cmd/compile: preserve loop depth when evaluating block

Add block method to preserve loop depth when evaluating statements in a
block, so escape analysis can handle looping label more precisely.

Updates #22438

Change-Id: I39b306544a6c0ee3fcbebbe0d0ee735cb71773e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/193517
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -201,7 +201,7 @@ func (e *Escape) walkFunc(fn *Node) {
 
 	e.curfn = fn
 	e.loopDepth = 1
-	e.stmts(fn.Nbody)
+	e.block(fn.Nbody)
 }
 
 // Below we implement the methods for walking the AST and recording
@@ -284,14 +284,14 @@ func (e *Escape) stmt(n *Node) {
 
 	case OIF:
 		e.discard(n.Left)
-		e.stmts(n.Nbody)
-		e.stmts(n.Rlist)
+		e.block(n.Nbody)
+		e.block(n.Rlist)
 
 	case OFOR, OFORUNTIL:
 		e.loopDepth++
 		e.discard(n.Left)
 		e.stmt(n.Right)
-		e.stmts(n.Nbody)
+		e.block(n.Nbody)
 		e.loopDepth--
 
 	case ORANGE:
@@ -311,7 +311,7 @@ func (e *Escape) stmt(n *Node) {
 			}
 		}
 
-		e.stmts(n.Nbody)
+		e.block(n.Nbody)
 		e.loopDepth--
 
 	case OSWITCH:
@@ -340,13 +340,13 @@ func (e *Escape) stmt(n *Node) {
 			}
 
 			e.discards(cas.List)
-			e.stmts(cas.Nbody)
+			e.block(cas.Nbody)
 		}
 
 	case OSELECT:
 		for _, cas := range n.List.Slice() {
 			e.stmt(cas.Left)
-			e.stmts(cas.Nbody)
+			e.block(cas.Nbody)
 		}
 	case OSELRECV:
 		e.assign(n.Left, n.Right, "selrecv", n)
@@ -398,12 +398,18 @@ func (e *Escape) stmt(n *Node) {
 }
 
 func (e *Escape) stmts(l Nodes) {
-	// TODO(mdempsky): Preserve and restore e.loopDepth? See also #22438.
 	for _, n := range l.Slice() {
 		e.stmt(n)
 	}
 }
 
+// block is like stmts, but preserves loopDepth.
+func (e *Escape) block(l Nodes) {
+	old := e.loopDepth
+	e.stmts(l)
+	e.loopDepth = old
+}
+
 // expr models evaluating an expression n and flowing the result into
 // hole k.
 func (e *Escape) expr(k EscHole, n *Node) {

test/escape_goto.go

+// errorcheck -0 -m -l
+
+// Copyright 2019 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.
+
+// Test escape analysis for goto statements.
+
+package escape
+
+var x bool
+
+func _() {
+	var p *int
+loop:
+	if x {
+		goto loop
+	}
+	// BAD: We should be able to recognize that there
+	// aren't any more "goto loop" after here.
+	p = new(int) // ERROR "escapes to heap"
+	_ = p
+}
+
+func _() {
+	var p *int
+	if x {
+	loop:
+		goto loop
+	} else {
+		p = new(int) // ERROR "does not escape"
+	}
+	_ = p
+}
+
+func _() {
+	var p *int
+	if x {
+	loop:
+		goto loop
+	}
+	p = new(int) // ERROR "does not escape"
+	_ = p
+}