support safe reassignments in `reduce_vars` (#1823)

`var a=1;a=2;x(a)` => `x(2)`

fix pre-existing issues
- reference counting on assignment
- walking of anonymous functions
- chained assignment

1 files changed, 37 insertions, 14 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 0dfe2a3c..596b03fa 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 (d.fixed === undefined || safe_to_assign(d, node.value)) {
                         if (node.value) {
                             d.fixed = function() {
                                 return node.value;
@@ -297,7 +297,8 @@ 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 (safe_to_assign(d, node.right)) {
+                        d.references.push(node.left);
                         d.fixed = function() {
                             return node.right;
                         };
@@ -309,7 +310,7 @@ 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;
@@ -321,13 +322,12 @@ merge(Compressor.prototype, {
                     safe_ids = save_ids;
                     return true;
                 }
-                var iife;
-                if (node instanceof AST_Function
-                    && (iife = tw.parent()) instanceof AST_Call
-                    && iife.expression === node) {
-                    if (node.name) {
-                        node.name.definition().fixed = node;
-                    } else {
+                if (node instanceof AST_Function) {
+                    push();
+                    var iife;
+                    if (!node.name
+                        && (iife = tw.parent()) instanceof AST_Call
+                        && iife.expression === node) {
                         // Virtually turn IIFE parameters into variable definitions:
                         //   (function(a,b) {...})(c,d) => (function() {var a=c,b=d; ...})()
                         // So existing transformation rules can work on them.
@@ -339,6 +339,9 @@ merge(Compressor.prototype, {
                             mark(d, true);
                         });
                     }
+                    descend();
+                    pop();
+                    return true;
                 }
                 if (node instanceof AST_Binary
                     && (node.operator == "&&" || node.operator == "||")) {
@@ -385,11 +388,19 @@ 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 +437,7 @@ 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 +448,18 @@ merge(Compressor.prototype, {
             }
         }
 
+        function safe_to_assign(def, value) {
+            if (!HOP(safe_ids, def.id)) return false;
+            if (!safe_to_read(def)) return false;
+            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);
         }