Bug 1143508 (part 2) - Optimize pair returning from makeNodeElement() in JSDOMParser. r=bnicholson.

In one workload this avoids allocating 2.3 MB of short-lived arrays.

toolkit/components/reader/JSDOMParser.js

@@ -632,6 +632,11 @@
     // over and over for this purpose uses less memory than using a new array
     // for each string.
     this.strBuf = [];
+
+    // Similarly, we reuse this array to return the two arguments from
+    // makeElementNode(), which saves us from having to allocate a new array
+    // every time.
+    this.retPair = [];
   };
 
   JSDOMParser.prototype = {
@@ -712,7 +717,7 @@
      *          the second index is a boolean indicating whether this is a void
      *          Element
      */
-    makeElementNode: function () {
+    makeElementNode: function (retPair) {
       let c = this.nextChar();
 
       // Read the Element tag name
@@ -720,21 +725,21 @@
       strBuf.length = 0;
       while (c !== " " && c !== ">" && c !== "/") {
         if (c === undefined)
-          return null;
+          return false;
         strBuf.push(c);
         c = this.nextChar();
       }
       let tag = strBuf.join('');
 
       if (!tag)
-        return null;
+        return false;
 
       let node = new Element(tag);
 
       // Read Element attributes
       while (c !== "/" && c !== ">") {
         if (c === undefined)
-          return null;
+          return false;
         while (this.match(" "));
         c = this.nextChar();
         if (c !== "/" && c !== ">") {
@@ -750,11 +755,13 @@
         c = this.nextChar();
         if (c !== ">") {
           error("expected '>'");
-          return null;
+          return false;
         }
       }
 
-      return [node, closed];
+      retPair[0] = node;
+      retPair[1] = closed;
+      return true
     },
 
     /**
@@ -855,11 +862,12 @@
       }
 
       // Otherwise, we're looking at an Element node
-      let result = this.makeElementNode();
-      if (result === null)
+      let result = this.makeElementNode(this.retPair);
+      if (!result)
         return null;
 
-      let [node, closed] = result;
+      let node = this.retPair[0];
+      let closed = this.retPair[1];
       let localName = node.localName;
 
       // If this isn't a void Element, read its child nodes