Do constant folding during parsing since it is cheap to do that at this stage then in optimizer during separated phase so even the interpreter will take advantage of the folding.

js/rhino/src/org/mozilla/javascript/IRFactory.java

@@ -20,6 +20,7 @@
  *
  * Contributor(s):
  * Norris Boyd
+ * Igor Bukanov
  *
  * Alternatively, the contents of this file may be used under the
  * terms of the GNU Public License (the "GPL"), in which case the
@@ -771,7 +772,93 @@ public class IRFactory {
             // OPT: could optimize to GETPROP iff string can't be a number
             nodeType = Token.GETELEM;
             break;
+
+          case Token.ADD:
+            // numerical addition and string concatenation
+            if (left.type == Token.STRING) {
+                String s2;
+                if (right.type == Token.STRING) {
+                    s2 = right.getString();
+                } else if (right.type == Token.NUMBER) {
+                    s2 = ScriptRuntime.numberToString(right.getDouble(), 10);
+                } else {
+                    break;
+                }
+                String s1 = left.getString();
+                return  Node.newString(s1.concat(s2));
+            } else if (left.type == Token.NUMBER) {
+                if (right.type == Token.NUMBER) {
+                    double x = left.getDouble() + right.getDouble();
+                    return Node.newNumber(x);
+                } else if (right.type == Token.STRING) {
+                    String s1, s2;
+                    s1 = ScriptRuntime.numberToString(left.getDouble(), 10);
+                    s2 = right.getString();
+                    return  Node.newString(s1.concat(s2));
+                }
+            }
+            // can't do anything if we don't know  both types - since
+            // 0 + object is supposed to call toString on the object and do
+            // string concantenation rather than addition
+            break;
+
+          case Token.SUB:
+            // numerical subtraction
+            if (left.type == Token.NUMBER) {
+                double ld = left.getDouble();
+                if (right.type == Token.NUMBER) {
+                    //both numbers
+                    return Node.newNumber(ld - right.getDouble());
+                } else if (ld == 0.0) {
+                    // first 0: 0-x -> -x
+                    return new Node(Token.NEG, right);
+                }
+            } else if (right.type == Token.NUMBER) {
+                if (right.getDouble() == 0.0) {
+                    //second 0: x - 0 -> +x
+                    // can not make simply x because x - 0 must be number
+                    return new Node(Token.POS, left);
+                }
+            }
+            break;
+
+          case Token.MUL:
+            // numerical multiplication
+            if (left.type == Token.NUMBER) {
+                double ld = left.getDouble();
+                if (right.type == Token.NUMBER) {
+                    //both numbers
+                    return Node.newNumber(ld * right.getDouble());
+                } else if (ld == 1.0) {
+                    // first 1: 1 *  x -> +x
+                    return new Node(Token.POS, right);
+                }
+            } else if (right.type == Token.NUMBER) {
+                if (right.getDouble() == 1.0) {
+                    //second 1: x * 1 -> +x
+                    // can not make simply x because x - 0 must be number
+                    return new Node(Token.POS, left);
+                }
+            }
+            // can't do x*0: Infinity * 0 gives NaN, not 0
+            break;
+
+          case Token.DIV:
+            // number division
+            if (right.type == Token.NUMBER) {
+                double rd = right.getDouble();
+                if (left.type == Token.NUMBER) {
+                    // both constants -- just divide, trust Java to handle x/0
+                    return Node.newNumber(left.getDouble() / rd);
+                } else if (rd == 1.0) {
+                    // second 1: x/1 -> +x
+                    // not simply x to force number convertion
+                    return new Node(Token.POS, left);
+                }
+            }
+            break;
         }
+
         return new Node(nodeType, left, right);
     }