cmd/compile: ensure we evaluate side effects of len() arg

For any len() which requires the evaluation of its arg (according to the spec).

Update golang#72844

Change-Id: Id2b0bcc78073a6d5051abd000131dafdf65e7f26
Reviewed-on: https://go-review.googlesource.com/c/go/+/658097
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

Author: randall77

src/cmd/compile/internal/ssagen/ssa.go

@@ -3469,19 +3469,28 @@ func (s *state) exprCheckPtr(n ir.Node, checkPtrOK bool) *ssa.Value {
 
 	case ir.OLEN, ir.OCAP:
 		n := n.(*ir.UnaryExpr)
+		// Note: all constant cases are handled by the frontend. If len or cap
+		// makes it here, we want the side effects of the argument. See issue 72844.
+		a := s.expr(n.X)
+		t := n.X.Type()
 		switch {
-		case n.X.Type().IsSlice():
+		case t.IsSlice():
 			op := ssa.OpSliceLen
 			if n.Op() == ir.OCAP {
 				op = ssa.OpSliceCap
 			}
-			return s.newValue1(op, types.Types[types.TINT], s.expr(n.X))
-		case n.X.Type().IsString(): // string; not reachable for OCAP
-			return s.newValue1(ssa.OpStringLen, types.Types[types.TINT], s.expr(n.X))
-		case n.X.Type().IsMap(), n.X.Type().IsChan():
-			return s.referenceTypeBuiltin(n, s.expr(n.X))
-		default: // array
-			return s.constInt(types.Types[types.TINT], n.X.Type().NumElem())
+			return s.newValue1(op, types.Types[types.TINT], a)
+		case t.IsString(): // string; not reachable for OCAP
+			return s.newValue1(ssa.OpStringLen, types.Types[types.TINT], a)
+		case t.IsMap(), t.IsChan():
+			return s.referenceTypeBuiltin(n, a)
+		case t.IsArray():
+			return s.constInt(types.Types[types.TINT], t.NumElem())
+		case t.IsPtr() && t.Elem().IsArray():
+			return s.constInt(types.Types[types.TINT], t.Elem().NumElem())
+		default:
+			s.Fatalf("bad type in len/cap: %v", t)
+			return nil
 		}
 
 	case ir.OSPTR:

src/cmd/compile/internal/walk/builtin.go

@@ -278,12 +278,13 @@ func walkLenCap(n *ir.UnaryExpr, init *ir.Nodes) ir.Node {
 	// replace len(*[10]int) with 10.
 	// delayed until now to preserve side effects.
 	t := n.X.Type()
-
 	if t.IsPtr() {
 		t = t.Elem()
 	}
 	if t.IsArray() {
-		safeExpr(n.X, init)
+		// evaluate any side effects in n.X. See issue 72844.
+		appendWalkStmt(init, ir.NewAssignStmt(base.Pos, ir.BlankNode, n.X))
+
 		con := ir.NewConstExpr(constant.MakeInt64(t.NumElem()), n)
 		con.SetTypecheck(1)
 		return con

test/fixedbugs/issue72844.go

@@ -47,11 +47,11 @@ func testRange4() {
 }
 
 func main() {
-	//shouldPanic(testLen1)
+	shouldPanic(testLen1)
 	shouldNotPanic(testLen2)
 	shouldNotPanic(testLen3)
 	shouldNotPanic(testLen4)
-	//shouldPanic(testRange1)
+	shouldPanic(testRange1)
 	shouldNotPanic(testRange2)
 	shouldNotPanic(testRange3)
 	shouldNotPanic(testRange4)