Use an optimized HashTable instead of a LinearTable

The two implementation classes of LinearTable had so much in common that
they could be combined. Since the implementation is now fixed, we can
use the usual mutable interface for a table.

Author: odersky

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

@@ -1552,14 +1552,14 @@ object SymDenotations {
     initPrivateWithin: Symbol)
     extends SymDenotation(symbol, maybeOwner, name, initFlags, initInfo, initPrivateWithin) {
 
-    import util.LinearTable
+    import util.HashTable
 
     // ----- caches -------------------------------------------------------
 
     private var myTypeParams: List[TypeSymbol] = null
     private var fullNameCache: SimpleIdentityMap[QualifiedNameKind, Name] = SimpleIdentityMap.Empty
 
-    private var myMemberCache: LinearTable[Name, PreDenotation] = null
+    private var myMemberCache: HashTable[Name, PreDenotation] = null
     private var myMemberCachePeriod: Period = Nowhere
 
     /** A cache from types T to baseType(T, C) */
@@ -1570,9 +1570,9 @@ object SymDenotations {
     private var baseDataCache: BaseData = BaseData.None
     private var memberNamesCache: MemberNames = MemberNames.None
 
-    private def memberCache(using Context): LinearTable[Name, PreDenotation] = {
+    private def memberCache(using Context): HashTable[Name, PreDenotation] = {
       if (myMemberCachePeriod != ctx.period) {
-        myMemberCache = LinearTable.empty
+        myMemberCache = HashTable()
         myMemberCachePeriod = ctx.period
       }
       myMemberCache
@@ -1859,7 +1859,7 @@ object SymDenotations {
         var denots: PreDenotation = memberCache.lookup(name)
         if (denots == null) {
           denots = computeNPMembersNamed(name)
-          myMemberCache = memberCache.enter(name, denots)
+          memberCache.enter(name, denots)
         }
         else if (Config.checkCacheMembersNamed) {
           val denots1 = computeNPMembersNamed(name)

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

@@ -0,0 +1,139 @@
+package dotty.tools.dotc.util
+
+object HashTable:
+  inline val MaxDense = 8
+
+/** A hash table using open hashing with linear scan which is also very space efficient
+ *  at small sizes.
+ *  @param  initialCapacity  Indicates the initial number of slots in the hash table.
+ *                           The actual number of slots is always a power of 2, so the
+ *                           initial size of the table will be the smallest power of two
+ *                           that is equal or greater than the given `initialCapacity`.
+ *                           Minimum value is 4.
+ *  @param  loadFactor       The maximum fraction of used elements relative to capacity.
+ *                           The hash table will be re-sized once the number of elements exceeds
+ *                           the current size of the hash table multiplied by loadFactor.
+ *                           However, a table of size up to MaxDense will be re-sized to only
+ *                           once the number of elements reaches the table's size.
+ */
+class HashTable[Key >: Null <: AnyRef, Value >: Null <: AnyRef]
+    (initialCapacity: Int = 8: Int, loadFactor: Float = 0.33f):
+  import HashTable.MaxDense
+  private var used: Int = _
+  private var limit: Int = _
+  private var table: Array[AnyRef] = _
+  clear()
+
+  private def allocate(capacity: Int) =
+    table = new Array[AnyRef](capacity * 2)
+    limit = if capacity <= MaxDense then capacity - 1 else (capacity * loadFactor).toInt
+
+  private def roundToPower(n: Int) =
+    if Integer.bitCount(n) == 1 then n
+    else
+      def recur(n: Int): Int =
+        if n == 1 then 2
+        else recur(n >>> 1) << 1
+      recur(n)
+
+  /** Remove all elements from this table and set back to initial configuration */
+  def clear(): Unit =
+    used = 0
+    allocate(roundToPower(initialCapacity max 4))
+
+  /** The number of elements in the set */
+  def size: Int = used
+
+  private def isDense = limit < MaxDense
+
+  /** Hashcode, by default `System.identityHashCode`, but can be overriden */
+  protected def hash(x: Key): Int = System.identityHashCode(x)
+
+  /** Equality, by default `eq`,  but can be overridden */
+  protected def isEqual(x: Key, y: Key): Boolean = x eq y
+
+  /** Turn hashcode `x` into a table index */
+  private def index(x: Int): Int = x & (table.length - 2)
+
+  private def firstIndex(key: Key) = if isDense then 0 else index(hash(key))
+  private def nextIndex(idx: Int) = index(idx + 2)
+
+  private def keyAt(idx: Int): Key = table(idx).asInstanceOf[Key]
+  private def valueAt(idx: Int): Value = table(idx + 1).asInstanceOf[Value]
+
+  /** Find entry such that `isEqual(x, entry)`. If it exists, return it.
+   *  If not, enter `x` in set and return `x`.
+   */
+  def lookup(key: Key): Value =
+    var idx = firstIndex(key)
+    var k = keyAt(idx)
+    while k != null do
+      if isEqual(k, key) then return valueAt(idx)
+      idx = nextIndex(idx)
+      k = keyAt(idx)
+    null
+
+  def enter(key: Key, value: Value): Unit =
+    var idx = firstIndex(key)
+    var k = keyAt(idx)
+    while k != null do
+      if isEqual(k, key) then
+        table(idx + 1) = value
+        return
+      idx = nextIndex(idx)
+      k = keyAt(idx)
+    table(idx) = key
+    table(idx + 1) = value
+    used += 1
+    if used > limit then growTable()
+
+  def invalidate(key: Key): Unit =
+    var idx = firstIndex(key)
+    var k = keyAt(idx)
+    while k != null do
+      if isEqual(k, key) then
+        var hole = idx
+        if !isDense then
+          while
+            idx = nextIndex(idx)
+            k = keyAt(idx)
+            k != null && index(hash(k)) != idx
+          do
+            table(hole) = k
+            table(hole + 1) = valueAt(idx)
+            hole = idx
+        table(hole) = null
+        used -= 1
+        return
+      idx = nextIndex(idx)
+      k = keyAt(idx)
+
+  private def addOld(key: Key, value: AnyRef): Unit =
+    var idx = firstIndex(key)
+    var k = keyAt(idx)
+    while k != null do
+      idx = nextIndex(idx)
+      k = keyAt(idx)
+    table(idx) = key
+    table(idx + 1) = value
+
+  private def growTable(): Unit =
+    val oldTable = table
+    allocate(table.length)
+    if isDense then
+      Array.copy(oldTable, 0, table, 0, oldTable.length)
+    else
+      var idx = 0
+      while idx < oldTable.length do
+        val key = oldTable(idx).asInstanceOf[Key]
+        if key != null then addOld(key, oldTable(idx + 1))
+        idx += 2
+
+  def iterator: Iterator[(Key, Value)] =
+    for idx <- (0 until table.length by 2).iterator
+        if keyAt(idx) != null
+    yield (keyAt(idx), valueAt(idx))
+
+  override def toString: String =
+    iterator.map((k, v) => s"$k -> $v").mkString("LinearTable(", ", ", ")")
+end HashTable