[SPARK-18999][SQL][MINOR] simplify Literal codegen

## What changes were proposed in this pull request?

`Literal` can use `CodegenContex.addReferenceObj` to implement codegen, instead of `CodegenFallback`.  This can also simplify the generated code a little bit, before we will generate: `((Expression) references[1]).eval(null)`, now it's just `references[1]`.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #16402 from cloud-fan/minor.

4 files changed, 18 insertions, 35 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
index 09007b7c89..d7746ca7a0 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
@@ -92,10 +92,10 @@ class CodegenContext {
    * This is for minor objects not to store the object into field but refer it from the references
    * field at the time of use because number of fields in class is limited so we should reduce it.
    */
-  def addReferenceObj(obj: Any): String = {
+  def addReferenceMinorObj(obj: Any, className: String = null): String = {
     val idx = references.length
     references += obj
-    val clsName = obj.getClass.getName
+    val clsName = Option(className).getOrElse(obj.getClass.getName)
     s"(($clsName) references[$idx])"
   }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala
index 1985e68c94..ab45c41bc0 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/literals.scala
@@ -220,7 +220,7 @@ object DecimalLiteral {
 /**
  * In order to do type checking, use Literal.create() instead of constructor
  */
-case class Literal (value: Any, dataType: DataType) extends LeafExpression with CodegenFallback {
+case class Literal (value: Any, dataType: DataType) extends LeafExpression {
 
   override def foldable: Boolean = true
   override def nullable: Boolean = value == null
@@ -271,45 +271,28 @@ case class Literal (value: Any, dataType: DataType) extends LeafExpression with
       ev.isNull = "true"
       ev.copy(s"final ${ctx.javaType(dataType)} ${ev.value} = ${ctx.defaultValue(dataType)};")
     } else {
-      dataType match {
-        case BooleanType =>
-          ev.isNull = "false"
-          ev.value = value.toString
-          ev.copy("")
+      ev.isNull = "false"
+      ev.value = dataType match {
+        case BooleanType | IntegerType | DateType => value.toString
         case FloatType =>
           val v = value.asInstanceOf[Float]
           if (v.isNaN || v.isInfinite) {
-            super[CodegenFallback].doGenCode(ctx, ev)
+            ctx.addReferenceMinorObj(v)
           } else {
-            ev.isNull = "false"
-            ev.value = s"${value}f"
-            ev.copy("")
+            s"${value}f"
           }
         case DoubleType =>
           val v = value.asInstanceOf[Double]
           if (v.isNaN || v.isInfinite) {
-            super[CodegenFallback].doGenCode(ctx, ev)
+            ctx.addReferenceMinorObj(v)
           } else {
-            ev.isNull = "false"
-            ev.value = s"${value}D"
-            ev.copy("")
+            s"${value}D"
           }
-        case ByteType | ShortType =>
-          ev.isNull = "false"
-          ev.value = s"(${ctx.javaType(dataType)})$value"
-          ev.copy("")
-        case IntegerType | DateType =>
-          ev.isNull = "false"
-          ev.value = value.toString
-          ev.copy("")
-        case TimestampType | LongType =>
-          ev.isNull = "false"
-          ev.value = s"${value}L"
-          ev.copy("")
-        // eval() version may be faster for non-primitive types
-        case other =>
-          super[CodegenFallback].doGenCode(ctx, ev)
+        case ByteType | ShortType => s"(${ctx.javaType(dataType)})$value"
+        case TimestampType | LongType => s"${value}L"
+        case other => ctx.addReferenceMinorObj(value, ctx.javaType(dataType))
       }
+      ev.copy("")
     }
   }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/misc.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/misc.scala
index a874a1cf37..bb9368cf6d 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/misc.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/misc.scala
@@ -78,7 +78,7 @@ case class AssertTrue(child: Expression) extends UnaryExpression with ImplicitCa
 
     // Use unnamed reference that doesn't create a local field here to reduce the number of fields
     // because errMsgField is used only when the value is null or false.
-    val errMsgField = ctx.addReferenceObj(errMsg)
+    val errMsgField = ctx.addReferenceMinorObj(errMsg)
     ExprCode(code = s"""${eval.code}
        |if (${eval.isNull} || !${eval.value}) {
        |  throw new RuntimeException($errMsgField);
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala
index fc323693a2..36bf3017d4 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala
@@ -961,7 +961,7 @@ case class AssertNotNull(child: Expression, walkedTypePath: Seq[String])
 
     // Use unnamed reference that doesn't create a local field here to reduce the number of fields
     // because errMsgField is used only when the value is null.
-    val errMsgField = ctx.addReferenceObj(errMsg)
+    val errMsgField = ctx.addReferenceMinorObj(errMsg)
 
     val code = s"""
       ${childGen.code}
@@ -998,7 +998,7 @@ case class GetExternalRowField(
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     // Use unnamed reference that doesn't create a local field here to reduce the number of fields
     // because errMsgField is used only when the field is null.
-    val errMsgField = ctx.addReferenceObj(errMsg)
+    val errMsgField = ctx.addReferenceMinorObj(errMsg)
     val row = child.genCode(ctx)
     val code = s"""
       ${row.code}
@@ -1038,7 +1038,7 @@ case class ValidateExternalType(child: Expression, expected: DataType)
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     // Use unnamed reference that doesn't create a local field here to reduce the number of fields
     // because errMsgField is used only when the type doesn't match.
-    val errMsgField = ctx.addReferenceObj(errMsg)
+    val errMsgField = ctx.addReferenceMinorObj(errMsg)
     val input = child.genCode(ctx)
     val obj = input.value