implement directives

README.md

@@ -620,6 +620,8 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
 
 - `dead_code` (default: `true`) -- remove unreachable code
 
+- `directives` (default: `true`) -- remove redundant or non-standard directives
+
 - `drop_console` (default: `false`) -- Pass `true` to discard calls to
   `console.*` functions. If you wish to drop a specific function call
   such as `console.info` and/or retain side effects from function arguments

lib/compress.js

@@ -54,6 +54,7 @@ function Compressor(options, false_by_default) {
         comparisons   : !false_by_default,
         conditionals  : !false_by_default,
         dead_code     : !false_by_default,
+        directives    : !false_by_default,
         drop_console  : false,
         drop_debugger : !false_by_default,
         evaluate      : !false_by_default,
@@ -3264,8 +3265,10 @@ merge(Compressor.prototype, {
 
     /* -----[ optimizers ]----- */
 
+    var directives = makePredicate(["use asm", "use strict"]);
     OPT(AST_Directive, function(self, compressor) {
-        if (compressor.has_directive(self.value) !== self) {
+        if (compressor.option("directives")
+            && (!directives[self.value] || compressor.has_directive(self.value) !== self)) {
             return make_node(AST_EmptyStatement, self);
         }
         return self;

test/compress/functions.js

@@ -2025,6 +2025,7 @@ deduplicate_parenthesis: {
 
 drop_lone_use_strict: {
     options = {
+        directives: true,
         side_effects: true,
     }
     input: {
@@ -2052,6 +2053,27 @@ drop_lone_use_strict: {
     }
 }
 
+issue_3166: {
+    options = {
+        directives: true,
+    }
+    input: {
+        "foo";
+        "use strict";
+        function f() {
+            "use strict";
+            "bar";
+            "use asm";
+        }
+    }
+    expect: {
+        "use strict";
+        function f() {
+            "use asm";
+        }
+    }
+}
+
 issue_3016_1: {
     options = {
         inline: true,

test/mocha/cli.js

@@ -8,7 +8,7 @@ function read(path) {
 
 describe("bin/uglifyjs", function() {
     var uglifyjscmd = '"' + process.argv[0] + '" bin/uglifyjs';
-    it("should produce a functional build when using --self", function(done) {
+    it("Should produce a functional build when using --self", function(done) {
         this.timeout(30000);
         var command = uglifyjscmd + ' --self -cm --wrap WrappedUglifyJS';
         exec(command, function(err, stdout) {
@@ -68,15 +68,15 @@ describe("bin/uglifyjs", function() {
             done();
         });
     });
-    it("should not append source map to output when not using --source-map url=inline", function(done) {
+    it("Should not append source map to output when not using --source-map url=inline", function(done) {
         var command = uglifyjscmd + ' test/input/issue-1323/sample.js';
         exec(command, function(err, stdout) {
             if (err) throw err;
             assert.strictEqual(stdout, "var bar=function(){function foo(bar){return bar}return foo}();\n");
             done();
         });
     });
-    it("should not consider source map file content as source map file name (issue #2082)", function(done) {
+    it("Should not consider source map file content as source map file name (issue #2082)", function(done) {
         var command = [
             uglifyjscmd,
             "test/input/issue-2082/sample.js",