Address comments.

Author: viirya

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala

@@ -118,7 +118,7 @@ abstract class Expression extends TreeNode[Expression] {
   private def reduceCodeSize(ctx: CodegenContext, eval: ExprCode): Unit = {
     // TODO: support whole stage codegen too
     if (eval.code.trim.length > 1024 && ctx.INPUT_ROW != null && ctx.currentVars == null) {
-      val setIsNull = if ("false" != s"${eval.isNull}" && "true" != s"${eval.isNull}") {
+      val setIsNull = if (!eval.isNull.isInstanceOf[LiteralValue]) {
         val globalIsNull = ctx.addMutableState(ctx.JAVA_BOOLEAN, "globalIsNull")
         val localIsNull = eval.isNull
         eval.isNull = GlobalValue(globalIsNull)

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/nullExpressions.scala

@@ -322,7 +322,7 @@ case class IsNull(child: Expression) extends UnaryExpression with Predicate {
 
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val eval = child.genCode(ctx)
-    val value = if ("true" == s"${eval.isNull}" || "false" == s"${eval.isNull}") {
+    val value = if (eval.isNull.isInstanceOf[LiteralValue]) {
       LiteralValue(eval.isNull)
     } else {
       VariableValue(eval.isNull)
@@ -353,9 +353,9 @@ case class IsNotNull(child: Expression) extends UnaryExpression with Predicate {
 
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     val eval = child.genCode(ctx)
-    val value = if ("true" == s"${eval.isNull}") {
+    val value = if (eval.isNull == LiteralValue("true")) {
       LiteralValue("false")
-    } else if ("false" == s"${eval.isNull}") {
+    } else if (eval.isNull == LiteralValue("false")) {
       LiteralValue("true")
     } else {
       StatementValue(s"(!(${eval.isNull}))")