CodeGen-Expression

BoundReference: override def genCode(ctx: CodegenContext, ev: ExprCode): String = {

val javaType = ctx.javaType(dataType)
// return the code to retrieve a vlue from the current input row at the ordinal.
val value = ctx.getValue(ctx.INPUT_ROW, dataType, ordinal.toString)
// if we already have everything ready.
if (ctx.currentVars != null && ctx.currentVars(ordinal) != null) {
  val oev = ctx.currentVars(ordinal)
  ev.isNull = oev.isNull
  ev.value = oev.value
  val code = oev.code
  // the code to retrieve the value has already invoked, no need to call it again.
  oev.code = ""
  code
} else if (nullable) {
  s"""
    boolean ${ev.isNull} = ${ctx.INPUT_ROW}.isNullAt($ordinal);
    $javaType ${ev.value} = ${ev.isNull} ? ${ctx.defaultValue(dataType)} : ($value);
  """
} else {
  ev.isNull = "false"
  s"""
    $javaType ${ev.value} = $value;
  """
}

} }

Now let's look at CreateStruct:

override def genCode(ctx: CodegenContext, ev: ExprCode): String = {

  val rowClass = classOf[GenericInternalRow].getName
val values = ctx.freshName("values")
s"""
  boolean ${ev.isNull} = false;
  final Object[] $values = new Object[${children.size}];
""" +
  children.zipWithIndex.map { case (e, i) =>
    val eval = e.gen(ctx)
    eval.code + s"""
    // set the value in the array at the ordinal
      if (${eval.isNull}) {
        $values[$i] = null;
      } else {
        $values[$i] = ${eval.value};
      }
     """
  }.mkString("\n") +
  // Construct the GenericInternalRow with the input array (values) retrieved above.
  s"final InternalRow ${ev.value} = new $rowClass($values);"

}

Now let's look at top level Expression:

def gen(ctx: CodegenContext): ExprCode = {
  ctx.subExprEliminationExprs.get(this).map { subExprState =>
    // This expression is repeated meaning the code to evaluated has already been added
    // as a function and called in advance. Just use it.
    val code = s"/* ${toCommentSafeString(this.toString)} */"
    ExprCode(code, subExprState.isNull, subExprState.value)
  }.getOrElse {
    val isNull = ctx.freshName("isNull")
    val value = ctx.freshName("value")
    // We construct a new ExprCode, and genCode will fill the code, which also 
    // set the value, null etc.
    val ve = ExprCode("", isNull, value)
    ve.code = genCode(ctx, ve)
    if (ve.code != "") {
      // Add `this` in the comment.
      ve.copy(s"/* ${toCommentSafeString(this.toString)} */\n" + ve.code.trim)
    } else {
      ve
    }
  }
}