Third version of function specialization

- Fix isPlainFunctionClass

Previous implementation is incorrect, as scala.Function1$ would qualify.

- Create the symbol at the next phase

Create the symbol at the next phase, so that it is a valid member of the
corresponding function for all valid periods of its SymDenotations.
Otherwise, the valid period will offset by 1, which causes a stale symbol
in compiling stdlib.

- Handle abstract apply and multiple applys

- Don't specialize abstract apply

- Fast specialization

We avoid going through InfoTransformer, which will cause all symbols to be checked.
The reason why it works is that the specialized base classes, i.e. Function0-2
already have all the relevant definitions.

- Use StringBuilder instead of StringBuffer (thanks @smarter)

StringBuffer is synchronized thus is slower.

Author: liufengyun

community-build/community-projects/stdLib213

@@ -69,21 +69,23 @@ class Compiler {
          new CacheAliasImplicits,    // Cache RHS of parameterless alias implicits
          new ByNameClosures,         // Expand arguments to by-name parameters to closures
          new HoistSuperArgs,         // Hoist complex arguments of supercalls to enclosing scope
+         new SpecializeApplyMethods, // Adds specialized methods to FunctionN
          new RefChecks) ::           // Various checks mostly related to abstract members and overriding
     List(new ElimOpaque,             // Turn opaque into normal aliases
          new TryCatchPatterns,       // Compile cases in try/catch
          new PatternMatcher,         // Compile pattern matches
          new sjs.ExplicitJSClasses,  // Make all JS classes explicit (Scala.js only)
          new ExplicitOuter,          // Add accessors to outer classes from nested ones.
          new ExplicitSelf,           // Make references to non-trivial self types explicit as casts
+         new ElimByName,             // Expand by-name parameter references
          new StringInterpolatorOpt) :: // Optimizes raw and s string interpolators by rewriting them to string concatentations
     List(new PruneErasedDefs,        // Drop erased definitions from scopes and simplify erased expressions
          new InlinePatterns,         // Remove placeholders of inlined patterns
          new VCInlineMethods,        // Inlines calls to value class methods
          new SeqLiterals,            // Express vararg arguments as arrays
          new InterceptedMethods,     // Special handling of `==`, `|=`, `getClass` methods
          new Getters,                // Replace non-private vals and vars with getter defs (fields are added later)
-         new ElimByName,             // Expand by-name parameter references
+         new SpecializeFunctions,    // Specialized Function{0,1,2} by replacing super with specialized super
          new LiftTry,                // Put try expressions that might execute on non-empty stacks into their own methods
          new CollectNullableFields,  // Collect fields that can be nulled out after use in lazy initialization
          new ElimOuterSelect,        // Expand outer selections

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -784,8 +784,8 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
   def tupleArgs(tree: Tree)(using Context): List[Tree] = tree match {
     case Block(Nil, expr) => tupleArgs(expr)
     case Inlined(_, Nil, expr) => tupleArgs(expr)
-    case Apply(fn, args)
-    if fn.symbol.name == nme.apply &&
+    case Apply(fn: NameTree, args)
+    if fn.name == nme.apply &&
         fn.symbol.owner.is(Module) &&
         defn.isTupleClass(fn.symbol.owner.companionClass) => args
     case _ => Nil

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -1209,7 +1209,11 @@ class Definitions {
       else funType(n)
     ).symbol.asClass
 
-  @tu lazy val Function0_apply: Symbol = FunctionClass(0).requiredMethod(nme.apply)
+  @tu lazy val Function0_apply: Symbol = Function0.requiredMethod(nme.apply)
+
+  @tu lazy val Function0: Symbol = FunctionClass(0)
+  @tu lazy val Function1: Symbol = FunctionClass(1)
+  @tu lazy val Function2: Symbol = FunctionClass(2)
 
   def FunctionType(n: Int, isContextual: Boolean = false, isErased: Boolean = false)(using Context): TypeRef =
     FunctionClass(n, isContextual && !ctx.erasedTypes, isErased).typeRef
@@ -1244,7 +1248,7 @@ class Definitions {
 
   def isBottomClassAfterErasure(cls: Symbol): Boolean = cls == NothingClass || cls == NullClass
 
-  /** Is a function class.
+  /** Is any function class where
    *   - FunctionXXL
    *   - FunctionN for N >= 0
    *   - ContextFunctionN for N >= 0
@@ -1253,6 +1257,11 @@ class Definitions {
    */
   def isFunctionClass(cls: Symbol): Boolean = scalaClassName(cls).isFunction
 
+  /** Is a function class where
+   *    - FunctionN for N >= 0 and N != XXL
+   */
+  def isPlainFunctionClass(cls: Symbol) = isVarArityClass(cls, str.Function)
+
   /** Is an context function class.
    *   - ContextFunctionN for N >= 0
    *   - ErasedContextFunctionN for N > 0
@@ -1488,6 +1497,25 @@ class Definitions {
         false
     })
 
