dart2js: improve performance of TypeMask.intersection

pkg/compiler/lib/src/types/union_type_mask.dart

Index: pkg/compiler/lib/src/types/union_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/union_type_mask.dart b/pkg/compiler/lib/src/types/union_type_mask.dart
index 24a019144a87e3d76477db32517f90d80fc4f17a..6f8c81367584a09ac669ef49f8e5eebfb414dc86 100644
--- a/pkg/compiler/lib/src/types/union_type_mask.dart
+++ b/pkg/compiler/lib/src/types/union_type_mask.dart
@@ -152,12 +152,17 @@ class UnionTypeMask implements TypeMask {
   TypeMask intersection(var other, ClassWorld classWorld) {
     other = TypeMask.nonForwardingMask(other);
     if (!other.isUnion && disjointMasks.contains(other)) return other;
+    if (other.isUnion && this == other) return this;

[Siggi Cherem (dart-lang), 2016/01/21 01:07:12]

These are common cases we hit during type propagation (here and below), so I
decided to add special cases here.

Either one of these changes here or the change in FlatTypeMask was enough to
make intersection practically disappear from the profile. Together, there is
little extra benefits (4s on some local tests, but that might be within the
noise). It might still be worth including the changes here though. Thoughts? My
inclination is to add them all.

[asgerf, 2016/01/21 01:15:45]

I say add both.

 
     List<TypeMask> intersections = <TypeMask>[];
     for (TypeMask current in disjointMasks) {
       if (other.isUnion) {
-        for (FlatTypeMask flatOther in other.disjointMasks) {
-          intersections.add(current.intersection(flatOther, classWorld));
+        if (other.disjointMasks.contains(current)) {
+          intersections.add(current);
+        } else {
+          for (FlatTypeMask flatOther in other.disjointMasks) {
+            intersections.add(current.intersection(flatOther, classWorld));
+          }
         }
       } else {
         intersections.add(current.intersection(other, classWorld));