Increase loadFactor

Reduce capacityMultiple for HashSets from 4 to 2 and for HashMaps from 3 to 2.
We observed not a large increase in misses, for half the space used, and consequently
half as many re-enter operations.

Author: odersky

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -174,10 +174,10 @@ object Config {
   /** If set, enables tracing */
   final val tracingEnabled = false
 
-  /** Initial capacity of uniques HashMap.
-   *  Note: This MUST BE a power of two to work with util.HashSet
+  /** Initial capacity of the uniques HashMap.
+   *  Note: This should be a power of two to work with util.HashSet
    */
-  final val initialUniquesCapacity = 65536
+  final val initialUniquesCapacity = 0x8000
 
   /** How many recursive calls to NamedType#underlying are performed before logging starts. */
   final val LogPendingUnderlyingThreshold = 50

compiler/src/dotty/tools/dotc/core/Uniques.scala

@@ -32,7 +32,7 @@ object Uniques:
     if tp.hash == NotCached then tp
     else ctx.uniques.put(tp).asInstanceOf[T]
 
-  final class NamedTypeUniques extends HashSet[NamedType](Config.initialUniquesCapacity) with Hashable:
+  final class NamedTypeUniques extends HashSet[NamedType](Config.initialUniquesCapacity * 4) with Hashable:
     override def hash(x: NamedType): Int = x.hash
 
     def enterIfNew(prefix: Type, designator: Designator, isTerm: Boolean)(using Context): NamedType =
@@ -53,7 +53,7 @@ object Uniques:
         addEntryAt(idx, newType)
   end NamedTypeUniques
 
-  final class AppliedUniques extends HashSet[AppliedType](Config.initialUniquesCapacity) with Hashable:
+  final class AppliedUniques extends HashSet[AppliedType](Config.initialUniquesCapacity * 2) with Hashable:
     override def hash(x: AppliedType): Int = x.hash
 
     def enterIfNew(tycon: Type, args: List[Type]): AppliedType =

compiler/src/dotty/tools/dotc/util/HashSet.scala

@@ -19,7 +19,7 @@ object HashSet:
  *                           However, a table of size up to DenseLimit will be re-sized only
  *                           once the number of elements reaches the table's size.
  */
-class HashSet[T >: Null <: AnyRef](initialCapacity: Int = 8, capacityMultiple: Int = 4) extends MutableSet[T] {
+class HashSet[T >: Null <: AnyRef](initialCapacity: Int = 8, capacityMultiple: Int = 2) extends MutableSet[T] {
   import HashSet.DenseLimit
 
   private var used: Int = _
@@ -57,6 +57,8 @@ class HashSet[T >: Null <: AnyRef](initialCapacity: Int = 8, capacityMultiple: I
   /** Turn hashcode `x` into a table index */
   protected def index(x: Int): Int = x & (table.length - 1)
 
+  protected def currentTable: Array[AnyRef] = table
+
   protected def firstIndex(x: T) = if isDense then 0 else index(hash(x))
   protected def nextIndex(idx: Int) =
     Stats.record(statsItem("miss"))
@@ -136,7 +138,7 @@ class HashSet[T >: Null <: AnyRef](initialCapacity: Int = 8, capacityMultiple: I
   protected def growTable(): Unit =
     val oldTable = table
     val newLength =
-      if oldTable.length == DenseLimit then DenseLimit * roundToPower(capacityMultiple)
+      if oldTable.length == DenseLimit then DenseLimit * 2 * roundToPower(capacityMultiple)
       else table.length * 2
     allocate(newLength)
     copyFrom(oldTable)