[SPARK-28916][SQL] Split subexpression elimination functions code for Generate[Mutable|Unsafe]Projection

### What changes were proposed in this pull request?

The PR proposes to split the code for subexpression elimination before inlining the function calls all in the apply method for `Generate[Mutable|Unsafe]Projection`.

### Why are the changes needed?

Before this PR, code generation can fail due to the 64KB code size limit if a lot of subexpression elimination functions are generated. The added UT is a reproducer for the issue (thanks to the JIRA reporter and HyukjinKwon for it).

### Does this PR introduce any user-facing change?

No.

### How was this patch tested?

added UT

Closes #25642 from mgaido91/SPARK-28916.

Authored-by: Marco Gaido <marcogaido91@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -403,13 +403,14 @@ class CodegenContext {
    *  equivalentExpressions will match the tree containing `col1 + col2` and it will only
    *  be evaluated once.
    */
-  val equivalentExpressions: EquivalentExpressions = new EquivalentExpressions
+  private val equivalentExpressions: EquivalentExpressions = new EquivalentExpressions
 
   // Foreach expression that is participating in subexpression elimination, the state to use.
-  var subExprEliminationExprs = Map.empty[Expression, SubExprEliminationState]
+  // Visible for testing.
+  private[expressions] var subExprEliminationExprs = Map.empty[Expression, SubExprEliminationState]
 
   // The collection of sub-expression result resetting methods that need to be called on each row.
-  val subexprFunctions = mutable.ArrayBuffer.empty[String]
+  private val subexprFunctions = mutable.ArrayBuffer.empty[String]
 
   val outerClassName = "OuterClass"
 
@@ -993,6 +994,15 @@ class CodegenContext {
     }
   }
 
+  /**
+   * Returns the code for subexpression elimination after splitting it if necessary.
+   */
+  def subexprFunctionsCode: String = {
+    // Whole-stage codegen's subexpression elimination is handled in another code path
+    assert(currentVars == null || subexprFunctions.isEmpty)
+    splitExpressions(subexprFunctions, "subexprFunc_split", Seq("InternalRow" -> INPUT_ROW))
+  }
+
   /**
    * Perform a function which generates a sequence of ExprCodes with a given mapping between
    * expressions and common expressions, instead of using the mapping in current context.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/GenerateMutableProjection.scala

@@ -92,7 +92,7 @@ object GenerateMutableProjection extends CodeGenerator[Seq[Expression], MutableP
     }
 
     // Evaluate all the subexpressions.
-    val evalSubexpr = ctx.subexprFunctions.mkString("\n")
+    val evalSubexpr = ctx.subexprFunctionsCode
 
     val allProjections = ctx.splitExpressionsWithCurrentInputs(projectionCodes.map(_._1))
     val allUpdates = ctx.splitExpressionsWithCurrentInputs(projectionCodes.map(_._2))

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/GenerateUnsafeProjection.scala

@@ -299,7 +299,7 @@ object GenerateUnsafeProjection extends CodeGenerator[Seq[Expression], UnsafePro
       v => s"$v = new $rowWriterClass(${expressions.length}, ${numVarLenFields * 32});")
 
     // Evaluate all the subexpression.
-    val evalSubexpr = ctx.subexprFunctions.mkString("\n")
+    val evalSubexpr = ctx.subexprFunctionsCode
 
     val writeExpressions = writeExpressionsToBuffer(
       ctx, ctx.INPUT_ROW, exprEvals, exprSchemas, rowWriter, isTopLevel = true)

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CodeGenerationSuite.scala

@@ -545,6 +545,18 @@ class CodeGenerationSuite extends SparkFunSuite with ExpressionEvalHelper {
     }
     assert(appender.seenMessage)
   }
+
+  test("SPARK-28916: subexrepssion elimination can cause 64kb code limit on UnsafeProjection") {
+    val numOfExprs = 10000
+    val exprs = (0 to numOfExprs).flatMap(colIndex =>
+      Seq(Add(BoundReference(colIndex, DoubleType, true),
+        BoundReference(numOfExprs + colIndex, DoubleType, true)),
+        Add(BoundReference(colIndex, DoubleType, true),
+          BoundReference(numOfExprs + colIndex, DoubleType, true))))
+    // these should not fail to compile due to 64K limit
+    GenerateUnsafeProjection.generate(exprs, true)
+    GenerateMutableProjection.generate(exprs, true)
+  }
 }
 
 case class HugeCodeIntExpression(value: Int) extends Expression {