fixes #131, use before define from variables to classes

lib/src/codegen/js_module_item_order.dart

+// Copyright (c) 2015, 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.
+
+import 'dart:collection' show HashMap;
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:analyzer/src/generated/element.dart';
+import 'package:dev_compiler/src/dependency_graph.dart' show corelibOrder;
+
+typedef void ModuleItemEmitter(AstNode item);
+
+/// Helper that tracks order of elements visited by the compiler, detecting
+/// if the top level item can be loaded eagerly or not.
+class ModuleItemLoadOrder {
+
+  /// The order that elements should be emitted in, with a bit indicating if
+  /// the element should be generated lazily. The value will be `false` if
+  /// the item could not be loaded, and needs to be made lazy.
+  ///
+  /// The order will match the original source order, unless something needed to
+  /// be moved sooner to satisfy dependencies.
+  ///
+  /// Sometimes it's impossible to ensure an ordering when confronting library
+  /// cycles. In that case, we mark the definition as lazy.
+  final _loaded = new Map<Element, bool>.identity();

[vsm, 2015/05/12 18:00:11]

Can you add a comment on why identity?  Should all our Map<Element, *> be
identity?

[Jennifer Messerly, 2015/05/12 18:47:04]

ah, I can remove. This was to workaround the temp-identity problem.

+
+  final _declarationNodes = new HashMap<Element, AstNode>.identity();
+
+  /// The stack of currently emitting elements, if generating top-level code
+  /// for them. This is not used when inside method bodies, because order does
+  /// not matter for those.
+  final _topLevelElements = new List<Element>();
+
+  /// The current element being loaded.
+  /// We can use this to determine if we're loading top-level code or not:
+  ///
+  ///     _currentElements.last == _topLevelElements.last
+  final _currentElements = new List<Element>();
+
+  /// Memoized results of [_inLibraryCycle].
+  final _libraryCycleMemo = new HashMap<LibraryElement, bool>();
+
+  final ModuleItemEmitter _emitModuleItem;
+
+  LibraryElement _currentLibrary;
+
+  ModuleItemLoadOrder(this._emitModuleItem);
+
+  bool isLoaded(Element e) => _loaded[e] == true;
+
+  /// Collect top-level elements and nodes we need to emit.
+  void collectElements(
+      LibraryElement library, Iterable<CompilationUnit> partsThenLibrary) {
+    assert(_currentLibrary == null);
+    _currentLibrary = library;
+
+    for (var unit in partsThenLibrary) {
+      for (var decl in unit.declarations) {
+        _declarationNodes[decl.element] = decl;
+
+        if (decl is ClassDeclaration) {
+          for (var member in decl.members) {
+            if (member is FieldDeclaration && member.isStatic) {
+              _collectElementsForVariable(member.fields);
+            }
+          }
+        } else if (decl is TopLevelVariableDeclaration) {
+          _collectElementsForVariable(decl.variables);
+        }
+      }
+    }
+  }
+
+  void _collectElementsForVariable(VariableDeclarationList fields) {
+    for (var field in fields.variables) {
+      _declarationNodes[field.element] = field;
+    }
+  }
+
+  /// Start generating top-level code for the element [e].
+  /// Subsequent [loadElement] calls will cause those elements to be generated
+  /// before this one, until [finishElement] is called.
+  void startTopLevel(Element e) {
+    assert(isCurrentElement(e));
+    // Assume loading will succeed until proven otherwise.
+    _loaded[e] = true;
+    _topLevelElements.add(e);
+  }
+
+  /// Finishes the top-level code for the element [e].
+  void finishTopLevel(Element e) {
+    var last = _topLevelElements.removeLast();
+    assert(identical(e, last));
+  }
+
+  // Starts generating code for the element [e].
+  //
+  // Normally this is called implicitly by [loadDeclaration] and/or [loadElement].

[vsm, 2015/05/12 18:00:11]

line len?

[Jennifer Messerly, 2015/05/12 18:47:04]

Done.

