Implement shrinking reductions in CPS IR

sdk/lib/_internal/compiler/implementation/cps_ir/shrinking_reductions.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+part of dart2js.optimizers;
+
+/**
+ * [[ShrinkingReducer]] applies shrinking reductions to CPS terms as described
+ * in 'Compiling with Continuations, Continued' by Andrew Kennedy.
+ */
+class ShrinkingReducer implements Pass {
+  _RedexVisitor _redexVisitor;
+  Set<_ReductionTask> _worklist;
+
+  static final _DeletedNode _DELETED = new _DeletedNode();
+
+  /// Applies shrinking reductions to root, mutating root in the process.
+  void rewrite(FunctionDefinition root) {
+    _worklist = new Set<_ReductionTask>();
+    _redexVisitor = new _RedexVisitor(_worklist);
+
+    // Set all parent pointers.
+    new _ParentVisitor().visit(root);
+
+    // Sweep over the term, collecting redexes into the worklist.
+    _redexVisitor.visitFunctionDefinition(root);
+
+    // Process the worklist.
+    while (_worklist.isNotEmpty) {
+      _ReductionTask task = _worklist.first;
+      _worklist.remove(task);
+      _processTask(task);
+    }
+  }
+
+  /// Removes the given node from the CPS graph, replacing it with its body
+  /// and marking it as deleted. The node's parent must be a [[InteriorNode]].
+  void _removeNode(InteriorNode node) {
+    Node body           = node.body;
+    InteriorNode parent = node.parent;
+    assert(parent.body == node);
+
+    body.parent = parent;
+    parent.body = body;
+    node.parent = _DELETED;
+  }
+
+  void _processTask(_ReductionTask task) {
+    // Lazily skip tasks for deleted nodes.
+    if (task.node.parent == _DELETED) {
+      return;
+    }
+
+    switch (task.kind) {
+      case _ReductionKind.DEAD_VAL:
+        _reduceDeadVal(task);
+        break;
+      case _ReductionKind.DEAD_CONT:
+        _reduceDeadCont(task);
+        break;
+      case _ReductionKind.BETA_CONT_LIN:
+        _reduceBetaContLin(task);
+        break;
+      case _ReductionKind.ETA_CONT:
+        _reduceEtaCont(task);
+        break;
+      default:
+        assert(false);
+    }
+  }
+
+  /// Applies the dead-val reduction:
+  ///   letprim x = V in E -> E (x not free in E).
+  void _reduceDeadVal(_ReductionTask task) {
+    assert(_isDeadVal(task.node));
+
+    // Remove dead primitive.
+    LetPrim letPrim = task.node;;
+    _removeNode(letPrim);
+
+    // Perform bookkeeping on removed body and scan for new redexes.
+    new _RemovalRedexVisitor(_worklist).visit(letPrim.primitive);
+  }
+
+  /// Applies the dead-cont reduction:
+  ///   letcont k x = E0 in E1 -> E1 (k not free in E1).
+  void _reduceDeadCont(_ReductionTask task) {
+    assert(_isDeadCont(task.node));
+
+    // Remove dead continuation.
+    LetCont letCont = task.node;
+    _removeNode(letCont);
+
+    // Perform bookkeeping on removed body and scan for new redexes.
+    new _RemovalRedexVisitor(_worklist).visit(letCont.continuation);
+  }
+
+  /// Applies the beta-cont-lin reduction:
+  ///   letcont k x = E0 in E1[k y] -> E1[E0[y/x]] (k not free in E1).
+  void _reduceBetaContLin(_ReductionTask task) {
+    // Might have been mutated, recheck if reduction is still valid.
+    // In the following example, the beta-cont-lin reduction of k0 could have
+    // been invalidated by removal of the dead continuation k1:
+    //
+    //  letcont k0 x0 = E0 in
+    //    letcont k1 x1 = k0 x1 in
+    //      return x2
+    if (!_isBetaContLin(task.node)) {
+      return;
+    }
+
+    // Remove the continuation.
+    LetCont letCont = task.node;
+    Continuation cont = letCont.continuation;
+    _removeNode(letCont);
+
+    // Replace its invocation with the continuation body.
