drop unused compound assignments (#2230)

fixes #2226

lib/compress.js

@@ -790,6 +790,7 @@ merge(Compressor.prototype, {
                                     right: candidate.value
                                 });
                             }
+                            candidate.write_only = false;
                             return candidate;
                         }
                         // These node types have child nodes that execute sequentially,
@@ -2177,7 +2178,12 @@ merge(Compressor.prototype, {
         var drop_funcs = !(self instanceof AST_Toplevel) || compressor.toplevel.funcs;
         var drop_vars = !(self instanceof AST_Toplevel) || compressor.toplevel.vars;
         if (!drop_funcs && !drop_vars) return;
-        var assign_as_unused = !/keep_assign/.test(compressor.option("unused"));
+        var assign_as_unused = /keep_assign/.test(compressor.option("unused")) ? return_false : function(node) {
+            if (node instanceof AST_Assign && (node.write_only || node.operator == "=")) {
+                return node.left;
+            }
+            if (node instanceof AST_Unary && node.write_only) return node.expression;
+        };
         var in_use = [];
         var in_use_ids = Object.create(null); // avoid expensive linear scans of in_use
         if (self instanceof AST_Toplevel && compressor.top_retain) {
@@ -2227,12 +2233,8 @@ merge(Compressor.prototype, {
                     });
                     return true;
                 }
-                if (assign_as_unused
-                    && node instanceof AST_Assign
-                    && node.operator == "="
-                    && node.left instanceof AST_SymbolRef
-                    && scope === self) {
-                    node.right.walk(tw);
+                if (assign_as_unused(node) instanceof AST_SymbolRef && scope === self) {
+                    if (node instanceof AST_Assign) node.right.walk(tw);
                     return true;
                 }
                 if (node instanceof AST_SymbolRef) {
@@ -2396,15 +2398,18 @@ merge(Compressor.prototype, {
                         });
                     }
                 }
-                if (drop_vars && assign_as_unused
-                    && node instanceof AST_Assign
-                    && node.operator == "="
-                    && node.left instanceof AST_SymbolRef) {
-                    var def = node.left.definition();
-                    if (!(def.id in in_use_ids)
+                if (drop_vars) {
+                    var def = assign_as_unused(node);
+                    if (def instanceof AST_SymbolRef
+                        && !((def = def.definition()).id in in_use_ids)
                         && self.variables.get(def.name) === def) {
+                        if (node instanceof AST_Assign) {
                             return maintain_this_binding(tt.parent(), node, node.right.transform(tt));
                         }
+                        return make_node(AST_Number, node, {
+                            value: 0
+                        });
+                    }
                 }
                 // certain combination of unused name + side effect leads to:
                 //    https://github.com/mishoo/UglifyJS2/issues/44
@@ -2642,7 +2647,10 @@ merge(Compressor.prototype, {
                 return make_sequence(this, [ left, right ]);
             }
         });
-        def(AST_Assign, return_this);
+        def(AST_Assign, function(compressor){
+            this.write_only = !this.left.has_side_effects(compressor);
+            return this;
+        });
         def(AST_Conditional, function(compressor){
             var consequent = this.consequent.drop_side_effect_free(compressor);
             var alternative = this.alternative.drop_side_effect_free(compressor);
@@ -2663,7 +2671,10 @@ merge(Compressor.prototype, {
             return node;
         });
         def(AST_Unary, function(compressor, first_in_statement){
-            if (unary_side_effects(this.operator)) return this;
+            if (unary_side_effects(this.operator)) {
+                this.write_only = !this.expression.has_side_effects(compressor);
+                return this;
+            }
             if (this.operator == "typeof" && this.expression instanceof AST_SymbolRef) return null;
             var expression = this.expression.drop_side_effect_free(compressor, first_in_statement);
             if (first_in_statement
@@ -3485,7 +3496,7 @@ merge(Compressor.prototype, {
                                 operator: car.operator,
                                 expression: left
                             });
-                        }
+                        } else car.write_only = false;
                         if (parent) {
                             parent[field] = car;
                             expressions[i] = expressions[j];

test/compress/collapse_vars.js

@@ -863,7 +863,7 @@ collapse_vars_unary: {
     input: {
         function f0(o, p) {
             var x = o[p];
-            delete x;
+            return delete x;
         }
         function f1(n) {
             var k = !!n;
@@ -893,7 +893,7 @@ collapse_vars_unary: {
     expect: {
         function f0(o, p) {
             var x = o[p];
-            delete x;
+            return delete x;
         }
         function f1(n) {
             return n > +!!n

test/compress/drop-unused.js

@@ -1090,6 +1090,7 @@ var_catch_toplevel: {
             a--;
             try {
                 a++;
+                x();
             } catch(a) {
                 if (a) var a;
                 var a = 10;
@@ -1099,9 +1100,8 @@ var_catch_toplevel: {
     }
     expect: {
         !function() {
-            a--;
             try {
-                a++;
+                x();
             } catch(a) {
                 var a;
             }
@@ -1153,3 +1153,89 @@ issue_2105: {
     }
     expect_stdout: "PASS"
 }
+
+issue_2226_1: {
+    options = {
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        function f1() {
+            var a = b;
+            a += c;
+        }
+        function f2(a) {
+            a <<= b;
+        }
+        function f3(a) {
+            --a;
+        }
+        function f4() {
+            var a = b;
+            return a *= c;
+        }
+        function f5(a) {
+            x(a /= b);
+        }
+    }
+    expect: {
+        function f1() {
+            b;
+            c;
+        }
+        function f2(a) {
+            b;
+        }
+        function f3(a) {
+            0;
+        }
+        function f4() {
+            var a = b;
+            return a *= c;
+        }
+        function f5(a) {
+            x(a /= b);
+        }
+    }
+}
+
+issue_2226_2: {
+    options = {
+        cascade: true,
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        console.log(function(a, b) {
+            a += b;
+            return a;
+        }(1, 2));
+    }
+    expect: {
+        console.log(function(a, b) {
+            return a += b;
+        }(1, 2));
+    }
+    expect_stdout: "3"
+}
+
+issue_2226_3: {
+    options = {
+        collapse_vars: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        console.log(function(a, b) {
+            a += b;
+            return a;
+        }(1, 2));
+    }
+    expect: {
+        console.log(function(a, b) {
+            return a += 2;
+        }(1));
+    }
+    expect_stdout: "3"
+}