[SPARK-8117] [SQL] Push codegen implementation into each Expression

This PR move codegen implementation of expressions into Expression class itself, make it easy to manage.

It introduces two APIs in Expression:
```
def gen(ctx: CodeGenContext): GeneratedExpressionCode
def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code
```

gen(ctx) will call genSource(ctx, ev) to generate Java source code for the current expression. A expression needs to override genSource().

Here are the types:
```
type Term String
type Code String

/**
 * Java source for evaluating an [[Expression]] given a [[Row]] of input.
 */
case class GeneratedExpressionCode(var code: Code,
                               nullTerm: Term,
                               primitiveTerm: Term,
                               objectTerm: Term)
/**
 * A context for codegen, which is used to bookkeeping the expressions those are not supported
 * by codegen, then they are evaluated directly. The unsupported expression is appended at the
 * end of `references`, the position of it is kept in the code, used to access and evaluate it.
 */
class CodeGenContext {
  /**
   * Holding all the expressions those do not support codegen, will be evaluated directly.
   */
  val references: Seq[Expression] = new mutable.ArrayBuffer[Expression]()
}
```

This is basically apache#6660, but fixed style violation and compilation failure.

Author: Davies Liu <davies@databricks.com>
Author: Reynold Xin <rxin@databricks.com>

Closes apache#6690 from rxin/codegen and squashes the following commits:

e1368c2 [Reynold Xin] Fixed tests.
73db80e [Reynold Xin] Fixed compilation failure.
19d6435 [Reynold Xin] Fixed style violation.
9adaeaf [Davies Liu] address comments
f42c732 [Davies Liu] improve coverage and tests
bad6828 [Davies Liu] address comments
e03edaa [Davies Liu] consts fold
86fac2c [Davies Liu] fix style
02262c9 [Davies Liu] address comments
b5d3617 [Davies Liu] Merge pull request #5 from rxin/codegen
48c454f [Reynold Xin] Some code gen update.
2344bc0 [Davies Liu] fix test
12ff88a [Davies Liu] fix build
c5fb514 [Davies Liu] rename
8c6d82d [Davies Liu] update docs
b145047 [Davies Liu] fix style
e57959d [Davies Liu] add type alias
3ff25f8 [Davies Liu] refactor
593d617 [Davies Liu] pushing codegen into Expression

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

@@ -19,6 +19,7 @@ package org.apache.spark.sql.catalyst.expressions
 
 import org.apache.spark.Logging
 import org.apache.spark.sql.catalyst.errors.attachTree
+import org.apache.spark.sql.catalyst.expressions.codegen.{GeneratedExpressionCode, Code, CodeGenContext}
 import org.apache.spark.sql.types._
 import org.apache.spark.sql.catalyst.trees
 
@@ -41,6 +42,14 @@ case class BoundReference(ordinal: Int, dataType: DataType, nullable: Boolean)
   override def qualifiers: Seq[String] = throw new UnsupportedOperationException
 
   override def exprId: ExprId = throw new UnsupportedOperationException
+
+  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
+    s"""
+        boolean ${ev.isNull} = i.isNullAt($ordinal);
+        ${ctx.javaType(dataType)} ${ev.primitive} = ${ev.isNull} ?
+            ${ctx.defaultValue(dataType)} : (${ctx.getColumn(dataType, ordinal)});
+    """
+  }
 }
 
 object BindReferences extends Logging {

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

@@ -21,6 +21,7 @@ import java.sql.{Date, Timestamp}
 import java.text.{DateFormat, SimpleDateFormat}
 
 import org.apache.spark.Logging
+import org.apache.spark.sql.catalyst.expressions.codegen.{GeneratedExpressionCode, Code, CodeGenContext}
 import org.apache.spark.sql.catalyst.util.DateUtils
 import org.apache.spark.sql.types._
 
@@ -433,6 +434,47 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression w
     val evaluated = child.eval(input)
     if (evaluated == null) null else cast(evaluated)
   }
+
+  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
+    // TODO(cg): Add support for more data types.
+    (child.dataType, dataType) match {
+
+      case (BinaryType, StringType) =>
+        defineCodeGen (ctx, ev, c =>
+          s"new ${ctx.stringType}().set($c)")
+      case (DateType, StringType) =>
+        defineCodeGen(ctx, ev, c =>
+          s"""new ${ctx.stringType}().set(
+                org.apache.spark.sql.catalyst.util.DateUtils.toString($c))""")
+      // Special handling required for timestamps in hive test cases since the toString function
+      // does not match the expected output.
+      case (TimestampType, StringType) =>
+        super.genCode(ctx, ev)
+      case (_, StringType) =>
+        defineCodeGen(ctx, ev, c => s"new ${ctx.stringType}().set(String.valueOf($c))")
+
+      // fallback for DecimalType, this must be before other numeric types
+      case (_, dt: DecimalType) =>
+        super.genCode(ctx, ev)
+
+      case (BooleanType, dt: NumericType) =>
+        defineCodeGen(ctx, ev, c => s"(${ctx.javaType(dt)})($c ? 1 : 0)")
+      case (dt: DecimalType, BooleanType) =>
+        defineCodeGen(ctx, ev, c => s"$c.isZero()")
+      case (dt: NumericType, BooleanType) =>
+        defineCodeGen(ctx, ev, c => s"$c != 0")
+
+      case (_: DecimalType, IntegerType) =>
+        defineCodeGen(ctx, ev, c => s"($c).toInt()")
+      case (_: DecimalType, dt: NumericType) =>
+        defineCodeGen(ctx, ev, c => s"($c).to${ctx.boxedType(dt)}()")
+      case (_: NumericType, dt: NumericType) =>
+        defineCodeGen(ctx, ev, c => s"(${ctx.javaType(dt)})($c)")
+
+      case other =>
+        super.genCode(ctx, ev)
+    }
+  }
 }
 
