Re-use contexts when changing owners in MegaPhase

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -2,29 +2,36 @@ package dotty.tools.dotc.config
 
 object Config {
 
-  final val cacheMembersNamed = true
-  final val cacheAsSeenFrom = true
-  final val cacheMemberNames = true
-  final val cacheImplicitScopes = true
-  final val cacheMatchReduced = true
+  inline val cacheMembersNamed = true
+  inline val cacheAsSeenFrom = true
+  inline val cacheMemberNames = true
+  inline val cacheImplicitScopes = true
+  inline val cacheMatchReduced = true
 
-  final val checkCacheMembersNamed = false
+  /** If true, the `runWithOwner` operation uses a re-usable context,
+   *  similar to explore. This requires that the context does not escape
+   *  the call. If false, `runWithOwner` runs its operation argument
+   *  in a fresh context.
+   */
+  inline val reuseOwnerContexts = true
+
+  inline val checkCacheMembersNamed = false
 
   /** When updating a constraint bound, check that the constrained parameter
    *  does not appear at the top-level of either of its bounds.
    */
-  final val checkConstraintsNonCyclic = false
+  inline val checkConstraintsNonCyclic = false
 
   /** Check that each constraint resulting from a subtype test
    *  is satisfiable.
    */
-  final val checkConstraintsSatisfiable = false
+  inline val checkConstraintsSatisfiable = false
 
   /** Check that each constraint is fully propagated. i.e.
    *  If P <: Q then the upper bound of P is a subtype of the upper bound of Q
    *  and the lower bound of Q is a subtype of the lower bound of P.
    */
-  final val checkConstraintsPropagated = false
+  inline val checkConstraintsPropagated = false
 
   /** Check that constraints of globally committable typer states are closed.
    *  NOTE: When enabled, the check can cause CyclicReference errors because
@@ -35,51 +42,51 @@ object Config {
    *  It is recommended to turn this option on only when chasing down
    *  a TypeParamRef instantiation error. See comment in Types.TypeVar.instantiate.
    */
-  final val debugCheckConstraintsClosed = false
+  inline val debugCheckConstraintsClosed = false
 
   /** Check that no type appearing as the info of a SymDenotation contains
    *  skolem types.
    */
-  final val checkNoSkolemsInInfo = false
+  inline val checkNoSkolemsInInfo = false
 
   /** Check that Name#toString is not called directly from backend by analyzing
    *  the stack trace of each toString call on names. This is very expensive,
    *  so not suitable for continuous testing. But it can be used to find a problem
    *  when running a specific test.
    */
-  final val checkBackendNames = false
+  inline val checkBackendNames = false
 
   /** Check that re-used type comparers are in their initialization state */
-  final val checkTypeComparerReset = false
+  inline val checkTypeComparerReset = false
 
   /** Type comparer will fail with an assert if the upper bound
    *  of a constrained parameter becomes Nothing. This should be turned
    *  on only for specific debugging as normally instantiation to Nothing
    *  is not an error condition.
    */
-  final val failOnInstantiationToNothing = false
+  inline val failOnInstantiationToNothing = false
 
   /** Enable noDoubleDef checking if option "-YnoDoubleDefs" is set.
     * The reason to have an option as well as the present global switch is
     * that the noDoubleDef checking is done in a hotspot, and we do not
     * want to incur the overhead of checking an option each time.
     */
-  final val checkNoDoubleBindings = true
+  inline val checkNoDoubleBindings = true
 
   /** Check positions for consistency after parsing */
-  final val checkPositions = true
+  inline val checkPositions = true
 
   /** Check that typed trees don't point to untyped ones */
-  final val checkTreesConsistent = false
+  inline val checkTreesConsistent = false
 
   /** Show subtype traces for all deep subtype recursions */
-  final val traceDeepSubTypeRecursions = false
+  inline val traceDeepSubTypeRecursions = false
 
   /** When explaining subtypes and this flag is set, also show the classes of the compared types. */
-  final val verboseExplainSubtype = false
+  inline val verboseExplainSubtype = false
 
   /** If this flag is set, take the fast path when comparing same-named type-aliases and types */
-  final val fastPathForRefinedSubtype = true
+  inline val fastPathForRefinedSubtype = true
 
   /** If this flag is set, and we compute `T1[X1]` & `T2[X2]` as a new
    *  upper bound of a constrained parameter, try to align the arguments by computing
@@ -88,20 +95,20 @@ object Config {
    *
    *  For more info, see the comment in `TypeComparer#glbArgs`.
    */
-  final val alignArgsInAnd = true
+  inline val alignArgsInAnd = true
 
