Improve Array.shift() performance for small arrays.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 1cdca4c46efa14ac1cfb764d52fff9c641790749..85d1e7549fa38aa6a051ab0f0feaa970878674ec 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -50,6 +50,7 @@ class LChunkBuilder;
   V(ArgumentsElements)                         \
   V(ArgumentsLength)                           \
   V(ArgumentsObject)                           \
+  V(ArrayShift)                                \
   V(Bitwise)                                   \
   V(BlockEntry)                                \
   V(BoundsCheck)                               \
@@ -7077,6 +7078,51 @@ class HTransitionElementsKind V8_FINAL : public HTemplateInstruction<2> {
 };
 
 
+class HArrayShift V8_FINAL : public HTemplateInstruction<2> {
+ public:
+  static HArrayShift* New(Zone* zone,
+                          HValue* context,
+                          HValue* object,
+                          ElementsKind kind) {
+    return new(zone) HArrayShift(context, object, kind);
+  }
+
+  virtual Representation RequiredInputRepresentation(int index) V8_OVERRIDE {
+    return Representation::Tagged();
+  }
+
+  HValue* context() const { return OperandAt(0); }
+  HValue* object() const { return OperandAt(1); }
+  ElementsKind kind() const { return kind_; }
+
+  virtual void PrintDataTo(StringStream* stream) V8_OVERRIDE;
+
+  DECLARE_CONCRETE_INSTRUCTION(ArrayShift);
+
+ protected:
+  virtual bool DataEquals(HValue* other) V8_OVERRIDE {
+    HArrayShift* that = HArrayShift::cast(other);
+    return this->kind_ == that->kind_;
+  }
+
+ private:
+  HArrayShift(HValue* context, HValue* object, ElementsKind kind)
+      : kind_(kind) {
+    SetOperandAt(0, context);
+    SetOperandAt(1, object);
+    SetChangesFlag(kNewSpacePromotion);
+    set_representation(Representation::Tagged());
+    if (IsFastSmiOrObjectElementsKind(kind)) {
+      SetChangesFlag(kArrayElements);
+    } else {
+      SetChangesFlag(kDoubleArrayElements);
+    }
+  }
+
+  ElementsKind kind_;
+};
+
+
 class HStringAdd V8_FINAL : public HBinaryOperation {
  public:
   static HInstruction* New(Zone* zone,