Update period in place when transforming denotations

Author: odersky

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

@@ -313,6 +313,12 @@ object Contexts {
     final def withPhase(phase: Phase): Context =
       withPhase(phase.id)
 
+    inline def evalAt[T](phase: Phase)(inline op: Context ?=> T): T =
+      val saved = period
+      this.asInstanceOf[FreshContext].period = Period(runId, phase.id)
+      try op(using this)
+      finally period = saved
+
     // `creationTrace`-related code. To enable, uncomment the code below and the
     // call to `setCreationTrace()` in this file.
     /*

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

@@ -797,7 +797,13 @@ object Denotations {
               val transformer = ctx.base.denotTransformers(nextTransformerId)
               //println(s"transforming $this with $transformer")
               try
-                next = atPhase(transformer)(transformer.transform(cur))
+                util.Stats.record("denot transform")
+                next = ctx.evalAt(transformer)(transformer.transform(cur))
+                  // We temporarily update the context with the new phase instead of creating a
+                  // new one. This is done for performance. We cut down on about 30% of context
+                  // creations that way, and also avoid phase caches in contexts to get large.
+                  // To work correctly, we need to demand that the context with the new phase
+                  // is not retained in the result.
               catch {
                 case ex: CyclicReference =>
                   println(s"error while transforming $this") // DEBUG