Micro-optimization: Symbol#denot

Author: odersky

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

@@ -398,19 +398,30 @@ object Symbols {
 
     /** The last denotation of this symbol */
     private[this] var lastDenot: SymDenotation = _
+    private[this] var checkedPeriod: Period = 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
+      val lastd = lastDenot
+      if (checkedPeriod == ctx.period) lastd
+      else computeDenot(lastd)
+    }
+
+    private def computeDenot(lastd: SymDenotation)(implicit ctx: Context): SymDenotation = {
+      val now = ctx.period
+      checkedPeriod = now
+      if (lastd.validFor contains now) lastd
+      else {
+        val newd = lastd.current.asInstanceOf[SymDenotation]
+        lastDenot = newd
+        newd
       }
-      denot
     }
 
     /** The initial denotation of this symbol, without going through `current` */