   /** If this flag is set, higher-kinded applications are checked for validity
    */
-  final val checkHKApplications = false
+  inline val checkHKApplications = false
 
   /** If this flag is set, method types are checked for valid parameter references
    */
-  final val checkMethodTypes = false
+  inline val checkMethodTypes = false
 
   /** If this flag is set, it is checked that TypeRefs don't refer directly
    *  to themselves.
    */
-  final val checkTypeRefCycles = false
+  inline val checkTypeRefCycles = false
 
   /** If this flag is set, we check that types assigned to trees are error types only
    *  if some error was already reported. There are complicicated scenarios where this
@@ -113,35 +120,35 @@ object Config {
    *  this in all circumstances. But since it is almost always true it is useful to
    *  keep the Config option for debugging.
    */
-  final val checkUnreportedErrors = false
+  inline val checkUnreportedErrors = false
 
   /** If this flag is set, it is checked that class type parameters are
    *  only references with NoPrefix or ThisTypes as prefixes. This option
    *  is usually disabled, because there are still some legitimate cases where
    *  this can arise (e.g. for pos/Map.scala, in LambdaType.integrate).
    */
-  final val checkTypeParamRefs = false
+  inline val checkTypeParamRefs = false
 
   /** The recursion depth for showing a summarized string */
-  final val summarizeDepth = 2
+  inline val summarizeDepth = 2
 
   /** Check that variances of lambda arguments match the
    *  variance of the underlying lambda class.
    */
-  final val checkLambdaVariance = false
+  inline val checkLambdaVariance = false
 
   /** Check that certain types cannot be created in erasedTypes phases.
    *  Note: Turning this option on will get some false negatives, since it is
    *  possible that And/Or types are still created during erasure as the result
    *  of some operation on an existing type.
    */
-  final val checkUnerased = false
+  inline val checkUnerased = false
 
   /** Check that atoms-based comparisons match regular comparisons that do not
    *  take atoms into account. The two have to give the same results, since
    *  atoms comparison is intended to be just an optimization.
    */
-  final val checkAtomsComparisons = false
+  inline val checkAtomsComparisons = false
 
   /** In `derivedSelect`, rewrite
    *
@@ -151,39 +158,39 @@ object Config {
    *  Not sure whether this is useful. Preliminary measurements show a slowdown of about
    *  7% for the build when this option is enabled.
    */
-  final val splitProjections = false
+  inline val splitProjections = false
 
   /** If this flag is on, always rewrite an application `S[Ts]` where `S` is an alias for
    *  `[Xs] -> U` to `[Xs := Ts]U`.
    *  Turning this flag on was observed to give a ~6% speedup on the JUnit test suite.
    */
-  final val simplifyApplications = true
+  inline val simplifyApplications = true
 
   /** Assume -indent by default */
-  final val defaultIndent = true
+  inline val defaultIndent = true
 
   /** If set, prints a trace of all symbol completions */
-  final val showCompletions = false
+  inline val showCompletions = false
 
   /** If set, method results that are context functions are flattened by adding
    *  the parameters of the context function results to the methods themselves.
    *  This is an optimization that reduces closure allocations.
    */
-  final val flattenContextFunctionResults = true
+  inline val flattenContextFunctionResults = true
 
   /** If set, enables tracing */
-  final val tracingEnabled = false
+  inline val tracingEnabled = false
 
   /** Initial capacity of uniques HashMap.
    *  Note: This MUST BE a power of two to work with util.HashSet
    */
-  final val initialUniquesCapacity = 65536
+  inline val initialUniquesCapacity = 65536
 
   /** How many recursive calls to NamedType#underlying are performed before logging starts. */
-  final val LogPendingUnderlyingThreshold = 50
+  inline val LogPendingUnderlyingThreshold = 50
 
   /** How many recursive calls to isSubType are performed before logging starts. */
-  final val LogPendingSubTypesThreshold = 50
+  inline val LogPendingSubTypesThreshold = 50
 
   /** How many recursive calls to findMember are performed before logging names starts
    *  Note: this threshold has to be chosen carefully. Too large, and programs
@@ -194,8 +201,8 @@ object Config {
    *  dotty itself only causes small pending names lists to be generated (we measured
    *  at max 6 elements) and these lists are never searched with contains.
    */
-  final val LogPendingFindMemberThreshold = 9
+  inline val LogPendingFindMemberThreshold = 9
 
   /** When in IDE, turn StaleSymbol errors into warnings instead of crashing */
-  final val ignoreStaleInIDE = true
+  inline val ignoreStaleInIDE = true
 }

