introduce unsafe_regexp (#1970)

fixes #1964

README.md

@@ -462,6 +462,9 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
 - `unsafe_proto` (default: false) -- optimize expressions like
   `Array.prototype.slice.call(a)` into `[].slice.call(a)`
 
+- `unsafe_regexp` (default: false) -- enable substitutions of variables with
+  `RegExp` values the same way as if they are constants.
+
 - `conditionals` -- apply optimizations for `if`-s and conditional
   expressions
 
@@ -553,9 +556,9 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
   being compressed into `1/0`, which may cause performance issues on Chrome.
 
 - `side_effects` -- default `true`. Pass `false` to disable potentially dropping
-functions marked as "pure".  A function call is marked as "pure" if a comment 
-annotation `/*@__PURE__*/` or `/*#__PURE__*/` immediately precedes the call. For 
-example: `/*@__PURE__*/foo()`;
+  functions marked as "pure".  A function call is marked as "pure" if a comment
+  annotation `/*@__PURE__*/` or `/*#__PURE__*/` immediately precedes the call. For
+  example: `/*@__PURE__*/foo();`
 
 ## Mangle options
 

lib/compress.js

@@ -83,6 +83,7 @@ function Compressor(options, false_by_default) {
         unsafe_comps  : false,
         unsafe_math   : false,
         unsafe_proto  : false,
+        unsafe_regexp : false,
         unused        : !false_by_default,
         warnings      : false,
     }, true);
@@ -3765,7 +3766,7 @@ merge(Compressor.prototype, {
             if (fixed) {
                 if (d.should_replace === undefined) {
                     var init = fixed.evaluate(compressor);
-                    if (init !== fixed) {
+                    if (init !== fixed && (compressor.option("unsafe_regexp") || !(init instanceof RegExp))) {
                         init = make_node_from_constant(init, fixed);
                         var value_length = init.optimize(compressor).print_to_string().length;
                         var fn;

test/compress/evaluate.js

@@ -990,3 +990,50 @@ Infinity_NaN_undefined_LHS: {
         "}",
     ]
 }
+
+issue_1964_1: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unsafe_regexp: false,
+        unused: true,
+    }
+    input: {
+        function f() {
+            var long_variable_name = /\s/;
+            return "a b c".split(long_variable_name)[1];
+        }
+        console.log(f());
+    }
+    expect: {
+        function f() {
+            var long_variable_name = /\s/;
+            return "a b c".split(long_variable_name)[1];
+        }
+        console.log(f());
+    }
+    expect_stdout: "b"
+}
+
+issue_1964_2: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unsafe_regexp: true,
+        unused: true,
+    }
+    input: {
+        function f() {
+            var long_variable_name = /\s/;
+            return "a b c".split(long_variable_name)[1];
+        }
+        console.log(f());
+    }
+    expect: {
+        function f() {
+            return "a b c".split(/\s/)[1];
+        }
+        console.log(f());
+    }
+    expect_stdout: "b"
+}