+    InvokeContinuation invoke = cont.firstRef.parent;
+    InteriorNode invokeParent = invoke.parent;
+
+    cont.body.parent = invokeParent;
+    invokeParent.body = cont.body;
+
+    // Substitute the invocation argument for the continuation parameter.
+    for (int i = 0; i < invoke.arguments.length; i++) {
+      Reference argRef = invoke.arguments[i];
+      argRef.definition.substituteFor(cont.parameters[i]);
+    }
+
+    // Perform bookkeeping on removed body and scan for new redexes.
+    new _RemovalRedexVisitor(_worklist).visit(invoke);
+  }
+
+  /// Applies the eta-cont reduction:
+  ///   letcont k x = j x in E -> E[j/k].
+  /// If k is unused, degenerates to dead-cont.
+  void _reduceEtaCont(_ReductionTask task) {
+    // Might have been mutated, recheck if reduction is still valid.
+    // In the following example, the eta-cont reduction of k1 could have been
+    // invalidated by an earlier beta-cont-lin reduction of k0.
+    //
+    //  letcont k0 x0 = E0 in
+    //    letcont k1 x1 = k0 x1 in E1
+    if (!_isEtaCont(task.node)) {
+      return;
+    }
+
+    // Remove the continuation.
+    LetCont letCont   = task.node;
+    Continuation cont = letCont.continuation;
+    _removeNode(letCont);
+
+    InvokeContinuation invoke = cont.body;
+    Continuation wrappedCont = invoke.continuation.definition;
+
+    // Replace all occurrences with the wrapped continuation.
+    wrappedCont.substituteFor(cont);
+
+    // Perform bookkeeping on removed body and scan for new redexes.
+    new _RemovalRedexVisitor(_worklist).visit(cont);
+  }
+}
+
+/// Returns true iff the bound primitive is unused.
+bool _isDeadVal(LetPrim node) => !node.primitive.hasAtLeastOneUse;
+
+/// Returns true iff the bound continuation is unused.
+bool _isDeadCont(LetCont node) => !node.continuation.hasAtLeastOneUse;
+
+/// Returns true iff the bound continuation is used exactly once, and that
+/// use is as the receiver of a continuation invocation.
+bool _isBetaContLin(LetCont node) {
+  Continuation cont = node.continuation;
+  if (!cont.hasExactlyOneUse) {
+    return false;
+  }
+
+  if (cont.firstRef.parent is InvokeContinuation) {
+    InvokeContinuation invoke = cont.firstRef.parent;
+    return (cont == invoke.continuation.definition);
+  }
+
+  return false;
+
+}
+
+/// Returns true iff the bound continuation consists of a continuation
+/// invocation, passing on all parameters. Special cases exist (see below).
+bool _isEtaCont(LetCont node) {
+  Continuation cont = node.continuation;
+  if (!(cont.body is InvokeContinuation)) {
+    return false;
+  }
+
+  // Special case for continuations passed into one of { InvokeConstructor,
+  // InvokeMethod, InvokeStatic, ConcatenateStrings, TypeOperator,
+  // InvokeSuperMethod }, since their direct-style translation require a
+  // continuation that is used exactly once.
+  // TODO(kmillikin): Modify direct-style translation to handle multiply-used
+  // continuations for Invoke
+  // (see [[tree_ir_builder.Builder.continueWithExpression]]), and subsequently
+  // mark the following forms as eta-cont:
+  //    let cont k(v) = k'(v) in ... InvokeMethod(v, f, args, k).
+  if (cont.hasExactlyOneUse) {
+    if (cont.firstRef.parent is InvokeConstructor) {
+      InvokeConstructor parent = cont.firstRef.parent;
+      if (parent.continuation == cont.firstRef) {
+        return false;
+      }
+    } else if (cont.firstRef.parent is InvokeMethod) {
+      InvokeMethod parent = cont.firstRef.parent;
+      if (parent.continuation == cont.firstRef) {
+        return false;
+      }
+    } else if (cont.firstRef.parent is InvokeStatic) {
+      InvokeStatic parent = cont.firstRef.parent;
+      if (parent.continuation == cont.firstRef) {
+        return false;
+      }
+    } else if (cont.firstRef.parent is ConcatenateStrings) {
+      ConcatenateStrings parent = cont.firstRef.parent;
+      if (parent.continuation == cont.firstRef) {
+        return false;
+      }
+    } else if (cont.firstRef.parent is TypeOperator) {
+      TypeOperator parent = cont.firstRef.parent;
+      if (parent.continuation == cont.firstRef) {
+        return false;
+      }
+    } else if (cont.firstRef.parent is InvokeSuperMethod) {
+      InvokeSuperMethod parent = cont.firstRef.parent;
+      if (parent.continuation == cont.firstRef) {
+        return false;
+      }
+    }
+  }
+
+  InvokeContinuation invoke = cont.body;
+
+  // Translation to direct style generates different statements for recursive
+  // and non-recursive invokes. It should be possible to apply eta-cont, but
+  // higher order continuations require escape analysis, left as a possibility
+  // for future improvements.
