Fix a bug where we would emit a noSuchMethod handler in the Object class for Object methods because…

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

 // Copyright (c) 2012, 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 js_backend;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 2800 matching lines @@
       DartType objectType = objectClass.computeType(compiler);
 
       for (Selector selector in selectors) {
         // If the selector is typed, we check to see if that type may
         // have a user-defined noSuchMethod implementation. If not, we
         // skip the selector altogether.
 
         TypeMask mask = selector.mask;
         if (mask == null) {
           mask = new TypeMask.subclass(compiler.objectClass.rawType);
+        } else if (mask.isEmpty && !mask.isNullable) {

[ahe, 2013/08/20 08:00:33]

I'm a little concerned about this. Perhaps it is just a sign that I don't fully
understand the type inferrer.

We saw this bug in code using mirrors which means that type inferencing is
disabled. That makes me wonder if this is a symptom fix for something else.

Also, if this kind of type is meaningless, why is it inferred?

[ngeoffray, 2013/08/20 08:03:48]

The bug is not necessarily related to type inferencing. The SSA backend can also
create these selectors with empty receiver type. It just happens that it does
not do anything with it and registers  the selector to the world. Maybe that's
the point where you would want the filtering to happen?

I think it belongs in this file, filtering earlier seems more like an
optimization.

[ahe, 2013/08/20 08:11:13]

I don't understand why it would be an optimization. What is the contract between
providers of type masks and the consumers of type masks? How do we guarantee
correctness?

I would say it is reasonable for the consumer (emitter in this case) to assume
the provider (SSA backend/inferrer) only produce valid type masks.

Otherwise, you have to check all places where you consume a typemask that it
isn't bogus.

[ngeoffray, 2013/08/20 08:24:03]

Why does it have to be bogus? There's an optimization phase that realizes the
type of an instruction is empty (take for example, a = 42; if (a is List)
a.toString();). If we don't do dead code elimination, the codegen will see this
call to a.toString, type the selector with a's type and register it to the
world. Checking if it's an empty selector seems like an optimization to me.

[ahe, 2013/08/20 08:31:44]

Your 
Let's talk about this face to face. Perhaps we need a whiteboard. I'm having a
really hard time understanding lines 2813-2877 (except, I understand lines
2872-2875).

[pavelj, 2013/08/20 14:51:44]

Peter, that version was not using mirrors.

On 2013/08/20 08:00:33, ahe wrote:

+          // We know a non-null empty mask cannot happen at runtime.
+          // We may have those selectors in the set of seen selectors
+          // if the backend did not optimize them.
+          continue;
         }
 
         // If the receiver is guaranteed to have a member that
         // matches what we're looking for, there's no need to
         // introduce a noSuchMethod handler. It will never be called.
         //
         // As an example, consider this class hierarchy:
         //
         //                   A    <-- noSuchMethod
         //                  / \
@@ skipping 1321 matching lines @@
 
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;