Allow to reuse table of a util.{MutableHashSet,MutableHashMap}

On demand, fill the array with zeroes instead of creating a fresh one.
This can save some array allocations.

Author: odersky

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

@@ -360,7 +360,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
     def adjustReferenced(bound: Type, isLower: Boolean, add: Boolean) =
       adjuster.variance = if isLower then 1 else -1
       adjuster.add = add
-      adjuster.seen.clear()
+      adjuster.seen.clear(resetToInitial = false)
       adjuster.traverse(bound)
 
     /** Use an optimized strategy to adjust dependencies to account for the delta

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -630,7 +630,7 @@ trait ImplicitRunInfo:
 
       def apply(tp: Type): collection.Set[Type] =
         parts = mutable.LinkedHashSet()
-        partSeen.clear()
+        partSeen.clear(resetToInitial = false)
         traverse(tp)
         parts
     end collectParts

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

@@ -42,9 +42,10 @@ abstract class GenericHashMap[Key, Value]
     else 1 << (32 - Integer.numberOfLeadingZeros(n))
 
   /** Remove all elements from this table and set back to initial configuration */
-  def clear(): Unit =
+  def clear(resetToInitial: Boolean): Unit =
     used = 0
-    allocate(roundToPower(initialCapacity))
+    if resetToInitial then allocate(roundToPower(initialCapacity))
+    else java.util.Arrays.fill(table, null)
 
   /** The number of elements in the set */
   def size: Int = used

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

@@ -44,11 +44,10 @@ class HashSet[T](initialCapacity: Int = 8, capacityMultiple: Int = 2) extends Mu
     else if Integer.bitCount(n) == 1 then n
     else 1 << (32 - Integer.numberOfLeadingZeros(n))
 
-  /** Remove all elements from this set and set back to initial configuration */
-  def clear(): Unit = {
+  def clear(resetToInitial: Boolean): Unit =
     used = 0
-    allocate(roundToPower(initialCapacity))
-  }
+    if resetToInitial then allocate(roundToPower(initialCapacity))
+    else java.util.Arrays.fill(table, null)
 
   /** The number of elements in the set */
   def size: Int = used

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

@@ -13,6 +13,10 @@ abstract class MutableMap[Key, Value] extends ReadOnlyMap[Key, Value]:
     remove(k)
     this
 
-  def clear(): Unit
+  /** Remove all bindings from this map.
+   *  @param resetToInitial If true, set back to initial configuration, which includes
+   *                        reallocating tables.
+   */
+  def clear(resetToInitial: Boolean = true): Unit
 
   def getOrElseUpdate(key: Key, value: => Value): Value

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

@@ -15,7 +15,11 @@ abstract class MutableSet[T] extends ReadOnlySet[T]:
   /** Remove element `x` from the set */
   def -=(x: T): Unit
 
-  def clear(): Unit
+  /** Remove all elements from this set.
+   *  @param resetToInitial If true, set back to initial configuration, which includes
+   *                        reallocating tables.
+   */
+  def clear(resetToInitial: Boolean = true): Unit
 
   def ++= (xs: IterableOnce[T]): Unit =
     xs.iterator.foreach(this += _)

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

@@ -204,7 +204,7 @@ abstract class WeakHashSet[A <: AnyRef](initialCapacity: Int = 8, loadFactor: Do
       linkedListLoop(null, table(bucket))
   }
 
-  def clear(): Unit = {
+  def clear(resetToInitial: Boolean): Unit = {
     table = new Array[Entry[A] | Null](table.size)
     threshold = computeThreshold
     count = 0