Tweaks to improve performance of type-inference.

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index a5680663b42babb1f0b732cb0e6a77ce72a21d3c..b58855d8b2b2b113311b60e242e141afad23535c 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -538,11 +538,17 @@ class TypeInformationSystem extends TypeSystem<TypeInformation> {
   }
 
   TypeMask joinTypeMasks(Iterable<TypeMask> masks) {
+    var dynamicType = compiler.typesTask.dynamicType;
+    // Don't do any work on computing unions if we know that after all that work
+    // the result will be `dynamic`.
+    if (masks.any((m) => m == dynamicType)) return dynamicType;
     TypeMask newType = const TypeMask.nonNullEmpty();
     for (TypeMask mask in masks) {

[sra1, 2015/12/16 05:06:17]

what is the input iterable?
If its is mapped, this could be made cheaper by collecting values in the first
scan.

[Siggi Cherem (dart-lang), 2015/12/17 01:02:56]

Done. It's mapped, although I couldn't observe any extra savings here since the
operation was just reading a field (see call in line 537).

       newType = newType.union(mask, classWorld);

[Johnni Winther, 2015/12/16 09:07:08]

How about:

TypeMask newType;
for (TypeMask mask in masks) {
  if (newType == null) {
    newType = mask;
  } else {
    newType = newType.union(mask, classWorld);
  }
  ...
}
return newType ?? const TypeMask.nonNullEmpty();

to avoid union of empty and the first mask.

Maybe also avoid unions where [newType] is empty.

[Siggi Cherem (dart-lang), 2015/12/17 01:02:57]

Done. Also not much savings from this: empty is handled efficiently already
(e.g. FlatTypeMask.empty knows to return the other mask, and the union operation
skips empty masks too).

+      // Likewise - stop early if we already reach dynamic.
+      if (newType == dynamicType) return dynamicType;
     }
-    return newType.containsAll(classWorld) ? dynamicType.type : newType;
+    return newType;
   }
 }