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

@@ -701,6 +701,28 @@ object Contexts {
     val ectx = exploreCtx
     try op(using ectx) finally wrapUpExplore(ectx)
 
+  private def changeOwnerCtx(owner: Symbol)(using Context): Context =
+    val base = ctx.base
+    import base._
+    val nestedCtx =
+      if changeOwnersInUse < changeOwnerContexts.size then
+        changeOwnerContexts(changeOwnersInUse).reuseIn(ctx)
+      else
+        val c = FreshContext(ctx.base).init(ctx, ctx)
+        changeOwnerContexts += c
+        c
+    changeOwnersInUse += 1
+    nestedCtx.setOwner(owner).setTyperState(ctx.typerState)
+
+  /** Run `op` in current context, with a mode is temporarily set as specified.
+   */
+  inline def runWithOwner[T](owner: Symbol)(inline op: Context ?=> T)(using Context): T =
+    if Config.reuseOwnerContexts then
+      try op(using changeOwnerCtx(owner))
+      finally ctx.base.changeOwnersInUse -= 1
+    else
+      op(using ctx.fresh.setOwner(owner))
+
   /** The type comparer of the kind created by `maker` to be used.
    *  This is the currently active type comparer CMP if
    *   - CMP is associated with the current context, and
@@ -887,6 +909,9 @@ object Contexts {
     private[Contexts] val exploreContexts = new mutable.ArrayBuffer[FreshContext]
     private[Contexts] var exploresInUse: Int = 0
 
+    private[Contexts] val changeOwnerContexts = new mutable.ArrayBuffer[FreshContext]
+    private[Contexts] var changeOwnersInUse: Int = 0
+
     private[Contexts] val comparers = new mutable.ArrayBuffer[TypeComparer]
     private[Contexts] var comparersInUse: Int = 0
 

compiler/src/dotty/tools/dotc/transform/MegaPhase.scala

@@ -213,11 +213,10 @@ class MegaPhase(val miniPhases: Array[MiniPhase]) extends Phase {
 
   /** Transform full tree using all phases in this group that have idxInGroup >= start */
   def transformTree(tree: Tree, start: Int)(using Context): Tree = {
-    def localContext(using Context) = {
+
+    inline def inLocalContext[T](inline op: Context ?=> T)(using Context): T =
       val sym = tree.symbol
-      val owner = if (sym.is(PackageVal)) sym.moduleClass else sym
-      ctx.fresh.setOwner(owner)
-    }
+      runWithOwner(if (sym.is(PackageVal)) sym.moduleClass else sym)(op)
 
     def transformNamed(tree: Tree, start: Int, outerCtx: Context): Tree = tree match {
       case tree: Ident =>
@@ -236,8 +235,10 @@ class MegaPhase(val miniPhases: Array[MiniPhase]) extends Phase {
             val rhs = transformTree(tree.rhs, start)
             cpy.ValDef(tree)(tree.name, tpt, rhs)
           }
-          if (tree.isEmpty) tree
-          else goValDef(mapValDef(using if (tree.symbol.exists) localContext else ctx), start)
+          if tree.isEmpty then tree
+          else goValDef(
+            if tree.symbol.exists then inLocalContext(mapValDef) else mapValDef,
+            start)
         }
       case tree: DefDef =>
         inContext(prepDefDef(tree, start)(using outerCtx)) {
@@ -248,11 +249,11 @@ class MegaPhase(val miniPhases: Array[MiniPhase]) extends Phase {
             val rhs = transformTree(tree.rhs, start)
             cpy.DefDef(tree)(tree.name, tparams, vparamss, tpt, rhs)
           }
-          goDefDef(mapDefDef(using localContext), start)
+          goDefDef(inLocalContext(mapDefDef), start)
         }
       case tree: TypeDef =>
         inContext(prepTypeDef(tree, start)(using outerCtx)) {
-          val rhs = transformTree(tree.rhs, start)(using localContext)
+          val rhs = inLocalContext(transformTree(tree.rhs, start))
           goTypeDef(cpy.TypeDef(tree)(tree.name, rhs), start)
         }
       case tree: Labeled =>
@@ -381,7 +382,7 @@ class MegaPhase(val miniPhases: Array[MiniPhase]) extends Phase {
             val stats = transformStats(tree.stats, tree.symbol, start)
             cpy.PackageDef(tree)(pid, stats)
           }
-          goPackageDef(mapPackage(using localContext), start)
+          goPackageDef(inLocalContext(mapPackage), start)
         }
       case tree: Try =>
         inContext(prepTry(tree, start)(using outerCtx)) {