+  if (invoke.isRecursive) {
+    return false;
+  }
+
+  if (cont.parameters.length != invoke.arguments.length) {
+    return false;
+  }
+
+  // TODO(jgruber): Linear in the parameter count. Can be improved to near
+  // constant time by using union-find data structure.
+  for (int i = 0; i < cont.parameters.length; i++) {
+    if (invoke.arguments[i].definition != cont.parameters[i]) {
+      return false;
+      }
+    }
+
+    return true;
+  }
+
+/// Traverses a term and adds any found redexes to the worklist.
+class _RedexVisitor extends RecursiveVisitor {
+  final Set<_ReductionTask> worklist;
+
+  _RedexVisitor(this.worklist);
+
+  void processLetPrim(LetPrim node) {
+    if (node.parent == ShrinkingReducer._DELETED) {
+      return;
+    } else if (_isDeadVal(node)) {
+      worklist.add(new _ReductionTask(_ReductionKind.DEAD_VAL, node));
+    }
+  }
+
+  void processLetCont(LetCont node) {
+    if (node.parent == ShrinkingReducer._DELETED) {
+      return;
+    } else if (_isDeadCont(node)) {
+      worklist.add(new _ReductionTask(_ReductionKind.DEAD_CONT, node));
+    } else if (_isEtaCont(node)) {
+      worklist.add(new _ReductionTask(_ReductionKind.ETA_CONT, node));
+    } else if (_isBetaContLin(node)){
+      worklist.add(new _ReductionTask(_ReductionKind.BETA_CONT_LIN, node));
+    }
+  }
+}
+
+/// Traverses a deleted CPS term, marking existing tasks associated with a node
+/// within the term as deleted (which causes them to be skipped lazily when
+/// popped from the worklist), and adding newly created redexes to the worklist.
+class _RemovalRedexVisitor extends _RedexVisitor {
+  _RemovalRedexVisitor(Set<_ReductionTask> worklist) : super(worklist);
+
+  void processLetPrim(LetPrim node) {
+    node.parent = ShrinkingReducer._DELETED;
+  }
+
+  void processLetCont(LetCont node) {
+    node.parent = ShrinkingReducer._DELETED;
+  }
+
+  void processReference(Reference reference) {
+    reference.unlink();
+
+    if (reference.definition is Primitive) {
+      Primitive primitive = reference.definition;
+      Node parent = primitive.parent;
+      if (parent is LetPrim && _isDeadVal(parent)) {
+        worklist.add(new _ReductionTask(_ReductionKind.DEAD_VAL, parent));
+      }
+    } else if (reference.definition is Continuation) {
+      Continuation cont = reference.definition;
+      if (cont.isRecursive && cont.hasAtMostOneUse) {
+        // Convert recursive to nonrecursive continuations.
+        // If the continuation is still in use, it is either dead and will be
+        // removed, or it is called nonrecursively outside its body.
+        cont.isRecursive = false;
+      }
+      Node parent = cont.parent;
+      if (parent is LetCont && _isDeadCont(parent)) {
+        worklist.add(new _ReductionTask(_ReductionKind.DEAD_CONT, parent));
+      }
+    }
+  }
+}
+
+/// Traverses the CPS term and sets node.parent for each visited node.
+class _ParentVisitor extends RecursiveVisitor {
+
+  processFunctionDefinition(FunctionDefinition node) {
+    node.body.parent = node;
+    node.parameters.forEach((Parameter p) => p.parent = node);
+  }
+
+  // Expressions.
