enhance conditionals (#5575)

lib/compress.js

@@ -9662,10 +9662,20 @@ Compressor.prototype.compress = function(node) {
             var alt_index = last_index(alt_stats);
             for (var stats = []; body_index >= 0 && alt_index >= 0;) {
                 var stat = body_stats[body_index];
-                if (!stat.equals(alt_stats[alt_index])) break;
+                var alt_stat = alt_stats[alt_index];
+                if (stat.equals(alt_stat)) {
                     body_stats.splice(body_index--, 1);
                     alt_stats.splice(alt_index--, 1);
                     stats.unshift(stat);
+                } else {
+                    if (!(stat instanceof AST_SimpleStatement)) break;
+                    if (!(alt_stat instanceof AST_SimpleStatement)) break;
+                    var expr = stat.body.tail_node();
+                    if (!expr.equals(alt_stat.body.tail_node())) break;
+                    body_index = pop_expr(body_stats, stat.body, body_index);
+                    alt_index = pop_expr(alt_stats, alt_stat.body, alt_index);
+                    stats.unshift(make_node(AST_SimpleStatement, expr, { body: expr }));
+                }
             }
             if (stats.length > 0) {
                 self.body = body_stats.length > 0 ? make_node(AST_BlockStatement, self, {
@@ -9692,6 +9702,17 @@ Compressor.prototype.compress = function(node) {
             return index;
         }
 
+        function pop_expr(stats, body, index) {
+            if (body instanceof AST_Sequence) {
+                stats[index] = make_node(AST_SimpleStatement, body, {
+                    body: make_sequence(body, body.expressions.slice(0, -1)),
+                });
+            } else {
+                stats.splice(index--, 1);
+            }
+            return index;
+        }
+
         function sequencesize(stat, defuns, var_defs, refs) {
             if (stat == null) return [];
             if (stat instanceof AST_BlockStatement) {

test/compress/conditionals.js

@@ -278,6 +278,96 @@ merge_tail_2: {
     ]
 }
 
+merge_tail_sequence_1: {
+    options = {
+        conditionals: true,
+    }
+    input: {
+        function f(a) {
+            var b = "foo";
+            if (a) {
+                while (console.log("bar"));
+                console.log(b);
+            } else {
+                c = "baz";
+                while (console.log(c));
+                console.log("bar"),
+                console.log(b);
+                var c;
+            }
+        }
+        f();
+        f(42);
+    }
+    expect: {
+        function f(a) {
+            var b = "foo";
+            if (a)
+                while (console.log("bar"));
+            else {
+                c = "baz";
+                while (console.log(c));
+                console.log("bar");
+                var c;
+            }
+            console.log(b);
+        }
+        f();
+        f(42);
+    }
+    expect_stdout: [
+        "baz",
+        "bar",
+        "foo",
+        "bar",
+        "foo",
+    ]
+}
+
+merge_tail_sequence_2: {
+    options = {
+        conditionals: true,
+    }
+    input: {
+        function f(a) {
+            var b = "foo";
+            if (a) {
+                console.log("bar");
+                console.log(b);
+            } else {
+                c = "baz";
+                while (console.log(c));
+                console.log("bar"),
+                console.log(b);
+                var c;
+            }
+        }
+        f();
+        f(42);
+    }
+    expect: {
+        function f(a) {
+            var b = "foo";
+            if (!a) {
+                c = "baz";
+                while (console.log(c));
+                var c;
+            }
+            console.log("bar");
+            console.log(b);
+        }
+        f();
+        f(42);
+    }
+    expect_stdout: [
+        "baz",
+        "bar",
+        "foo",
+        "bar",
+        "foo",
+    ]
+}
+
 cond_1: {
     options = {
         conditionals: true,