Optimize is-check to instanceof when it eliminates an interceptor

pkg/compiler/lib/src/ssa/codegen.dart

Index: pkg/compiler/lib/src/ssa/codegen.dart
diff --git a/pkg/compiler/lib/src/ssa/codegen.dart b/pkg/compiler/lib/src/ssa/codegen.dart
index 4c97c464adcaf3c91da6339ce6fe6fda4845645d..58ff83099fa2b7d92218c8b7e21e72accbe12631 100644
--- a/pkg/compiler/lib/src/ssa/codegen.dart
+++ b/pkg/compiler/lib/src/ssa/codegen.dart
@@ -2304,6 +2304,18 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     if (!negative) push(new js.Prefix('!', pop()));
   }
 
+  void checkTypeViaInstanceof(
+      HInstruction input, DartType type, bool negative) {
+    registry.registerIsCheck(type);
+
+    use(input);
+
+    js.Expression jsClassReference =
+        backend.emitter.constructorAccess(type.element);

[floitsch, 2015/01/16 14:14:33]

Please confirm that `constructorAccess` is correct. (I believe it is, but a
second opinion is helpful).
In the new emitter the constructor-access might return the stub (at least until
there really was a construction).
Normally the test should simply fail in these cases (which makes sense, since no
instantiated object exists yet).

Later the rhs might change (probably leading to a deopt), but at that point we
are obviously safe.

[sra1, 2015/01/20 20:07:27]

Verified.
Good points.
Many instances are in methods of the class, e.g.  Duration.== has the check
"other is Duration".  These cases can only be reached after construction.

I think the risks are low since we are using 'instanceof' only if we can
eliminate a getInterceptor call.

I wonder if instanceof might be better than a property test for
self-intercepting classes where we 'know' that the property test will be
megamorphic?

+    push(js.js('# instanceof #', [pop(), jsClassReference]));
+    if (negative) push(new js.Prefix('!', pop()));
+  }
+
   void handleNumberOrStringSupertypeCheck(HInstruction input,
                                           HInstruction interceptor,
                                           DartType type,
@@ -2421,6 +2433,10 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
         js.Expression numTest = pop();
         checkBigInt(input, relation);
         push(new js.Binary(negative ? '||' : '&&', numTest, pop()), node);
+      } else if (node.useInstanceOf) {
+        assert(interceptor == null);
+        checkTypeViaInstanceof(input, type, negative);
+        attachLocationToLast(node);
       } else if (Elements.isNumberOrStringSupertype(element, compiler)) {
         handleNumberOrStringSupertypeCheck(
             input, interceptor, type, negative: negative);