expand symbol space to improve compression (#2460)

- give globally distinct names to distinct variables
- improve ability to compress cross-scoped
- introduce `options.rename` to `minify()`
- default `true` if both `compress` & `mangle`

lib/minify.js

@@ -55,6 +55,7 @@ function minify(files, options) {
             nameCache: null,
             output: {},
             parse: {},
+            rename: undefined,
             sourceMap: false,
             timings: false,
             toplevel: false,
@@ -64,6 +65,9 @@ function minify(files, options) {
         var timings = options.timings && {
             start: Date.now()
         };
+        if (options.rename === undefined) {
+            options.rename = options.compress && options.mangle;
+        }
         set_shorthand("ie8", options, [ "compress", "mangle", "output" ]);
         set_shorthand("keep_fnames", options, [ "compress", "mangle" ]);
         set_shorthand("toplevel", options, [ "compress", "mangle" ]);
@@ -137,6 +141,11 @@ function minify(files, options) {
         if (options.wrap) {
             toplevel = toplevel.wrap_commonjs(options.wrap);
         }
+        if (timings) timings.rename = Date.now();
+        if (options.rename) {
+            toplevel.figure_out_scope(options.mangle);
+            toplevel.expand_names(options.mangle);
+        }
         if (timings) timings.compress = Date.now();
         if (options.compress) toplevel = new Compressor(options.compress).compress(toplevel);
         if (timings) timings.scope = Date.now();
@@ -197,7 +206,8 @@ function minify(files, options) {
         if (timings) {
             timings.end = Date.now();
             result.timings = {
-                parse: 1e-3 * (timings.compress - timings.parse),
+                parse: 1e-3 * (timings.rename - timings.parse),
+                rename: 1e-3 * (timings.compress - timings.rename),
                 compress: 1e-3 * (timings.scope - timings.compress),
                 scope: 1e-3 * (timings.mangle - timings.scope),
                 mangle: 1e-3 * (timings.properties - timings.mangle),