Optimise ClassTag lookup

  - Optimize ClassTag.apply to avoid testing for primitive classes one-by-one.
  - Share instance of ClassTag in a ClassValue based cache and rely on
    this in the compiler where we had previously hoisted hot instances.

Author: retronym

library/src/scala/reflect/ClassTag.scala

@@ -121,29 +121,38 @@ object ClassTag {
   val Nothing : ClassTag[scala.Nothing]    = Manifest.Nothing
   val Null    : ClassTag[scala.Null]       = Manifest.Null
 
+  private[this] val cache = new ClassValue[ClassTag[_]] {
+    override def computeValue(runtimeClass: jClass[_]): ClassTag[_] = {
+      runtimeClass match {
+        case x if x.isPrimitive => primitiveClassTag(runtimeClass)
+        case ObjectTYPE         => ClassTag.Object
+        case NothingTYPE        => ClassTag.Nothing
+        case NullTYPE           => ClassTag.Null
+        case _                  => new GenericClassTag[AnyRef](runtimeClass)
+      }
+    }
+
+    private def primitiveClassTag[T](runtimeClass: Class[_]): ClassTag[_] = runtimeClass match {
+      case java.lang.Byte.TYPE      => ClassTag.Byte
+      case java.lang.Short.TYPE     => ClassTag.Short
+      case java.lang.Character.TYPE => ClassTag.Char
+      case java.lang.Integer.TYPE   => ClassTag.Int
+      case java.lang.Long.TYPE      => ClassTag.Long
+      case java.lang.Float.TYPE     => ClassTag.Float
+      case java.lang.Double.TYPE    => ClassTag.Double
+      case java.lang.Boolean.TYPE   => ClassTag.Boolean
+      case java.lang.Void.TYPE      => ClassTag.Unit
+    }
+  }
+
   @SerialVersionUID(1L)
   private class GenericClassTag[T](val runtimeClass: jClass[_]) extends ClassTag[T] {
     override def newArray(len: Int): Array[T] = {
       java.lang.reflect.Array.newInstance(runtimeClass, len).asInstanceOf[Array[T]]
     }
   }
 
-  def apply[T](runtimeClass1: jClass[_]): ClassTag[T] =
-    runtimeClass1 match {
-      case java.lang.Byte.TYPE      => ClassTag.Byte.asInstanceOf[ClassTag[T]]
-      case java.lang.Short.TYPE     => ClassTag.Short.asInstanceOf[ClassTag[T]]
-      case java.lang.Character.TYPE => ClassTag.Char.asInstanceOf[ClassTag[T]]
-      case java.lang.Integer.TYPE   => ClassTag.Int.asInstanceOf[ClassTag[T]]
-      case java.lang.Long.TYPE      => ClassTag.Long.asInstanceOf[ClassTag[T]]
-      case java.lang.Float.TYPE     => ClassTag.Float.asInstanceOf[ClassTag[T]]
-      case java.lang.Double.TYPE    => ClassTag.Double.asInstanceOf[ClassTag[T]]
-      case java.lang.Boolean.TYPE   => ClassTag.Boolean.asInstanceOf[ClassTag[T]]
-      case java.lang.Void.TYPE      => ClassTag.Unit.asInstanceOf[ClassTag[T]]
-      case ObjectTYPE               => ClassTag.Object.asInstanceOf[ClassTag[T]]
-      case NothingTYPE              => ClassTag.Nothing.asInstanceOf[ClassTag[T]]
-      case NullTYPE                 => ClassTag.Null.asInstanceOf[ClassTag[T]]
-      case _                        => new GenericClassTag[T](runtimeClass1)
-    }
+  def apply[T](runtimeClass1: jClass[_]): ClassTag[T] = cache.get(runtimeClass1).asInstanceOf[ClassTag[T]]
 
   def unapply[T](ctag: ClassTag[T]): Option[Class[_]] = Some(ctag.runtimeClass)
 }