dart2dart: Preserve variable names throughout the IR.

sdk/lib/_internal/compiler/implementation/ir/ir_nodes.dart

 // Copyright (c) 2013, 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.
 
 // IrNodes are kept in a separate library to have precise control over their
 // dependencies on other parts of the system.
 library dart2js.ir_nodes;
 
 import '../dart2jslib.dart' as dart2js show Constant;
 import '../elements/elements.dart'
-    show FunctionElement, LibraryElement, ParameterElement, ClassElement;
+    show FunctionElement, LibraryElement, ParameterElement, ClassElement,
+    Element, VariableElement;
 import '../universe/universe.dart' show Selector, SelectorKind;
 import '../dart_types.dart' show DartType, GenericType;
+import '../helpers/helpers.dart';
 
 abstract class Node {
   static int hashCount = 0;
   final int hashCode = hashCount = (hashCount + 1) & 0x3fffffff;
 
   accept(Visitor visitor);
 }
 
 abstract class Expression extends Node {
   Expression plug(Expression expr) => throw 'impossible';
@@ skipping 16 matching lines @@
     do {
       current.definition = this;
       previous = current;
       current = current.nextRef;
     } while (current != null);
     previous.nextRef = firstRef;
     firstRef = other.firstRef;
   }
 }
 
+/// A pure expression that cannot throw or diverge.
+/// All primitives are named using the identity of the [Primitive] object.
 abstract class Primitive extends Definition {
+  /// The [VariableElement] or [ParameterElement] from which the primitive
+  /// binding originated.
+  Element element;
+
+  /// Register in which the variable binding this primitive can be allocated.
+  /// Separate register spaces are used for primitives with different [element].
+  /// Assigned by [RegisterAllocator], is null before that phase.
+  int registerIndex;
 }
 
 /// Operands to invocations and primitives are always variables.  They point to
 /// their definition and are linked into a list of occurrences.
 class Reference {
   Definition definition;
   Reference nextRef = null;
 
   Reference(this.definition) {
     nextRef = definition.firstRef;
@@ skipping 230 matching lines @@
   List<Reference> values;
 
   LiteralMap(List<Primitive> keys, List<Primitive> values)
       : this.keys = _referenceList(keys),
         this.values = _referenceList(values);
 
   accept(Visitor visitor) => visitor.visitLiteralMap(this);
 }
 
 class Parameter extends Primitive {
-  final ParameterElement element;
-
-  Parameter(this.element);
+  Parameter(Element element) {
+    super.element = element;
+  }
 
   accept(Visitor visitor) => visitor.visitParameter(this);
 }
 
 /// Continuations are normally bound by 'let cont'.  A continuation with no
 /// parameter (or body) is used to represent a function's return continuation.
 /// The return continuation is bound by the Function, not by 'let cont'.
 class Continuation extends Definition {
   final List<Parameter> parameters;
   Expression body = null;
@@ skipping 180 matching lines @@
   String visitContinuation(Continuation node) {
     // Continuations are visited directly in visitLetCont.
     return '(Unexpected Continuation)';
   }
 
   String visitIsTrue(IsTrue node) {
     String value = names[node.value.definition];
     return '(IsTrue $value)';
   }
 }
+
+/// Keeps track of currently unused register indices.
+class RegisterArray {
+  int nextIndex = 0;
+  final List<int> freeStack = <int>[];
+
+  int makeIndex() {
+    if (freeStack.isEmpty) {
+      return nextIndex++;
+    } else {
+      return freeStack.removeLast();
+    }
+  }
+
+  void releaseIndex(int index) {
+    freeStack.add(index);
+  }
+}
+
+/// Assigns indices to each primitive in the IR such that primitives that are
+/// live simultaneously never get assigned the same index.
+/// This information is used by the dart tree builder to generate fewer
+/// redundant variables.
+/// Currently, the liveness analysis is very simple and is often inadequate
+/// for removing all of the redundant variables.
+class RegisterAllocator extends Visitor {
+  /// Separate register spaces for each source-level variable/parameter.
+  /// Note that null is used as key for primitives without elements.
+  final Map<Element, RegisterArray> elementRegisters =
+      <Element, RegisterArray>{};
+
+  RegisterArray getRegisterArray(Element element) {
+    RegisterArray registers = elementRegisters[element];
+    if (registers == null) {
+      registers = new RegisterArray();
+      elementRegisters[element] = registers;
+    }
+    return registers;
+  }
+
+  void allocate(Primitive primitive) {
+    if (primitive.registerIndex == null) {
+      primitive.registerIndex = getRegisterArray(primitive.element).makeIndex();
+    }
+  }
+
+  void release(Primitive primitive) {
+    // Do not share indices for temporaries as this may obstruct inlining.
+    if (primitive.element == null) return;
+    if (primitive.registerIndex != null) {
+      getRegisterArray(primitive.element).releaseIndex(primitive.registerIndex);
+    }
+  }
+
+  void visitReference(Reference reference) {
+    allocate(reference.definition);
+  }
+
+  void visitFunctionDefinition(FunctionDefinition node) {
+    visit(node.body);
+    node.parameters.forEach(allocate); // Assign indices to unused parameters.
+    elementRegisters.clear();
+  }
+
+  void visitLetPrim(LetPrim node) {
+    visit(node.body);
+    release(node.primitive);
+    visit(node.primitive);
+  }
+
+  void visitLetCont(LetCont node) {
+    visit(node.continuation);
+    visit(node.body);
+  }
+
+  void visitInvokeStatic(InvokeStatic node) {
+    node.arguments.forEach(visitReference);
+  }
+
+  void visitInvokeContinuation(InvokeContinuation node) {
+    node.arguments.forEach(visitReference);
+  }
+
+  void visitInvokeMethod(InvokeMethod node) {
+    visitReference(node.receiver);
+    node.arguments.forEach(visitReference);
+  }
+
+  void visitInvokeConstructor(InvokeConstructor node) {
+    node.arguments.forEach(visitReference);
+  }
+
+  void visitConcatenateStrings(ConcatenateStrings node) {
+    node.arguments.forEach(visitReference);
+  }
+
+  void visitBranch(Branch node) {
+    visit(node.condition);
+  }
+
+  void visitInvokeConstConstructor(InvokeConstConstructor node) {
+    node.arguments.forEach(visitReference);
+  }
+
+  void visitLiteralList(LiteralList node) {
+    node.values.forEach(visitReference);
+  }
+
+  void visitLiteralMap(LiteralMap node) {
+    for (int i = 0; i < node.keys.length; ++i) {
+      visitReference(node.keys[i]);
+      visitReference(node.values[i]);
+    }
+  }
+
+  void visitConstant(Constant node) {
+  }
+
+  void visitParameter(Parameter node) {
+    throw "Parameters should not be visited by RegisterAllocator";
+  }
+
+  void visitContinuation(Continuation node) {
+    visit(node.body);
+
+    // Arguments get allocated left-to-right, so we release parameters
+    // right-to-left. This increases the likelihood that arguments can be
+    // transferred without intermediate assignments.
+    for (int i = node.parameters.length - 1; i >= 0; --i) {
+      release(node.parameters[i]);
+    }
+  }
+
+  void visitIsTrue(IsTrue node) {
+    visitReference(node.value);
+  }
+
+}