+  @tu lazy val Function0SpecializedApplyNames: collection.Set[TermName] =
+    for r <- Function0SpecializedReturnTypes
+    yield nme.apply.specializedFunction(r, Nil).asTermName
+
+  @tu lazy val Function1SpecializedApplyNames: collection.Set[TermName] =
+    for
+      r  <- Function1SpecializedReturnTypes
+      t1 <- Function1SpecializedParamTypes
+    yield
+      nme.apply.specializedFunction(r, List(t1)).asTermName
+
+  @tu lazy val Function2SpecializedApplyNames: collection.Set[TermName] =
+    for
+      r  <- Function2SpecializedReturnTypes
+      t1 <- Function2SpecializedParamTypes
+      t2 <- Function2SpecializedParamTypes
+    yield
+      nme.apply.specializedFunction(r, List(t1, t2)).asTermName
+
   def functionArity(tp: Type)(using Context): Int = tp.dropDependentRefinement.dealias.argInfos.length - 1
 
   /** Return underlying context function type (i.e. instance of an ContextFunctionN class)

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

@@ -231,6 +231,11 @@ object NameOps {
     def isFunction: Boolean =
       (name eq tpnme.FunctionXXL) || checkedFunArity(functionSuffixStart) >= 0
 
+    /** Is a function name
+     *    - FunctionN for N >= 0
+     */
+    def isPlainFunction: Boolean = functionArity >= 0
+
     /** Is an context function name, i.e one of ContextFunctionN or ErasedContextFunctionN for N >= 0
      */
     def isContextFunction: Boolean =
@@ -279,7 +284,7 @@ object NameOps {
     /** This method is to be used on **type parameters** from a class, since
      *  this method does sorting based on their names
      */
-    def specializedFor(classTargs: List[Types.Type], classTargsNames: List[Name], methodTargs: List[Types.Type], methodTarsNames: List[Name])(implicit ctx: Context): Name = {
+    def specializedFor(classTargs: List[Type], classTargsNames: List[Name], methodTargs: List[Type], methodTarsNames: List[Name])(using Context): N = {
       val methodTags: Seq[Name] = (methodTargs zip methodTarsNames).sortBy(_._2).map(x => defn.typeTag(x._1))
       val classTags: Seq[Name] = (classTargs zip classTargsNames).sortBy(_._2).map(x => defn.typeTag(x._1))
 
@@ -294,10 +299,15 @@ object NameOps {
      *
      *  `<return type><first type><second type><...>`
      */
-    def specializedFunction(ret: Types.Type, args: List[Types.Type])(implicit ctx: Context): Name =
-      name ++ nme.specializedTypeNames.prefix ++
-      nme.specializedTypeNames.separator ++ defn.typeTag(ret) ++
-      args.map(defn.typeTag).fold(nme.EMPTY)(_ ++ _) ++ nme.specializedTypeNames.suffix
+    def specializedFunction(ret: Type, args: List[Type])(using Context): Name =
+      val sb = new StringBuilder
+      sb.append(name.toString)
+      sb.append(nme.specializedTypeNames.prefix.toString)
+      sb.append(nme.specializedTypeNames.separator)
+      sb.append(defn.typeTag(ret).toString)
+      args.foreach { arg => sb.append(defn.typeTag(arg)) }
+      sb.append(nme.specializedTypeNames.suffix)
+      termName(sb.toString)
 
     /** If name length exceeds allowable limit, replace part of it by hash */
     def compactified(using Context): TermName = termName(compactify(name.toString))

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

@@ -162,7 +162,7 @@ class ExpandSAMs extends MiniPhase {
         cpy.Block(tree)(pfDef :: Nil, New(pfSym.typeRef, Nil))
 
       case _ =>
-        val found = tpe.baseType(defn.FunctionClass(1))
+        val found = tpe.baseType(defn.Function1)
         report.error(TypeMismatch(found, tpe), tree.srcPos)
         tree
     }

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

@@ -39,18 +39,20 @@ class FunctionXXLForwarders extends MiniPhase with IdentityDenotTransformer {
       ref(receiver.symbol).appliedToArgss(argss).cast(defn.ObjectType)
     }
 
+    if impl.symbol.owner.is(Trait) then return impl
+
     val forwarders =
       for {
-        tree <- if (impl.symbol.owner.is(Trait)) Nil else impl.body
-        if tree.symbol.is(Method) && tree.symbol.name == nme.apply &&
-           tree.symbol.signature.paramsSig.size > MaxImplementedFunctionArity &&
-           tree.symbol.allOverriddenSymbols.exists(sym => defn.isXXLFunctionClass(sym.owner))
+        (ddef: DefDef) <- impl.body
+        if ddef.name == nme.apply && ddef.symbol.is(Method) &&
+           ddef.symbol.signature.paramsSig.size > MaxImplementedFunctionArity &&
+           ddef.symbol.allOverriddenSymbols.exists(sym => defn.isXXLFunctionClass(sym.owner))
       }
       yield {
         val xsType = defn.ArrayType.appliedTo(List(defn.ObjectType))
         val methType = MethodType(List(nme.args))(_ => List(xsType), _ => defn.ObjectType)
-        val meth = newSymbol(tree.symbol.owner, nme.apply, Synthetic | Method, methType)
-        DefDef(meth, paramss => forwarderRhs(tree, paramss.head.head))
+        val meth = newSymbol(ddef.symbol.owner, nme.apply, Synthetic | Method, methType)
+        DefDef(meth, paramss => forwarderRhs(ddef, paramss.head.head))
       }
 
     cpy.Template(impl)(body = forwarders ::: impl.body)

