optimize unmodified variables

lib/compress.js

@@ -67,6 +67,7 @@ function Compressor(options, false_by_default) {
         if_return     : !false_by_default,
         join_vars     : !false_by_default,
         collapse_vars : false,
+        reduce_vars   : false,
         cascade       : !false_by_default,
         side_effects  : !false_by_default,
         pure_getters  : false,
@@ -1107,7 +1108,7 @@ merge(Compressor.prototype, {
             this._evaluating = true;
             try {
                 var d = this.definition();
-                if (d && d.constant && d.init) {
+                if (d && (d.constant || compressor.option("reduce_vars") && !d.global && !d.modified) && d.init) {
                     return ev(d.init, compressor);
                 }
             } finally {

lib/scope.js

@@ -197,7 +197,8 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
         }
         if (node instanceof AST_SymbolRef) {
             var name = node.name;
-            if (name == "eval" && tw.parent() instanceof AST_Call) {
+            var parent = tw.parent();
+            if (name == "eval" && parent instanceof AST_Call) {
                 for (var s = node.scope; s && !s.uses_eval; s = s.parent_scope) {
                     s.uses_eval = true;
                 }
@@ -213,12 +214,14 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
                     g.global = true;
                     globals.set(name, g);
                 }
-                node.thedef = g;
+                sym = g;
                 if (func && name == "arguments") {
                     func.uses_arguments = true;
                 }
-            } else {
+            }
             node.thedef = sym;
+            if (parent instanceof AST_Assign || parent instanceof AST_Unary && (parent.operator === '++' || parent.operator === '--')) {
+                sym.modified = true;
             }
             node.reference();
             return true;

test/compress/reduce_vars.js

@@ -0,0 +1,60 @@
+reduce_vars: {
+    options = {
+        conditionals  : true,
+        evaluate      : true,
+        global_defs   : {
+            C : 0
+        },
+        reduce_vars   : true,
+        unused        : true
+    }
+    input: {
+        var A = 1;
+        (function f0() {
+            var a = 2;
+            console.log(a - 5);
+            console.log(A - 5);
+        })();
+        (function f1() {
+            var a = 2;
+            console.log(a - 5);
+            eval("console.log(a);");
+        })();
+        (function f2(eval) {
+            var a = 2;
+            console.log(a - 5);
+            eval("console.log(a);");
+        })(eval);
+        (function f3() {
+            var b = typeof C !== "undefined";
+            var c = 4;
+            if (b) {
+                return 'yes';
+            } else {
+                return 'no';
+            }
+        })();
+        console.log(A + 1);
+    }
+    expect: {
+        var A = 1;
+        (function() {
+            console.log(-3);
+            console.log(A - 5);
+        })();
+        (function f1() {
+            var a = 2;
+            console.log(-3);
+            eval("console.log(a);");
+        })();
+        (function f2(eval) {
+            var a = 2;
+            console.log(-3);
+            eval("console.log(a);");
+        })(eval);
+        (function() {
+            return "yes";
+        })();
+        console.log(A + 1);
+    }
+}