Linter Demo

Errors: 6

Warnings: 14

File: /home/fstrocco/Dart/dart/benchmark/devcompiler/lib/src/codegen/js_metalet.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. library dev_compiler.src.codegen.js_metalet; // TODO(jmesserly): import from its own package import 'package:dev_compiler/src/js/js_ast.dart'; import 'package:dev_compiler/src/js/precedence.dart'; import 'js_names.dart' show TemporaryId; /// A synthetic `let*` node, similar to that found in Scheme. /// /// For example, postfix increment can be desugared as: /// /// // psuedocode mix of Scheme and JS: /// (let* (x1=expr1, x2=expr2, t=x1[x2]) { x1[x2] = t + 1; t }) /// /// [MetaLet] will simplify itself automatically when [toExpression], /// [toStatement], or [toReturn] is called. /// /// * variables used once will be inlined. /// * if used in a statement context they can emit as blocks. /// * if return value is not used it can be eliminated, see [statelessResult]. /// * if there are no variables, the codegen will be simplified. /// /// Because this deals with JS AST nodes, it is not aware of any Dart semantics /// around statelessness (such as `final` variables). [variables] should not /// be created for these Dart expressions. /// class MetaLet extends Expression { /// Creates a temporary to contain the value of [expr]. The temporary can be /// used multiple times in the resulting expression. For example: /// `expr ** 2` could be compiled as `expr * expr`. The temporary scope will /// ensure `expr` is only evaluated once: `(x => x * x)(expr)`. /// /// If the expression does not end up using `x` more than once, or if those /// expressions can be treated as [stateless] (e.g. they are non-mutated /// variables), then the resulting code will be simplified automatically. final Map<String, Expression> variables; /// A list of expressions in the body. /// Conceptually this is like a comma expression: the last value is returned. final List<Expression> body; /// True if the final expression in [body] can be skipped in [toStatement]. final bool statelessResult; /// We run [toExpression] implicitly when the JS AST is visited, to get the /// transformation to happen before the tree is printed. /// This happens multiple times, so ensure the expression form is cached. Expression _expression; MetaLet(this.variables, this.body, {this.statelessResult: false}); /// Returns an expression that ignores the result. This is a cross between /// [toExpression] and [toStatement]. Used for C-style for-loop updaters, /// which is an expression syntactically, but functions more like a statement. Expression toVoidExpression() { var block = toStatement(); var s = block.statements; if (s.length == 1 && s.first is ExpressionStatement) { ExpressionStatement es = s.first; return es.expression; } return new Call(new ArrowFun([], block), []); } Expression toAssignExpression(Expression left) { if (left is Identifier) { var simple = _simplifyAssignment(left); if (simple != null) return simple; var exprs = body.toList(); exprs.add(exprs.removeLast().toAssignExpression(left)); return new MetaLet(variables, exprs); } return super.toAssignExpression(left); } Statement toVariableDeclaration(Identifier name) { var simple = _simplifyAssignment(name, isDeclaration: true); if (simple != null) return simple.toStatement(); return super.toVariableDeclaration(name); } Expression toExpression() { if (_expression != null) return _expression; var block = toReturn(); var s = block.statements; if (s.length == 1 && s.first is Return) { Return es = s.first; return _expression = es.value; } // Wrap it in an immediately called function to get in expression context. // TODO(jmesserly): return _expression = new Call(new ArrowFun([], block), []); } Block toStatement() { // Skip return value if not used. var statements = body.map((e) => e.toStatement()).toList(); if (statelessResult) statements.removeLast(); return _finishStatement(statements); } Block toReturn() { var statements = body .map((e) => e == body.last ? e.toReturn() : e.toStatement()) .toList(); return _finishStatement(statements); } accept(NodeVisitor visitor) => toExpression().accept(visitor); void visitChildren(NodeVisitor visitor) { toExpression().visitChildren(visitor); } /// This generates as either a comma expression or a call. int get precedenceLevel => variables.isEmpty ? EXPRESSION : CALL; Block _finishStatement(List<Statement> statements) { var params = []; var values = []; var block = _build(params, values, new Block(statements)); if (params.isEmpty) return block; var vars = []; for (int i = 0; i < params.length; i++) { vars.add(new VariableInitialization(params[i], values[i])); } return new Block(<Statement>[ new VariableDeclarationList('let', vars).toStatement(), block ]); } Node _build(List<TemporaryId> params, List<Expression> values, Node node) { // Visit the tree and count how many times each temp was used. var counter = new _VariableUseCounter(); node.accept(counter); // Also count the init expressions. for (var init in variables.values) init.accept(counter); var substitutions = {}; _substitute(node) => new Template(null, node).safeCreate(substitutions); variables.forEach((name, init) { // Since this is let*, subsequent variables can refer to previous ones, // so we need to substitute here. init = _substitute(init); int n = counter.counts[name]; if (n == null || n < 2) { substitutions[name] = _substitute(init); } else { params.add(substitutions[name] = new TemporaryId(name)); values.add(init); } }); // Interpolate the body: // Replace interpolated exprs with their value, if it only occurs once. // Otherwise replace it with a temp, which will be assigned once. return _substitute(node); } /// If we finish with an assignment to an identifier, try to simplify the /// block. For example: /// /// ((_) => _.add(1), _.add(2), result = _)([]) /// /// Can be transformed to: /// /// (result = [], result.add(1), result.add(2), result) /// /// However we should not simplify in this case because `result` is read: /// /// ((_) => _.addAll(result), _.add(2), result = _)([]) /// MetaLet _simplifyAssignment(Identifier left, {bool isDeclaration: false}) { // See if the result value is a let* temporary variable. if (body.last is! InterpolatedExpression) return null; InterpolatedExpression last = body.last; String name = last.nameOrPosition; if (!variables.containsKey(name)) return null; // Variables declared can't be used inside their initializer. if (!isDeclaration) { var finder = new _IdentFinder(left.name); for (var expr in body) { if (finder.found) break; expr.accept(finder); } // If the identifier was used elsewhere, bail, because we're going to // change the order of when the assignment happens. if (finder.found) return null; } var vars = new Map<String, Expression>.from(variables); var value = vars.remove(name); Expression assign; if (isDeclaration) { // Technically, putting one of these in a comma expression is not // legal. However when isDeclaration is true, toStatement will be // called immediately on the MetaLet, which results in legal JS. assign = new VariableDeclarationList( 'let', [new VariableInitialization(left, value)]); } else { assign = value.toAssignExpression(left); } var newBody = new Expression.binary([assign]..addAll(body), ','); Binary comma = new Template(null, newBody).safeCreate({name: left}); return new MetaLet(vars, comma.commaToExpressionList(), statelessResult: statelessResult); } } class _VariableUseCounter extends BaseVisitor { final counts = <String, int>{}; visitInterpolatedExpression(InterpolatedExpression node) { int n = counts[node.nameOrPosition]; counts[node.nameOrPosition] = n == null ? 1 : n + 1; } } class _IdentFinder extends BaseVisitor { final String name; bool found = false; _IdentFinder(this.name); visitIdentifier(Identifier node) { if (node.name == name) found = true; } visitNode(Node node) { if (!found) super.visitNode(node); } }