cmd/compile: inline list storage stealing

It is only necessary in a few places, and this inlining will
simplify the transition away from NodeLists.

Passes toolstash -cmp.

Change-Id: I4ee9b4bf56ffa04df23e20a0a83b302d36b33510
Reviewed-on: https://go-review.googlesource.com/20290Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -684,7 +684,11 @@ func (p *parser) labeled_stmt(label *Node) *Node {
 	label.Name.Defn = ls
 	l := list1(label)
 	if ls != nil {
-		l = list(l, ls)
+		if ls.Op == OBLOCK && nodeSeqLen(ls.Ninit) == 0 {
+			l = concat(l, ls.List)
+		} else {
+			l = list(l, ls)
+		}
 	}
 	return liststmt(l)
 }
@@ -1043,7 +1047,12 @@ func (p *parser) if_stmt() *Node {
 		if p.tok == LIF {
 			setNodeSeq(&stmt.Rlist, []*Node{p.if_stmt()})
 		} else {
-			setNodeSeq(&stmt.Rlist, []*Node{p.compound_stmt(true)})
+			cs := p.compound_stmt(true)
+			if cs.Op == OBLOCK && cs.Ninit == nil {
+				setNodeSeq(&stmt.Rlist, cs.List)
+			} else {
+				setNodeSeq(&stmt.Rlist, []*Node{cs})
+			}
 		}
 	}
 
@@ -2538,7 +2547,11 @@ func (p *parser) stmt_list() (l *NodeList) {
 		if s == missing_stmt {
 			break
 		}
-		l = list(l, s)
+		if s != nil && s.Op == OBLOCK && nodeSeqLen(s.Ninit) == 0 {
+			l = concat(l, s.List)
+		} else {
+			l = list(l, s)
+		}
 		// customized version of osemi:
 		// ';' is optional before a closing ')' or '}'
 		if p.tok == ')' || p.tok == '}' {

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

@@ -391,15 +391,6 @@ func list1(n *Node) *NodeList {
 	if n == nil {
 		return nil
 	}
-	if n.Op == OBLOCK && nodeSeqLen(n.Ninit) == 0 {
-		// Flatten list and steal storage.
-		// Poison pointer to catch errant uses.
-		l := n.List
-
-		setNodeSeq(&n.List, nil)
-		return l
-	}
-
 	l := new(NodeList)
 	l.N = n
 	l.End = l
@@ -741,15 +732,6 @@ func setNodeSeq(a nodesOrNodeListPtr, b nodesOrNodeList) {
 // This is an interim function during the transition from NodeList to Nodes.
 // TODO(iant): Remove when transition is complete.
 func setNodeSeqNode(a nodesOrNodeListPtr, n *Node) {
-	// This is what the old list1 function did;
-	// the rest of the compiler has come to expect it.
-	if n.Op == OBLOCK && nodeSeqLen(n.Ninit) == 0 {
-		l := n.List
-		setNodeSeq(&n.List, nil)
-		setNodeSeq(a, l)
-		return
-	}
-
 	switch a := a.(type) {
 	case **NodeList:
 		*a = list1(n)
@@ -822,15 +804,6 @@ func appendNodeSeq(a nodesOrNodeListPtr, b nodesOrNodeList) {
 // This is an interim function during the transition from NodeList to Nodes.
 // TODO(iant): Remove when transition is complete.
 func appendNodeSeqNode(a nodesOrNodeListPtr, n *Node) {
-	// This is what the old list1 function did;
-	// the rest of the compiler has come to expect it.
-	if n.Op == OBLOCK && nodeSeqLen(n.Ninit) == 0 {
-		l := n.List
-		setNodeSeq(&n.List, nil)
-		appendNodeSeq(a, l)
-		return
-	}
-
 	switch a := a.(type) {
 	case **NodeList:
 		*a = list(*a, n)