 object Cast {

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

@@ -18,6 +18,7 @@
 package org.apache.spark.sql.catalyst.expressions
 
 import org.apache.spark.sql.catalyst.analysis.{TypeCheckResult, UnresolvedAttribute}
+import org.apache.spark.sql.catalyst.expressions.codegen.{GeneratedExpressionCode, Code, CodeGenContext, Term}
 import org.apache.spark.sql.catalyst.trees
 import org.apache.spark.sql.catalyst.trees.TreeNode
 import org.apache.spark.sql.types._
@@ -51,6 +52,44 @@ abstract class Expression extends TreeNode[Expression] {
   /** Returns the result of evaluating this expression on a given input Row */
   def eval(input: Row = null): Any
 
+  /**
+   * Returns an [[GeneratedExpressionCode]], which contains Java source code that
+   * can be used to generate the result of evaluating the expression on an input row.
+   *
+   * @param ctx a [[CodeGenContext]]
+   * @return [[GeneratedExpressionCode]]
+   */
+  def gen(ctx: CodeGenContext): GeneratedExpressionCode = {
+    val isNull = ctx.freshName("isNull")
+    val primitive = ctx.freshName("primitive")
+    val ve = GeneratedExpressionCode("", isNull, primitive)
+    ve.code = genCode(ctx, ve)
+    ve
+  }
+
+  /**
+   * Returns Java source code that can be compiled to evaluate this expression.
+   * The default behavior is to call the eval method of the expression. Concrete expression
+   * implementations should override this to do actual code generation.
+   *
+   * @param ctx a [[CodeGenContext]]
+   * @param ev an [[GeneratedExpressionCode]] with unique terms.
+   * @return Java source code
+   */
+  protected def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
+    ctx.references += this
+    val objectTerm = ctx.freshName("obj")
+    s"""
+      /* expression: ${this} */
+      Object ${objectTerm} = expressions[${ctx.references.size - 1}].eval(i);
+      boolean ${ev.isNull} = ${objectTerm} == null;
+      ${ctx.javaType(this.dataType)} ${ev.primitive} = ${ctx.defaultValue(this.dataType)};
+      if (!${ev.isNull}) {
+        ${ev.primitive} = (${ctx.boxedType(this.dataType)})${objectTerm};
+      }
+    """
+  }
+
   /**
    * Returns `true` if this expression and all its children have been resolved to a specific schema
    * and input data types checking passed, and `false` if it still contains any unresolved
@@ -116,6 +155,41 @@ abstract class BinaryExpression extends Expression with trees.BinaryNode[Express
   override def nullable: Boolean = left.nullable || right.nullable
 
   override def toString: String = s"($left $symbol $right)"
+
+  /**
+   * Short hand for generating binary evaluation code, which depends on two sub-evaluations of
+   * the same type.  If either of the sub-expressions is null, the result of this computation
+   * is assumed to be null.
+   *
+   * @param f accepts two variable names and returns Java code to compute the output.
+   */
+  protected def defineCodeGen(
+      ctx: CodeGenContext,
+      ev: GeneratedExpressionCode,
+      f: (Term, Term) => Code): String = {
+    // TODO: Right now some timestamp tests fail if we enforce this...
+    if (left.dataType != right.dataType) {
+      // log.warn(s"${left.dataType} != ${right.dataType}")
+    }
+
+    val eval1 = left.gen(ctx)
+    val eval2 = right.gen(ctx)
+    val resultCode = f(eval1.primitive, eval2.primitive)
+
+    s"""
+      ${eval1.code}
+      boolean ${ev.isNull} = ${eval1.isNull};
+      ${ctx.javaType(dataType)} ${ev.primitive} = ${ctx.defaultValue(dataType)};
+      if (!${ev.isNull}) {
+        ${eval2.code}
+        if(!${eval2.isNull}) {
+          ${ev.primitive} = $resultCode;
+        } else {
+          ${ev.isNull} = true;
+        }
+      }
+    """
+  }
 }
 
 private[sql] object BinaryExpression {
@@ -128,6 +202,32 @@ abstract class LeafExpression extends Expression with trees.LeafNode[Expression]
 
 abstract class UnaryExpression extends Expression with trees.UnaryNode[Expression] {
   self: Product =>
+
+  /**
+   * Called by unary expressions to generate a code block that returns null if its parent returns
+   * null, and if not not null, use `f` to generate the expression.
+   *
+   * As an example, the following does a boolean inversion (i.e. NOT).
+   * {{{
+   *   defineCodeGen(ctx, ev, c => s"!($c)")
+   * }}}
+   *
+   * @param f function that accepts a variable name and returns Java code to compute the output.
+   */
+  protected def defineCodeGen(
+      ctx: CodeGenContext,
+      ev: GeneratedExpressionCode,
+      f: Term => Code): Code = {
+    val eval = child.gen(ctx)
+    // reuse the previous isNull
+    ev.isNull = eval.isNull
+    eval.code + s"""
+      ${ctx.javaType(dataType)} ${ev.primitive} = ${ctx.defaultValue(dataType)};
+      if (!${ev.isNull}) {
+        ${ev.primitive} = ${f(eval.primitive)};
+      }
+    """
+  }
 }
 
 // TODO Semantically we probably not need GroupExpression