enhance side_effects (#4124)

- add documentation for `merge_vars`

README.md

@@ -688,6 +688,8 @@ to be `false` and all symbol names will be omitted.
 - `loops` (default: `true`) -- optimizations for `do`, `while` and `for` loops
   when we can statically determine the condition.
 
+- `merge_vars` (default: `true`) -- combine and reuse variables.
+
 - `negate_iife` (default: `true`) -- negate "Immediately-Called Function Expressions"
   where the return value is discarded, to avoid the parens that the
   code generator would insert.

lib/compress.js

@@ -3892,7 +3892,8 @@ merge(Compressor.prototype, {
             if (is_undeclared_ref(expr) && global_pure_fns[expr.name]) return true;
             if (expr instanceof AST_Dot && is_undeclared_ref(expr.expression)) {
                 var static_fn = static_fns[expr.expression.name];
-                return static_fn && static_fn[expr.property];
+                return static_fn && (static_fn[expr.property]
+                    || expr.expression.name == "Math" && expr.property == "random");
             }
         }
         return this.pure || !compressor.pure_funcs(this);

test/compress/side_effects.js

@@ -245,6 +245,31 @@ unsafe_builtin_2: {
     expect_stdout: "object PASS PASS"
 }
 
+unsafe_builtin_3: {
+    options = {
+        conditionals: true,
+        side_effects: true,
+        toplevel: true,
+        unsafe: true,
+    }
+    input: {
+        var o = {};
+        if (42 < Math.random())
+            o.p = "FAIL";
+        else
+            o.p = "PASS";
+        for (var k in o)
+            console.log(k, o[k]);
+    }
+    expect: {
+        var o = {};
+        o.p = 42 < Math.random() ? "FAIL" : "PASS";
+        for (var k in o)
+            console.log(k, o[k]);
+    }
+    expect_stdout: "p PASS"
+}
+
 unsafe_string_replace: {
     options = {
         side_effects: true,