+  // However, for synthetic elements (like temporary variables) it must be
+  // called explicitly, and paired with a call to [finishElement].
+  void startDeclaration(Element e) {
+    // Assume load will succeed until proven otherwise.
+    _loaded[e] = true;
+    _currentElements.add(e);
+  }
+
+  /// Returns true if all dependencies were loaded successfully.
+  bool finishDeclaration(Element e) {
+    var last = _currentElements.removeLast();
+    assert(identical(e, last));
+    return _loaded[e];
+  }
+
+  /// Loads a top-level declaration. This is similar to [loadElement], but it
+  /// ensures we always visit the node if it has not been emitted yet. In other
+  /// words, it handles nodes that do not need dependency resolution like
+  /// top-level functions.
+  bool loadDeclaration(AstNode node, Element e) {
+    assert(e.library == _currentLibrary);
+
+    // If we already tried to load it, see if we succeeded or not.
+    // Otherwise try and load now.
+    var loaded = _loaded[e];
+    if (loaded != null) return loaded;
+
+    // Otherwise, try to load it.
+    startDeclaration(e);
+    _emitModuleItem(node);
+    return finishDeclaration(e);
+  }
+
+  bool isCurrentElement(Element e) =>
+      _currentElements.isNotEmpty && identical(e, _currentElements.last);
+
+  /// To emit top-level module items, we sometimes need to reorder them.
+  ///
+  /// This function takes care of that, and also detects cases where reordering
+  /// failed, and we need to resort to lazy loading, by marking the element as
+  /// lazy. All elements need to be aware of this possibility and generate code
+  /// accordingly.
+  ///
+  /// If we are not emitting top-level code, this does nothing, because all
+  /// declarations are assumed to be available before we start execution.
+  /// See [startTopLevel].
+  void declareBeforeUse(Element e) {
+    if (e == null || _topLevelElements.isEmpty) return;
+    if (!isCurrentElement(_topLevelElements.last)) return;
+
+    // If the item is from our library, try to emit it now.
+    bool loaded;
+    if (e.library == _currentLibrary) {
+      // Type parameters are not in scope when generating hoisted fields.
+      if (e is TypeParameterElement &&
+          _currentElements.last is VariableElement) {
+        loaded = false;
+      } else {
+        var node = _declarationNodes[e];
+        loaded = node == null || loadDeclaration(node, e);
+      }
+    } else {
+      // We can't force things from other libraries to be emitted in a different
+      // order. Instead, we see if the library itself can be loaded before the
+      // current library. Generally that is possible, unless we have cyclic
+      // imports.
+      loaded = libraryIsLoaded(e.library);
+    }
+
+    if (loaded) return;
+
+    // If we failed to emit it, then we need to make sure all currently emitting
+    // elements are generated in a lazy way.
+    for (var current in _topLevelElements) {
+      // TODO(jmesserly): if we want to log what triggered laziness, this is
+      // the place to do so.
+      _loaded[current] = false;
+    }
+  }
+
+  bool libraryIsLoaded(LibraryElement library) {
+    assert(library != _currentLibrary);
+
+    if (library == null) return true;

[vsm, 2015/05/12 18:00:11]

When would this be null?

[Jennifer Messerly, 2015/05/12 18:47:04]

good catch. This showed up in the code at one point ... I think maybe our
synthetic elements for temps didn't have enclosingElement filled in, hence no
library. Anyways, fixed now, so removed this check :)

+
+    // The SDK is a special case: we optimize the order to prevent laziness.
+    if (library.isInSdk) {
+      // SDK is loaded before non-SDK libraries
+      if (!_currentLibrary.isInSdk) return true;
+
+      // Compute the order of both SDK libraries. If unknown, assume it's after.
+      var order = corelibOrder.indexOf(library.name);
+      if (order == -1) order = corelibOrder.length;
+
+      var currentOrder = corelibOrder.indexOf(_currentLibrary.name);
+      if (currentOrder == -1) currentOrder = corelibOrder.length;
+
+      // If the dart:* library we are currently compiling is loaded after the
+      // class's library, then we know the class is available.
+      if (order != currentOrder) return currentOrder > order;
+
+      // If we don't know the order of the class's library or the current
+      // library, do the normal cycle check. (Not all SDK libs are cycles.)
+    }
+
+    return !_inLibraryCycle(library);
+  }
+
+  /// Returns true if [library] depends on the [currentLibrary] via some
+  /// transitive import.
+  bool _inLibraryCycle(LibraryElement library) {
+    // SDK libs don't depend on things outside the SDK.
+    // (We can reach this via the recursive call below.)
+    if (library.isInSdk && !_currentLibrary.isInSdk) return false;
+
+    var result = _libraryCycleMemo[library];
+    if (result != null) return result;
+
+    result = library == _currentLibrary;
+    _libraryCycleMemo[library] = result;
+    for (var e in library.imports) {
+      if (result) break;
+      result = _inLibraryCycle(e.importedLibrary);
+    }
+    for (var e in library.exports) {
+      if (result) break;
+      result = _inLibraryCycle(e.exportedLibrary);
+    }
+    return _libraryCycleMemo[library] = result;
+  }
+}