enhance `assignments` & `unused` (#3428)

closes #3427

4 files changed, 123 insertions, 24 deletions

diff --git a/lib/compress.js b/lib/compress.js
index e99ce508..7bbb5a20 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2282,8 +2282,7 @@ merge(Compressor.prototype, {
     // returns true if this node may be null, undefined or contain `AST_Accessor`
     (function(def) {
         AST_Node.DEFMETHOD("may_throw_on_access", function(compressor) {
-            return !compressor.option("pure_getters")
-                || this._dot_throw(compressor);
+            return !compressor.option("pure_getters") || this._dot_throw(compressor);
         });
         function is_strict(compressor) {
             return /strict/.test(compressor.option("pure_getters"));
@@ -2291,7 +2290,15 @@ merge(Compressor.prototype, {
         def(AST_Node, is_strict);
         def(AST_Array, return_false);
         def(AST_Assign, function(compressor) {
-            return this.operator == "=" && this.right._dot_throw(compressor);
+            if (this.operator != "=") return false;
+            var rhs = this.right;
+            if (!rhs._dot_throw(compressor)) return false;
+            var sym = this.left;
+            if (!(sym instanceof AST_SymbolRef)) return true;
+            if (rhs instanceof AST_Binary && rhs.operator == "||" && sym.name == rhs.left.name) {
+                return rhs.right._dot_throw(compressor);
+            }
+            return true;
         });
         def(AST_Binary, function(compressor) {
             return lazy_op[this.operator] && (this.left._dot_throw(compressor) || this.right._dot_throw(compressor));
@@ -3618,7 +3625,7 @@ merge(Compressor.prototype, {
                     var value = null;
                     if (node instanceof AST_Assign) {
                         if (!in_use || node.left === sym && def.id in fixed_ids && fixed_ids[def.id] !== node) {
-                            value = node.right;
+                            value = get_rhs(node);
                         }
                     } else if (!in_use) {
                         value = make_node(AST_Number, node, {
@@ -3843,6 +3850,15 @@ merge(Compressor.prototype, {
             }
         }
 
