS2XXX(x bool, s string) {
+	if !x {
+		fmt.Printf("%s; got=(false); want=(true)\n", s)
+		panic("failed")
+	}
+}