compress loops with immediate break (#2746)

fixes #2740

lib/compress.js

@@ -3417,23 +3417,28 @@ merge(Compressor.prototype, {
     });
 
     function if_break_in_loop(self, compressor) {
-        function drop_it(rest) {
-            rest = as_statement_array(rest);
-            if (self.body instanceof AST_BlockStatement) {
-                self.body = self.body.clone();
-                self.body.body = rest.concat(self.body.body.slice(1));
-                self.body = self.body.transform(compressor);
-            } else {
-                self.body = make_node(AST_BlockStatement, self.body, {
-                    body: rest
-                }).transform(compressor);
-            }
-            if_break_in_loop(self, compressor);
-        }
         var first = self.body instanceof AST_BlockStatement ? self.body.body[0] : self.body;
+        if (compressor.option("dead_code") && is_break(first)) {
+            var body = [];
+            if (self.init instanceof AST_Statement) {
+                body.push(self.init);
+            } else if (self.init) {
+                body.push(make_node(AST_SimpleStatement, self.init, {
+                    body: self.init
+                }));
+            }
+            if (self.condition) {
+                body.push(make_node(AST_SimpleStatement, self.condition, {
+                    body: self.condition
+                }));
+            }
+            extract_declarations_from_unreachable_code(compressor, self.body, body);
+            return make_node(AST_BlockStatement, self, {
+                body: body
+            });
+        }
         if (first instanceof AST_If) {
-            if (first.body instanceof AST_Break
-                && compressor.loopcontrol_target(first.body) === compressor.self()) {
+            if (is_break(first.body)) {
                 if (self.condition) {
                     self.condition = make_node(AST_Binary, self.condition, {
                         left: self.condition,
@@ -3444,9 +3449,7 @@ merge(Compressor.prototype, {
                     self.condition = first.condition.negate(compressor);
                 }
                 drop_it(first.alternative);
-            }
-            else if (first.alternative instanceof AST_Break
-                     && compressor.loopcontrol_target(first.alternative) === compressor.self()) {
+            } else if (is_break(first.alternative)) {
                 if (self.condition) {
                     self.condition = make_node(AST_Binary, self.condition, {
                         left: self.condition,
@@ -3459,7 +3462,27 @@ merge(Compressor.prototype, {
                 drop_it(first.body);
             }
         }
-    };
+        return self;
+
+        function is_break(node) {
+            return node instanceof AST_Break
+                && compressor.loopcontrol_target(node) === compressor.self();
+        }
+
+        function drop_it(rest) {
+            rest = as_statement_array(rest);
+            if (self.body instanceof AST_BlockStatement) {
+                self.body = self.body.clone();
+                self.body.body = rest.concat(self.body.body.slice(1));
+                self.body = self.body.transform(compressor);
+            } else {
+                self.body = make_node(AST_BlockStatement, self.body, {
+                    body: rest
+                }).transform(compressor);
+            }
+            self = if_break_in_loop(self, compressor);
+        }
+    }
 
     OPT(AST_For, function(self, compressor){
         if (!compressor.option("loops")) return self;
@@ -3495,8 +3518,7 @@ merge(Compressor.prototype, {
                 }
             }
         }
-        if_break_in_loop(self, compressor);
-        return self;
+        return if_break_in_loop(self, compressor);
     });
 
     OPT(AST_If, function(self, compressor){

test/compress/loops.js

@@ -492,3 +492,116 @@ dead_code_condition: {
     }
     expect_stdout: "1"
 }
+
+issue_2740_1: {
+    options = {
+        dead_code: true,
+        loops: true,
+    }
+    input: {
+        for (; ; ) break;
+        for (a(); ; ) break;
+        for (; b(); ) break;
+        for (c(); d(); ) break;
+        for (; ; e()) break;
+        for (f(); ; g()) break;
+        for (; h(); i()) break;
+        for (j(); k(); l()) break;
+    }
+    expect: {
+        a();
+        b();
+        c();
+        d();
+        f();
+        h();
+        j();
+        k();
+    }
+}
+
+issue_2740_2: {
+    options = {
+        dead_code: true,
+        loops: true,
+        passes: 2,
+    }
+    input: {
+        L1: while (x()) {
+            break L1;
+        }
+    }
+    expect: {
+        x();
+    }
+}
+
+issue_2740_3: {
+    options = {
+        dead_code: true,
+        loops: true,
+    }
+    input: {
+        L1: for (var x = 0; x < 3; x++) {
+            L2: for (var y = 0; y < 2; y++) {
+                break L1;
+            }
+        }
+        console.log(x, y);
+    }
+    expect: {
+        L1: for (var x = 0; x < 3; x++)
+            for (var y = 0; y < 2; y++)
+                break L1;
+        console.log(x, y);
+    }
+    expect_stdout: "0 0"
+}
+
+issue_2740_4: {
+    options = {
+        dead_code: true,
+        loops: true,
+        passes: 2,
+    }
+    input: {
+        L1: for (var x = 0; x < 3; x++) {
+            L2: for (var y = 0; y < 2; y++) {
+                break L2;
+            }
+        }
+        console.log(x, y);
+    }
+    expect: {
+        for (var x = 0; x < 3; x++) {
+            var y = 0;
+            y < 2;
+        }
+        console.log(x, y);
+    }
+    expect_stdout: "3 0"
+}
+
+issue_2740_5: {
+    options = {
+        dead_code: true,
+        loops: true,
+        passes: 2,
+    }
+    input: {
+        L1: for (var x = 0; x < 3; x++) {
+            break L1;
+            L2: for (var y = 0; y < 2; y++) {
+                break L2;
+            }
+        }
+        console.log(x, y);
+    }
+    expect: {
+        var x = 0;
+        x < 3;
+        var y;
+        console.log(x,y);
+    }
+    expect_stdout: "0 undefined"
+}