dart2js cps: Pull SetFields into field initializer arguments.

pkg/compiler/lib/src/cps_ir/use_field_initializers.dart

+// Copyright (c) 2016, 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.
+library dart2js.cps_ir.use_field_initializers;
+
+import 'cps_ir_nodes.dart';
+import 'optimizers.dart';
+import '../elements/elements.dart';
+import '../js_backend/js_backend.dart';
+
+/// Eliminates [SetField] instructions when the value can instead be passed into
+/// the field initializer of a [CreateInstance] instruction.
+///
+/// This compensates for a somewhat common pattern where fields are initialized
+/// in the constructor body instead of using intializers. For example:
+///
+///     class Foo {
+///       var x, y;
+///       Foo(x, y) {
+///          this.x = x;
+///          this.y = y;
+///        }
+///      }
+///
+///   ==> (IR for Foo constructor)
+///
+///      foo = new D.Foo(null, null);
+///      foo.x = 'a';
+///      foo.y = 'b';
+///
+///   ==> (after this pass)
+///
+///      foo = new D.Foo('a', 'b');
+//
+// TODO(asgerf): Store forwarding and load elimination could most likely
+//   handle this more generally.
+//
+class UseFieldInitializers extends BlockVisitor implements Pass {
+  String get passName => 'Use field initializers';
+
+  final JavaScriptBackend backend;
+
+  final Set<CreateInstance> unescaped = new Set<CreateInstance>();
+
+  /// Continuation bindings separating the current traversal position from an
+  /// unescaped [CreateInstance].  When [CreateInstance] is sunk, these
+  /// continuations must sink as well to ensure the object remains in scope
+  /// inside the bound continuations.
+  final List<LetCont> letConts = <LetCont>[];
+
+  /// If non-null, the bindings in [letConts] should sink to immediately below
+  /// this node.
+  InteriorNode letContSinkTarget = null;
+  EscapeVisitor escapeVisitor;
+
+  UseFieldInitializers(this.backend);
+
+  void rewrite(FunctionDefinition node) {
+    escapeVisitor = new EscapeVisitor(this);
+    BlockVisitor.traverseInPreOrder(node, this);
+  }
+
+  void escape(Reference ref) {
+    Definition def = ref.definition;
+    if (def is CreateInstance) {
+      unescaped.remove(def);
+      if (unescaped.isEmpty) {
+        sinkLetConts();
+        letConts.clear();
+      }
+    }
+  }
+
+  void visitContinuation(Continuation node) {
+    endBasicBlock();
+  }
+  void visitLetHandler(LetHandler node) {
+    endBasicBlock();
+  }
+  void visitInvokeContinuation(InvokeContinuation node) {
+    endBasicBlock();
+  }
+  void visitBranch(Branch node) {
+    endBasicBlock();
+  }
+  void visitRethrow(Rethrow node) {
+    endBasicBlock();
+  }
+  void visitThrow(Throw node) {
+    endBasicBlock();
+  }
+  void visitUnreachable(Unreachable node) {
+    endBasicBlock();
+  }
+
+  void visitLetMutable(LetMutable node) {
+    escape(node.value);
+  }
+
+  void visitLetCont(LetCont node) {
+    if (unescaped.isNotEmpty) {
+      // Ensure we do not lift a LetCont if there is a sink target set above
+      // the current node.
+      sinkLetConts();
+      letConts.add(node);
+    }
+  }
+
+  void sinkLetConts() {
+    if (letContSinkTarget != null) {
+      for (LetCont letCont in letConts.reversed) {
+        letCont..remove()..insertBelow(letContSinkTarget);
+      }
+      letContSinkTarget = null;
+    }
+  }
+
+  void endBasicBlock() {
+    sinkLetConts();
+    letConts.clear();
+    unescaped.clear();
+  }
+
+  void visitLetPrim(LetPrim node) {
+    Primitive prim = node.primitive;
+    if (prim is CreateInstance) {
+      unescaped.add(prim);
+      prim.arguments.forEach(escape);
+      return;
+    }
+    if (unescaped.isEmpty) return;
+    if (prim is SetField) {
+      escape(prim.value);
+      Primitive object = prim.object.definition;
+      if (object is CreateInstance && unescaped.contains(object)) {
+        int index = getFieldIndex(object.classElement, prim.field);
+        if (index == -1) {
+          // This field is not initialized at creation time, so we cannot pull
+          // set SetField into the CreateInstance instruction.  We have to
+          // leave the instruction here, and this counts as a use of the object.
+          escape(prim.object);
+        } else {
+          // Replace the field initializer with the new value. There are no uses
+          // of the object before this, so the old value cannot have been seen.
+          object.arguments[index].changeTo(prim.value.definition);
+          prim.destroy();
+          // The right-hand side might not be in scope at the CreateInstance.
+          // Sink the creation down to this point.
+          rebindCreateInstanceAt(object, node);
+          letContSinkTarget = node;
+        }
+      }
+      return;
+    }
+    if (prim is GetField) {
+      // When reading the field of a newly created object, just use the initial
+      // value and destroy the GetField. This can unblock the other optimization
+      // since we remove a use of the object.
+      Primitive object = prim.object.definition;
+      if (object is CreateInstance && unescaped.contains(object)) {
+        int index = getFieldIndex(object.classElement, prim.field);
+        if (index == -1) {
+          escape(prim.object);
+        } else {
+          prim.replaceUsesWith(object.arguments[index].definition);
+          prim.destroy();
+          node.remove();
+        }
+      }
+      return;
+    }
+    escapeVisitor.visit(node.primitive);
+  }
+
+  void rebindCreateInstanceAt(CreateInstance prim, LetPrim newBinding) {
+    removeBinding(prim);
+    newBinding.primitive = prim;
+    prim.parent = newBinding;
+  }
+
+  /// Returns the index of [field] in the canonical initialization order in
+  /// [classElement], or -1 if the field is not initialized at creation time
+  /// for that class.
+  int getFieldIndex(ClassElement classElement, FieldElement field) {
+    // There is no stored map from a field to its index in a given class, so we
+    // have to iterate over all instance fields until we find it.
+    int current = -1, index = -1;
+    classElement.forEachInstanceField((host, currentField) {
+      if (!backend.isNativeOrExtendsNative(host)) {
+        ++current;
+        if (currentField == field) {
+          index = current;
+        }
+      }
+    }, includeSuperAndInjectedMembers: true);
+    return index;
+  }
+
+  void removeBinding(Primitive prim) {
+    LetPrim node = prim.parent;
+    node.remove();
+  }
+}
+
+class EscapeVisitor extends DeepRecursiveVisitor {
+  final UseFieldInitializers main;
+  EscapeVisitor(this.main);
+
+  processReference(Reference ref) {
+    main.escape(ref);
+  }
+}