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

@@ -0,0 +1,118 @@
+package dotty.tools.dotc
+package transform
+
+import ast.Trees._, ast.tpd, core._
+import Contexts._, Types._, Decorators._, Symbols._, DenotTransformers._
+import SymDenotations._, Scopes._, StdNames._, NameOps._, Names._
+import MegaPhase.MiniPhase
+
+import scala.collection.mutable
+
+
+/** This phase synthesizes specialized methods for FunctionN, this is done
+ *  since there are no scala signatures in the bytecode for the specialized
+ *  methods.
+ *
+ *  We know which specializations exist for the different arities, therefore we
+ *  can hardcode them. This should, however be removed once we're using a
+ *  different standard library.
+ */
+class SpecializeApplyMethods extends MiniPhase with InfoTransformer {
+  import ast.tpd._
+
+  val phaseName = "specializeApplyMethods"
+
+  override def isEnabled(using Context): Boolean =
+    !ctx.settings.scalajs.value
+
+  private def specApplySymbol(sym: Symbol, args: List[Type], ret: Type)(using Context): Symbol = {
+    val name = nme.apply.specializedFunction(ret, args)
+    // Create the symbol at the next phase, so that it is a valid member of the
+    // corresponding function for all valid periods of its SymDenotations.
+    // Otherwise, the valid period will offset by 1, which causes a stale symbol
+    // in compiling stdlib.
+    atNextPhase(newSymbol(sym, name, Flags.Method, MethodType(args, ret)))
+  }
+
+  private inline def specFun0(inline op: Type => Unit)(using Context): Unit = {
+    for (r <- defn.Function0SpecializedReturnTypes) do
+      op(r)
+  }
+
+  private inline def specFun1(inline op: (Type, Type) => Unit)(using Context): Unit = {
+    for
+      r  <- defn.Function1SpecializedReturnTypes
+      t1 <- defn.Function1SpecializedParamTypes
+    do
+      op(t1, r)
+  }
+
+  private inline def specFun2(inline op: (Type, Type, Type) => Unit)(using Context): Unit = {
+    for
+      r  <- defn.Function2SpecializedReturnTypes
+      t1 <- defn.Function2SpecializedParamTypes
+      t2 <- defn.Function2SpecializedParamTypes
+    do
+      op(t1, t2, r)
+  }
+
+  override def infoMayChange(sym: Symbol)(using Context) =
+    sym == defn.Function0
+    || sym == defn.Function1
+    || sym == defn.Function2
+
+  /** Add symbols for specialized methods to FunctionN */
+  override def transformInfo(tp: Type, sym: Symbol)(using Context) = tp match {
+    case tp: ClassInfo =>
+      if sym == defn.Function0 then
+        val scope = tp.decls.cloneScope
+        specFun0 { r => scope.enter(specApplySymbol(sym, Nil, r)) }
+        tp.derivedClassInfo(decls = scope)
+
+      else if sym == defn.Function1 then
+        val scope = tp.decls.cloneScope
+        specFun1 { (t1, r) => scope.enter(specApplySymbol(sym, t1 :: Nil, r)) }
+        tp.derivedClassInfo(decls = scope)
+
+      else if sym == defn.Function2 then
+        val scope = tp.decls.cloneScope
+        specFun2 { (t1, t2, r) => scope.enter(specApplySymbol(sym, t1 :: t2 :: Nil, r)) }
+        tp.derivedClassInfo(decls = scope)
+
+      else tp
+
+    case _ => tp
+  }
+
+  /** Create bridge methods for FunctionN with specialized applys */
+  override def transformTemplate(tree: Template)(using Context) = {
+    val cls = tree.symbol.owner.asClass
+
+    def synthesizeApply(names: collection.Set[TermName]): Tree = {
+      val applyBuf = new mutable.ListBuffer[DefDef]
+      names.foreach { name =>
+        val applySym = cls.info.decls.lookup(name)
+        val ddef = DefDef(
+          applySym.asTerm,
+          { vparamss =>
+              This(cls)
+                .select(nme.apply)
+                .appliedToArgss(vparamss)
+                .ensureConforms(applySym.info.finalResultType)
+          }
+        )
+        applyBuf += ddef
+      }
+      cpy.Template(tree)(body = tree.body ++ applyBuf)
+    }
+
+    if cls == defn.Function0 then
+      synthesizeApply(defn.Function0SpecializedApplyNames)
+    else if cls == defn.Function1 then
+      synthesizeApply(defn.Function1SpecializedApplyNames)
+    else if cls == defn.Function2 then
+      synthesizeApply(defn.Function2SpecializedApplyNames)
+    else
+      tree
+  }
+}