cmd/compile: hide NodeList details in evconst

The new code is a bit less efficient,
but it does not involve altering the structure
of any linked lists.
This will make it easier to replace NodeLists
with Node slices.
We can return to a more efficient algorithm
when NodeLists have been replaced.

Passes toolstash -cmp.

Change-Id: I0bb5ee75e7c0646e6d37fe558c8f0548729d8aa1
Reviewed-on: https://go-review.googlesource.com/20277
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -543,33 +543,34 @@ func evconst(n *Node) {
 
 		// merge adjacent constants in the argument list.
 	case OADDSTR:
-		var nr *Node
-		var nl *Node
-		var l2 *NodeList
-		for l1 := n.List; l1 != nil; l1 = l1.Next {
-			if Isconst(l1.N, CTSTR) && l1.Next != nil && Isconst(l1.Next.N, CTSTR) {
-				// merge from l1 up to but not including l2
-				var strs []string
-				l2 = l1
-				for l2 != nil && Isconst(l2.N, CTSTR) {
-					nr = l2.N
-					strs = append(strs, nr.Val().U.(string))
-					l2 = l2.Next
-				}
-
-				nl = Nod(OXXX, nil, nil)
-				*nl = *l1.N
-				nl.Orig = nl
-				nl.SetVal(Val{strings.Join(strs, "")})
-				l1.N = nl
-				l1.Next = l2
+		// TODO: We make a copy of n.List in order to abstract
+		// away the details of deleting elements.
+		// Once n.List is some kind of Node slice,
+		// re-implement using deletion.
+		var l *NodeList // replacement list
+		for l1 := n.List; l1 != nil; {
+			if !Isconst(l1.N, CTSTR) || l1.Next == nil || !Isconst(l1.Next.N, CTSTR) {
+				// non-constant string or solitary constant string
+				l = list(l, l1.N)
+				l1 = l1.Next
+				continue
+			}
+
+			first := l1.N
+
+			// merge run of constants
+			var strs []string
+			for ; l1 != nil && Isconst(l1.N, CTSTR); l1 = l1.Next {
+				strs = append(strs, l1.N.Val().U.(string))
 			}
-		}
 
-		// fix list end pointer.
-		for l2 := n.List; l2 != nil; l2 = l2.Next {
-			n.List.End = l2
+			nl := Nod(OXXX, nil, nil)
+			*nl = *first
+			nl.Orig = nl
+			nl.SetVal(Val{strings.Join(strs, "")})
+			l = list(l, nl)
 		}
+		n.List = l
 
 		// collapse single-constant list to single constant.
 		if count(n.List) == 1 && Isconst(n.List.N, CTSTR) {