improve compression of `if` conditions (#2544)

1 files changed, 25 insertions, 19 deletions

diff --git a/lib/compress.js b/lib/compress.js
index e6215f66..395c4ed0 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -3147,28 +3147,34 @@ merge(Compressor.prototype, {
         // “has no side effects”; also it doesn't work for cases like
         // `x && true`, though it probably should.
         var cond = self.condition.evaluate(compressor);
-        if (cond !== self.condition) {
-            if (cond) {
-                compressor.warn("Condition always true [{file}:{line},{col}]", self.condition.start);
-                if (compressor.option("dead_code")) {
-                    var a = [];
-                    if (self.alternative) {
-                        extract_declarations_from_unreachable_code(compressor, self.alternative, a);
-                    }
-                    a.push(self.body);
-                    return make_node(AST_BlockStatement, self, { body: a }).optimize(compressor);
-                }
-            } else {
+        if (!compressor.option("dead_code") && !(cond instanceof AST_Node)) {
+            var orig = self.condition;
+            self.condition = make_node_from_constant(cond, orig);
+            self.condition = best_of_expression(self.condition.transform(compressor), orig);
+        }
+        if (compressor.option("dead_code")) {
+            if (cond instanceof AST_Node) cond = self.condition.tail_node().evaluate(compressor);
+            if (!cond) {
                 compressor.warn("Condition always false [{file}:{line},{col}]", self.condition.start);
-                if (compressor.option("dead_code")) {
-                    var a = [];
-                    extract_declarations_from_unreachable_code(compressor, self.body, a);
-                    if (self.alternative) a.push(self.alternative);
-                    return make_node(AST_BlockStatement, self, { body: a }).optimize(compressor);
+                var body = [];
+                extract_declarations_from_unreachable_code(compressor, self.body, body);
+                body.push(make_node(AST_SimpleStatement, self.condition, {
+                    body: self.condition
+                }));
+                if (self.alternative) body.push(self.alternative);
+                return make_node(AST_BlockStatement, self, { body: body }).optimize(compressor);
+            } else if (!(cond instanceof AST_Node)) {
+                compressor.warn("Condition always true [{file}:{line},{col}]", self.condition.start);
+                var body = [];
+                if (self.alternative) {
+                    extract_declarations_from_unreachable_code(compressor, self.alternative, body);
                 }
+                body.push(make_node(AST_SimpleStatement, self.condition, {
+                    body: self.condition
+                }));
+                body.push(self.body);
+                return make_node(AST_BlockStatement, self, { body: body }).optimize(compressor);
             }
-            cond = make_node_from_constant(cond, self.condition).transform(compressor);
-            self.condition = best_of_expression(cond, self.condition);
         }
         var negated = self.condition.negate(compressor);
         var self_condition_length = self.condition.print_to_string().length;