cmd/compile: replace Order's use of NodeLists with slices

Order's "temp" and "free" fields use NodeLists in a rather
non-idiomatic way.  Instead of using the "list" or "concat" functions,
it manipulates them directly and without the normal invariants (e.g.,
it doesn't maintain the "End" field).

Rather than convert it to more typical usage, just replace with a
slice, which ends up much simpler anyway.

Passes toolstash/buildall.

Change-Id: Ibd0f24324bd674c0d5bb1bc40d073b01e7824ad5
Reviewed-on: https://go-review.googlesource.com/19776
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -42,8 +42,7 @@ import (
 // Order holds state during the ordering process.
 type Order struct {
 	out  *NodeList // list of generated statements
-	temp *NodeList // head of stack of temporary variables
-	free *NodeList // free list of NodeList* structs (for use in temp)
+	temp []*Node   // stack of temporary variables
 }
 
 // Order rewrites fn->nbody to apply the ordering constraints
@@ -68,14 +67,7 @@ func ordertemp(t *Type, order *Order, clear bool) *Node {
 		order.out = list(order.out, a)
 	}
 
-	l := order.free
-	if l == nil {
-		l = new(NodeList)
-	}
-	order.free = l.Next
-	l.Next = order.temp
-	l.N = var_
-	order.temp = l
+	order.temp = append(order.temp, var_)
 	return var_
 }
 
@@ -215,41 +207,34 @@ func orderaddrtemp(np **Node, order *Order) {
 	*np = ordercopyexpr(n, n.Type, order, 0)
 }
 
+type ordermarker int
+
 // Marktemp returns the top of the temporary variable stack.
-func marktemp(order *Order) *NodeList {
-	return order.temp
+func marktemp(order *Order) ordermarker {
+	return ordermarker(len(order.temp))
 }
 
 // Poptemp pops temporaries off the stack until reaching the mark,
 // which must have been returned by marktemp.
-func poptemp(mark *NodeList, order *Order) {
-	var l *NodeList
-
-	for {
-		l = order.temp
-		if l == mark {
-			break
-		}
-		order.temp = l.Next
-		l.Next = order.free
-		order.free = l
-	}
+func poptemp(mark ordermarker, order *Order) {
+	order.temp = order.temp[:mark]
 }
 
 // Cleantempnopop emits to *out VARKILL instructions for each temporary
 // above the mark on the temporary stack, but it does not pop them
 // from the stack.
-func cleantempnopop(mark *NodeList, order *Order, out **NodeList) {
+func cleantempnopop(mark ordermarker, order *Order, out **NodeList) {
 	var kill *Node
 
-	for l := order.temp; l != mark; l = l.Next {
-		if l.N.Name.Keepalive {
-			l.N.Name.Keepalive = false
-			kill = Nod(OVARLIVE, l.N, nil)
+	for i := len(order.temp) - 1; i >= int(mark); i-- {
+		n := order.temp[i]
+		if n.Name.Keepalive {
+			n.Name.Keepalive = false
+			kill = Nod(OVARLIVE, n, nil)
 			typecheck(&kill, Etop)
 			*out = list(*out, kill)
 		}
-		kill = Nod(OVARKILL, l.N, nil)
+		kill = Nod(OVARKILL, n, nil)
 		typecheck(&kill, Etop)
 		*out = list(*out, kill)
 	}
@@ -257,7 +242,7 @@ func cleantempnopop(mark *NodeList, order *Order, out **NodeList) {
 
 // Cleantemp emits VARKILL instructions for each temporary above the
 // mark on the temporary stack and removes them from the stack.
-func cleantemp(top *NodeList, order *Order) {
+func cleantemp(top ordermarker, order *Order) {
 	cleantempnopop(top, order, &order.out)
 	poptemp(top, order)
 }
@@ -289,13 +274,7 @@ func orderexprinplace(np **Node, outer *Order) {
 
 	// insert new temporaries from order
 	// at head of outer list.
-	lp := &order.temp
-
-	for *lp != nil {
-		lp = &(*lp).Next
-	}
-	*lp = outer.temp
-	outer.temp = order.temp
+	outer.temp = append(outer.temp, order.temp...)
 
 	*np = n
 }