cmd/compile: fix loop depth of range expression in escape analysis

ORANGE node's Right node is the expression it is ranging over,
which is evaluated before the loop. In the escape analysis,
we should walk this node without loop depth incremented.

Fixes #21709.

Change-Id: Idc1e4c76e39afb5a344d85f6b497930a488ce5cf
Reviewed-on: https://go-review.googlesource.com/80740
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -680,7 +680,18 @@ func (e *EscState) esc(n *Node, parent *Node) {
 	}
 
 	e.esc(n.Left, n)
+
+	if n.Op == ORANGE {
+		// ORANGE node's Right is evaluated before the loop
+		e.loopdepth--
+	}
+
 	e.esc(n.Right, n)
+
+	if n.Op == ORANGE {
+		e.loopdepth++
+	}
+
 	e.esclist(n.Nbody, n)
 	e.esclist(n.List, n)
 	e.esclist(n.Rlist, n)

test/fixedbugs/issue21709.go

+// errorcheck -0 -l -m
+
+// Copyright 2017 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.
+
+// Issue 21709: range expression overly escapes.
+
+package p
+
+type S struct{}
+
+func (s *S) Inc() {} // ERROR "\(\*S\).Inc s does not escape"
+var N int
+
+func F1() {
+	var s S // ERROR "moved to heap: s"
+	for i := 0; i < N; i++ {
+		fs := []func(){ // ERROR "F1 \[\]func\(\) literal does not escape"
+			s.Inc, // ERROR "F1 s.Inc does not escape" "s escapes to heap"
+		}
+		for _, f := range fs {
+			f()
+		}
+	}
+}
+
+func F2() {
+	var s S // ERROR "moved to heap: s"
+	for i := 0; i < N; i++ {
+		for _, f := range []func(){ // ERROR "F2 \[\]func\(\) literal does not escape"
+			s.Inc, // ERROR "F2 s.Inc does not escape" "s escapes to heap"
+		} {
+			f()
+		}
+	}
+}