Micro-optimization: Symbol#denot

Author: odersky

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

@@ -668,8 +668,10 @@ object Denotations {
     private[this] var myValidFor: Period = Nowhere
 
     def validFor = myValidFor
-    def validFor_=(p: Period) =
+    def validFor_=(p: Period) = {
       myValidFor = p
+      symbol.invalidateDenotCache()
+    }
 
     /** The next SingleDenotation in this run, with wrap-around from last to first.
      *
@@ -774,7 +776,10 @@ object Denotations {
 
       if (valid.runId != currentPeriod.runId)
         if (exists) initial.bringForward().current
-        else this
+        else {
+          myValidFor = Period.allInRun(currentPeriod.runId)
+          this
+        }
       else {
         var cur = this
         if (currentPeriod.code > valid.code) {

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

@@ -398,19 +398,36 @@ object Symbols {
 
     /** The last denotation of this symbol */
     private[this] var lastDenot: SymDenotation = _
+    private[this] var checkedPeriod: Period = Nowhere
+
+    private[core] def invalidateDenotCache() = { checkedPeriod = Nowhere }
 
     /** Set the denotation of this symbol */
-    private[core] def denot_=(d: SymDenotation) =
+    private[core] def denot_=(d: SymDenotation) = {
       lastDenot = d
+      checkedPeriod = Nowhere
+    }
 
     /** The current denotation of this symbol */
     final def denot(implicit ctx: Context): SymDenotation = {
-      var denot = lastDenot
-      if (!(denot.validFor contains ctx.period)) {
-        denot = denot.current.asInstanceOf[SymDenotation]
-        lastDenot = denot
-      }
-      denot
+      val lastd = lastDenot
+      if (checkedPeriod == ctx.period) lastd
+      else computeDenot(lastd)
+    }
+
+    private def computeDenot(lastd: SymDenotation)(implicit ctx: Context): SymDenotation = {
+      val now = ctx.period
+      if (lastd.validFor contains now) {
+        checkedPeriod = now
+        lastd
+      } else recomputeDenot(lastd, now)
+    }
+
+    private def recomputeDenot(lastd: SymDenotation, now: Period)(implicit ctx: Context): SymDenotation = {
+      val newd = lastd.current.asInstanceOf[SymDenotation]
+      lastDenot = newd
+      checkedPeriod = now
+      newd
     }
 
     /** The initial denotation of this symbol, without going through `current` */
@@ -631,7 +648,6 @@ object Symbols {
 
   @sharable object NoSymbol extends Symbol(NoCoord, 0) {
     denot = NoDenotation
-
     override def associatedFile(implicit ctx: Context): AbstractFile = NoSource.file
   }