+
+  processLetPrim(LetPrim node) {
+    node.primitive.parent = node;
+    node.body.parent = node;
+  }
+
+  processLetCont(LetCont node) {
+    node.continuation.parent = node;
+    node.body.parent = node;
+  }
+
+  processInvokeStatic(InvokeStatic node) {
+    node.continuation.parent = node;
+    node.arguments.forEach((Reference ref) => ref.parent = node);
+  }
+
+  processInvokeContinuation(InvokeContinuation node) {
+    node.continuation.parent = node;
+    node.arguments.forEach((Reference ref) => ref.parent = node);
+  }
+
+  processInvokeMethod(InvokeMethod node) {
+    node.receiver.parent = node;
+    node.continuation.parent = node;
+    node.arguments.forEach((Reference ref) => ref.parent = node);
+  }
+
+  processInvokeSuperMethod(InvokeSuperMethod node) {
+    node.continuation.parent = node;
+    node.arguments.forEach((Reference ref) => ref.parent = node);
+  }
+
+  processInvokeConstructor(InvokeConstructor node) {
+    node.continuation.parent = node;
+    node.arguments.forEach((Reference ref) => ref.parent = node);
+  }
+
+  processConcatenateStrings(ConcatenateStrings node) {
+    node.continuation.parent = node;
+    node.arguments.forEach((Reference ref) => ref.parent = node);
+  }
+
+  processBranch(Branch node) {
+    node.condition.parent = node;
+    node.trueContinuation.parent = node;
+    node.falseContinuation.parent = node;
+  }
+
+  processTypeOperator(TypeOperator node) {
+    node.continuation.parent = node;
+    node.receiver.parent = node;
+  }
+
+  processSetClosureVariable(SetClosureVariable node) {
+    node.body.parent = node;
+    node.value.parent = node;
+  }
+
+  processDeclareFunction(DeclareFunction node) {
+    node.definition.parent = node;
+    node.body.parent = node;
+  }
+
+  // Definitions.
+
+  processLiteralList(LiteralList node) {
+    node.values.forEach((Reference ref) => ref.parent = node);
+  }
+
+  processLiteralMap(LiteralMap node) {
+    node.values.forEach((Reference ref) => ref.parent = node);
+    node.keys.forEach((Reference ref) => ref.parent = node);
+  }
+
+  processCreateFunction(CreateFunction node) {
+    node.definition.parent = node;
+  }
+
+  processContinuation(Continuation node) {
+    node.body.parent = node;
+    node.parameters.forEach((Parameter param) => param.parent = node);
+  }
+
+  // Conditions.
+
+  processIsTrue(IsTrue node) {
+    node.value.parent = node;
+  }
+}
+
+class _ReductionKind {
+  final String name;
+  final int hashCode;
+
+  const _ReductionKind(this.name, this.hashCode);
+
+  static const _ReductionKind DEAD_VAL = const _ReductionKind('dead-val', 0);
+  static const _ReductionKind DEAD_CONT = const _ReductionKind('dead-cont', 1);
+  static const _ReductionKind BETA_CONT_LIN =
+      const _ReductionKind('beta-cont-lin', 2);
+  static const _ReductionKind ETA_CONT = const _ReductionKind('eta-cont', 3);
+
+  String toString() => name;
+}
+
+/// Represents a reduction task on the worklist. Implements both hashCode and
+/// operator== since instantiations are used as Set elements.
+class _ReductionTask {
+  final _ReductionKind kind;
+  final Node node;
+
+  int get hashCode {
+    assert(kind.hashCode < (1 << 2));
+    return (node.hashCode << 2) | kind.hashCode;
+  }
+
+  _ReductionTask(this.kind, this.node) {
+    // If new node types are added, they must be marked as deleted in
+    // [[_RemovalRedexVisitor]].
+    assert(node is LetCont || node is LetPrim);
+  }
+
+  bool operator==(_ReductionTask that) {
+    return (that.kind == this.kind && that.node == this.node);
+  }
+
+  String toString() => "$kind: $node";
+}
+
+/// A dummy class used solely to mark nodes as deleted once they are removed
+/// from a term.
+class _DeletedNode extends Node {
+  accept(_) => null;
+}