+        function get_rhs(assign) {
+            var rhs = assign.right;
+            if (!assign.write_only) return rhs;
+            if (!(rhs instanceof AST_Binary && lazy_op[rhs.operator])) return rhs;
+            var sym = assign.left;
+            if (!(sym instanceof AST_SymbolRef) || sym.name != rhs.left.name) return rhs;
+            return rhs.right;
+        }
+
         function scan_ref_scoped(node, descend) {
             var node_def, props = [], sym = assign_as_unused(node, props);
             if (sym && self.variables.get(sym.name) === (node_def = sym.definition())) {
@@ -3850,9 +3866,10 @@ merge(Compressor.prototype, {
                     prop.walk(tw);
                 });
                 if (node instanceof AST_Assign) {
-                    node.right.walk(tw);
+                    var right = get_rhs(node);
+                    right.walk(tw);
                     if (node.left === sym) {
-                        if (!node_def.chained && sym.fixed_value(true) === node.right) {
+                        if (!node_def.chained && sym.fixed_value(true) === right) {
                             fixed_ids[node_def.id] = node;
                         }
                         if (!node.write_only) {
@@ -4163,12 +4180,14 @@ merge(Compressor.prototype, {
         });
         def(AST_Assign, function(compressor) {
             var left = this.left;
-            if (left.has_side_effects(compressor)
-                || compressor.has_directive("use strict")
-                    && left instanceof AST_PropAccess
-                    && left.expression.is_constant()) {
-                return this;
+            if (left instanceof AST_PropAccess) {
+                var expr = left.expression;
+                if (expr instanceof AST_Assign && !expr.may_throw_on_access(compressor)) {
+                    expr.write_only = true;
+                }
+                if (compressor.has_directive("use strict") && expr.is_constant()) return this;
             }
+            if (left.has_side_effects(compressor)) return this;
             this.write_only = true;
             if (root_expr(left).is_constant_expression(compressor.find_parent(AST_Scope))) {
                 return this.right.drop_side_effect_free(compressor);
@@ -4243,8 +4262,9 @@ merge(Compressor.prototype, {
         });
         def(AST_Constant, return_null);
         def(AST_Dot, function(compressor, first_in_statement) {
-            if (this.expression.may_throw_on_access(compressor)) return this;
-            return this.expression.drop_side_effect_free(compressor, first_in_statement);
+            var expr = this.expression;
+            if (expr.may_throw_on_access(compressor)) return this;
+            return expr.drop_side_effect_free(compressor, first_in_statement);
         });
         def(AST_Function, function(compressor) {
             return this.name && compressor.option("ie8") ? this : null;
@@ -5556,20 +5576,29 @@ merge(Compressor.prototype, {
                 self.right = tmp;
             }
         }
-        if (commutativeOperators[self.operator]) {
-            if (self.right.is_constant()
-                && !self.left.is_constant()) {
-                // if right is a constant, whatever side effects the
-                // left side might have could not influence the
-                // result.  hence, force switch.
-
-                if (!(self.left instanceof AST_Binary
-                      && PRECEDENCE[self.left.operator] >= PRECEDENCE[self.operator])) {
-                    reverse();
-                }
+        if (commutativeOperators[self.operator] && self.right.is_constant() && !self.left.is_constant()) {
+            // if right is a constant, whatever side effects the
+            // left side might have could not influence the
+            // result.  hence, force switch.
+            if (!(self.left instanceof AST_Binary
+                    && PRECEDENCE[self.left.operator] >= PRECEDENCE[self.operator])) {
+                reverse();
             }
         }
         self = self.lift_sequences(compressor);
+        if (compressor.option("assignments") && lazy_op[self.operator]) {
+            var assign = self.right;
+            // a || (a = x) => a = a || x
+            // a && (a = x) => a = a && x
+            if (self.left instanceof AST_SymbolRef
+                && assign instanceof AST_Assign
+                && assign.operator == "="
+                && self.left.equivalent_to(assign.left)) {
+                self.right = assign.right;
+                assign.right = self;
+                return assign;
+            }
+        }
         if (compressor.option("comparisons")) switch (self.operator) {
           case "===":
           case "!==":
diff --git a/test/compress/assignment.js b/test/compress/assignment.js
index ece0185d..ecd0ac4a 100644
--- a/test/compress/assignment.js
+++ b/test/compress/assignment.js
@@ -311,3 +311,19 @@ issue_3375: {
     }
     expect_stdout: "string"
 }
+
+issue_3427: {
+    options = {
+        assignments: true,
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        (function() {
+            var a;
+            a || (a = {});
+        })();
+    }
+    expect: {}
+}
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index d7f3cf76..6ed193e9 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -2028,3 +2028,37 @@ issue_3375: {
     }
     expect_stdout: "0 0"
 }
+
+issue_3427_1: {
+    options = {
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        (function() {
+            var a;
+            a = a || {};
+        })();
+    }
+    expect: {}
+}
+
+issue_3427_2: {
+    options = {
+        unused: true,
+    }
+    input: {
+        (function() {
+            var s = "PASS";
+            console.log(s = s || "FAIL");
+        })();
+    }
+    expect: {
+        (function() {
+            var s = "PASS";
+            console.log(s = s || "FAIL");
+        })();
+    }
+    expect_stdout: "PASS"
+}
diff --git a/test/compress/pure_getters.js b/test/compress/pure_getters.js
index b6e42f9d..54c89c6d 100644
--- a/test/compress/pure_getters.js
+++ b/test/compress/pure_getters.js
@@ -1187,3 +1187,23 @@ drop_arguments: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3427: {
+    options = {
+        assignments: true,
+        collapse_vars: true,
+        inline: true,
+        pure_getters: "strict",
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        (function(b) {
+            b.p = 42;
+        })(a || (a = {}));
+    }
+    expect: {}
+}