cmd/compile: fix code duplication in race-instrumentation

instrumentnode() accidentally copies parent's already-instrumented nodes
into child's Ninit block. This generates repeated code in race-instrumentation.
This case surfaces only when it duplicates inline-labels, because of
compile time error. In other cases, it silently generates incorrect
instrumented code. This change prevents it from doing so.

Fixes #17449.

Change-Id: Icddf2198990442166307e176b7e20aa0cf6c171c
Reviewed-on: https://go-review.googlesource.com/31317
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -145,36 +145,21 @@ func instrumentnode(np **Node, init *Nodes, wr int, skip int) {
 		goto ret
 
 	case OBLOCK:
-		var out []*Node
 		ls := n.List.Slice()
-		for i := 0; i < len(ls); i++ {
-			switch ls[i].Op {
-			case OCALLFUNC, OCALLMETH, OCALLINTER:
-				instrumentnode(&ls[i], &ls[i].Ninit, 0, 0)
-				out = append(out, ls[i])
-				// Scan past OAS nodes copying results off stack.
-				// Those must not be instrumented, because the
-				// instrumentation calls will smash the results.
-				// The assignments are to temporaries, so they cannot
-				// be involved in races and need not be instrumented.
-				for i+1 < len(ls) && ls[i+1].Op == OAS && iscallret(ls[i+1].Right) {
-					i++
-					out = append(out, ls[i])
-				}
-			default:
-				var outn Nodes
-				outn.Set(out)
-				instrumentnode(&ls[i], &outn, 0, 0)
-				if ls[i].Op != OAS && ls[i].Op != OASWB && ls[i].Op != OAS2FUNC || ls[i].Ninit.Len() == 0 {
-					out = append(outn.Slice(), ls[i])
-				} else {
-					// Splice outn onto end of ls[i].Ninit
-					ls[i].Ninit.AppendNodes(&outn)
-					out = append(out, ls[i])
-				}
+		afterCall := false
+		for i := range ls {
+			op := ls[i].Op
+			// Scan past OAS nodes copying results off stack.
+			// Those must not be instrumented, because the
+			// instrumentation calls will smash the results.
+			// The assignments are to temporaries, so they cannot
+			// be involved in races and need not be instrumented.
+			if afterCall && op == OAS && iscallret(ls[i].Right) {
+				continue
 			}
+			instrumentnode(&ls[i], &ls[i].Ninit, 0, 0)
+			afterCall = (op == OCALLFUNC || op == OCALLMETH || op == OCALLINTER)
 		}
-		n.List.Set(out)
 		goto ret
 
 	case ODEFER:

test/fixedbugs/issue17449.go

@@ -0,0 +1,34 @@
+// errorcheck -0 -race
+
+// Copyright 2016 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 17449: race instrumentation copies over previous instrumented nodes from parents block into child's Ninit block.
+// This code surfaces the duplication at compile time because of generated inline labels.
+
+package master
+
+type PriorityList struct {
+    elems []interface{}
+}
+
+func (x *PriorityList) Len() int { return len(x.elems) }
+
+func (l *PriorityList) remove(i int) interface{} {
+    elem := l.elems[i]
+    l.elems = append(l.elems[:i], l.elems[i+1:]...)
+    return elem
+}
+
+func (l *PriorityList) Next() interface{} {
+    return l.remove(l.Len() - 1)
+}
+
+var l *PriorityList
+
+func Foo() {
+    // It would fail here if instrumented code (including inline-label) was copied.
+    for elem := l.Next(); elem != nil; elem = l.Next() {
+    }
+}