dart2js cps: Global value numbering and loop-invariant code motion.

pkg/compiler/lib/src/cps_ir/redundant_refinement.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 dart2js.cps_ir.redundant_refinement;
+
+import 'cps_ir_nodes.dart';
+import 'optimizers.dart' show Pass;
+import 'type_mask_system.dart';
+
+/// Removes [Refinement] nodes where the input value is already known to
+/// satisfy the refinement type.
+///
+/// Note: This pass improves loop-invariant code motion in the GVN pass because
+/// GVN will currently not hoist a primitive across a refinement guard.
+/// But some opportunities for hoisting are still missed.  A field access can
+/// safely be hoisted across a non-redundant refinement as long as the less
+/// refined value is still known to have the field.  For example:
+///
+///     class A { var field; }
+///     class B extends A {}
+///
+///     var x = getA();       // Return type is subclass of A.
+///     while (x is B) {      // Refinement to B is not redundant.
+///         x.field.baz++;    // x.field is safe for hoisting,
+///     }                     // but blocked by the refinement node.
+///
+/// Ideally, this pass should go away and GVN should handle refinements
+/// directly.
+class RedundantRefinementEliminator extends TrampolineRecursiveVisitor
+                                    implements Pass {
+  String get passName => 'Redundant refinement elimination';
+
+  TypeMaskSystem typeSystem;
+
+  RedundantRefinementEliminator(this.typeSystem);
+
+  void rewrite(FunctionDefinition node) {
+    visit(node);
+  }
+
+  Expression traverseLetPrim(LetPrim node) {
+    Expression next = node.body;
+    if (node.primitive is Refinement) {
+      Refinement refinement = node.primitive;
+      Primitive value = refinement.value.definition;
+      if (typeSystem.isMorePreciseOrEqual(value.type, refinement.refineType)) {
+        value.substituteFor(refinement);
+        refinement.destroy();
+        node.remove();
+        return next;
+      }
+    }
+    return next;
+  }
+}