Memoizing Context#withPhase

withPhase operations in contexts are now memoized.

Conflicts:
	src/dotty/tools/dotc/core/Contexts.scala

Author: DarkDimius

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

@@ -181,6 +181,30 @@ object Contexts {
     protected def searchHistory_= (searchHistory: SearchHistory) = _searchHistory = searchHistory
     def searchHistory: SearchHistory = _searchHistory
 
+    private var phasedCtx: Context = _
+    private var phasedCtxs: Array[Context] = _
+
+
+    /** This context at given phase.
+     *  This method will always return a phase period equal to phaseId, thus will never return squashed phases
+     */
+    final def withPhase(phaseId: PhaseId): Context = {
+      if (this.phaseId == phaseId) this
+      else if (phasedCtx.phaseId == phaseId) phasedCtx
+      else if (phasedCtxs != null && phasedCtxs(phaseId) != null) phasedCtxs(phaseId)
+      else {
+        val ctx1 = fresh.setPhase(phaseId)
+        if (phasedCtx eq this) phasedCtx = ctx1
+        else {
+          if (phasedCtxs == null) phasedCtxs = new Array[Context](base.phases.length)
+          phasedCtxs(phaseId) = ctx1
+        }
+        ctx1
+      }
+    }
+
+    final def withPhase(phase: Phase): Context =
+      withPhase(phase.id)
   /** If -Ydebug is on, the top of the stack trace where this context
      *  was created, otherwise `null`.
      */
@@ -266,32 +290,23 @@ object Contexts {
     }
     */
 
-    /** A fresh clone of this context. */
-    def fresh: FreshContext = {
-      val newctx: Context = super.clone.asInstanceOf[FreshContext]
-      newctx.outer = this
-      newctx.implicitsCache = null
-      newctx.setCreationTrace()
-        // Dotty deviation: Scala2x allows access to private members implicitCache and setCreationTrace
-        // even from a subclass prefix. Dotty (and Java) do not. It's confirmed as a bug in Scala2x.
-      newctx.asInstanceOf[FreshContext]
+    protected def init(outer: Context): this.type = {
+      this.outer = outer
+      this.implicitsCache = null
+      this.phasedCtx = this
+      this.phasedCtxs = null
+      setCreationTrace()
+      this
     }
+    /** A fresh clone of this context. */
+    def fresh: FreshContext = clone.asInstanceOf[FreshContext].init(this)
 
     final def withOwner(owner: Symbol): Context =
       if (owner ne this.owner) fresh.setOwner(owner) else this
 
     final def withMode(mode: Mode): Context =
       if (mode != this.mode) fresh.setMode(mode) else this
 
-    /**
-     * This method will always return a phase period equal to phaseId, thus will never return squashed phases
-     */
-    final def withPhase(phaseId: PhaseId): Context =
-      if (this.phaseId == phaseId) this else fresh.setPhase(phaseId)
-    final def withPhase(phase: Phase): Context =
-      if (this.period == phase.period) this else fresh.setPhase(phase)
-
-
     final def addMode(mode: Mode): Context = withMode(this.mode | mode)
     final def maskMode(mode: Mode): Context = withMode(this.mode & mode)
     final def retractMode(mode: Mode): Context = withMode(this.mode &~ mode)