From 9dc5247bc7756c5ee0577b6887de82a3ad715fc5 Mon Sep 17 00:00:00 2001
From: alexlamsl <alexlamsl@gmail.com>
Date: Tue, 18 Apr 2017 03:08:25 +0800
Subject: [PATCH] support safe reassignments in `reduce_vars` `var
 a=1;a=2;x(a)` => `x(2)`

---
 lib/compress.js              | 33 ++++++++++++++++++++++++++-------
 test/compress/reduce_vars.js | 25 ++++++++++---------------
 2 files changed, 36 insertions(+), 22 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 0dfe2a3c..802b28f0 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -267,7 +267,7 @@ merge(Compressor.prototype, {
                 if (node instanceof AST_SymbolRef) {
                     var d = node.definition();
                     d.references.push(node);
-                    if (d.fixed === undefined || !is_safe(d)
+                    if (d.fixed === undefined || !safe_to_read(d)
                         || is_modified(node, 0, is_immutable(node.fixed_value()))) {
                         d.fixed = false;
                     }
@@ -277,7 +277,7 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_VarDef) {
                     var d = node.name.definition();
-                    if (d.fixed == null) {
+                    if (safe_to_assign(d, node.value)) {
                         if (node.value) {
                             d.fixed = function() {
                                 return node.value;
@@ -297,7 +297,7 @@ merge(Compressor.prototype, {
                     && node.operator == "="
                     && node.left instanceof AST_SymbolRef) {
                     var d = node.left.definition();
-                    if (HOP(safe_ids, d.id) && d.fixed == null) {
+                    if (HOP(safe_ids, d.id) && safe_to_assign(d, node.right)) {
                         d.fixed = function() {
                             return node.right;
                         };
@@ -309,7 +309,7 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_Defun) {
                     var d = node.name.definition();
-                    if (!toplevel && d.global || is_safe(d)) {
+                    if (!toplevel && d.global || safe_to_read(d)) {
                         d.fixed = false;
                     } else {
                         d.fixed = node;
@@ -385,11 +385,19 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_For) {
                     if (node.init) node.init.walk(tw);
+                    if (node.condition) {
+                        push();
+                        node.condition.walk(tw);
+                        pop();
+                    }
                     push();
-                    if (node.condition) node.condition.walk(tw);
                     node.body.walk(tw);
-                    if (node.step) node.step.walk(tw);
                     pop();
+                    if (node.step) {
+                        push();
+                        node.step.walk(tw);
+                        pop();
+                    }
                     return true;
                 }
                 if (node instanceof AST_ForIn) {
@@ -426,7 +434,7 @@ merge(Compressor.prototype, {
             safe_ids[def.id] = safe;
         }
 
-        function is_safe(def) {
+        function safe_to_read(def) {
             if (safe_ids[def.id]) {
                 if (def.fixed == null) {
                     var orig = def.orig[0];
@@ -437,6 +445,17 @@ merge(Compressor.prototype, {
             }
         }
 
+        function safe_to_assign(def, value) {
+            if (def.fixed === undefined) return true;
+            if (def.fixed === false) return false;
+            if (def.fixed != null && (!value || def.references.length > 0)) return false;
+            return !def.orig.some(function(sym) {
+                return sym instanceof AST_SymbolConst
+                    || sym instanceof AST_SymbolDefun
+                    || sym instanceof AST_SymbolLambda;
+            });
+        }
+
         function push() {
             safe_ids = Object.create(safe_ids);
         }
diff --git a/test/compress/reduce_vars.js b/test/compress/reduce_vars.js
index ad2c90bc..0eeb80b3 100644
--- a/test/compress/reduce_vars.js
+++ b/test/compress/reduce_vars.js
@@ -66,7 +66,7 @@ modified: {
         conditionals  : true,
         evaluate      : true,
         reduce_vars   : true,
-        unused        : true
+        unused        : true,
     }
     input: {
         function f0() {
@@ -136,12 +136,11 @@ modified: {
         }
 
         function f2() {
-            var b = 2;
-            b = 3;
-            console.log(1 + b);
-            console.log(b + 3);
+            3;
             console.log(4);
-            console.log(1 + b + 3);
+            console.log(6);
+            console.log(4);
+            console.log(7);
         }
 
         function f3() {
@@ -375,12 +374,11 @@ passes: {
     }
     expect: {
         function f() {
-            var b = 2;
-            b = 3;
-            console.log(1 + b);
-            console.log(b + 3);
+            3;
             console.log(4);
-            console.log(1 + b + 3);
+            console.log(6);
+            console.log(4);
+            console.log(7);
         }
     }
 }
@@ -1828,10 +1826,7 @@ redefine_farg_3: {
         console.log(function(a) {
             var a;
             return typeof a;
-        }([]), "number", function(a) {
-            var a = void 0;
-            return typeof a;
-        }([]));
+        }([]), "number", "undefined");
     }
     expect_stdout: "object number undefined"
 }