Faster moving FixedArray elements around.

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index 8e88c28695599a1e71f54d633ca53febb545d76a..27a32fa01d272981a18c71d7a8e895672b461352 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -362,7 +362,56 @@ BUILTIN(ArrayShift) {
 }
 
 
+static Object* CallJsBuiltin(const char* name,
+                             BuiltinArguments<NO_EXTRA_ARGUMENTS> args) {
+  HandleScope handleScope;
+
+  Handle<Object> js_builtin =
+      GetProperty(Handle<JSObject>(Top::global_context()->builtins()),
+                  name);
+  ASSERT(js_builtin->IsJSFunction());
+  Handle<JSFunction> function(Handle<JSFunction>::cast(js_builtin));
+  Vector<Object**> argv(Vector<Object**>::New(args.length() - 1));
+  int n_args = args.length() - 1;
+  for (int i = 0; i < n_args; i++) {
+    argv[i] = &args[i + 1];
+  }
+  bool pending_exception = false;
+  Handle<Object> result = Execution::Call(function,
+                                          args.receiver(),
+                                          n_args,
+                                          argv.start(),
+                                          &pending_exception);
+  if (pending_exception) return Failure::Exception();
+  return *result;
+}
+
+
+static bool ArrayPrototypeHasNoElements() {
+  // This method depends on non writability of Object and Array prototype
+  // fields.
+  Context* global_context = Top::context()->global_context();
+  // Array.prototype
+  JSObject* proto =
+      JSObject::cast(global_context->array_function()->prototype());
+  if (proto->elements() != Heap::empty_fixed_array()) return false;
+  // Hidden prototype
+  proto = JSObject::cast(proto->GetPrototype());
+  ASSERT(proto->elements() == Heap::empty_fixed_array());
+  // Object.prototype
+  proto = JSObject::cast(proto->GetPrototype());
+  if (proto != global_context->initial_object_prototype()) return false;
+  if (proto->elements() != Heap::empty_fixed_array()) return false;
+  ASSERT(proto->GetPrototype()->IsNull());
+  return true;
+}
+
+
 BUILTIN(ArrayUnshift) {
+  if (!ArrayPrototypeHasNoElements()) {
+    return CallJsBuiltin("ArrayUnshift", args);
+  }
+
   JSArray* array = JSArray::cast(*args.receiver());
   ASSERT(array->HasFastElements());
 
@@ -379,45 +428,44 @@ BUILTIN(ArrayUnshift) {
 
   FixedArray* elms = FixedArray::cast(array->elements());
 
-  // Fetch the prototype.
-  JSFunction* array_function =
-      Top::context()->global_context()->array_function();
-  JSObject* prototype = JSObject::cast(array_function->prototype());
-
   if (new_length > elms->length()) {
     // New backing storage is needed.
     int capacity = new_length + (new_length >> 1) + 16;
-    Object* obj = Heap::AllocateFixedArrayWithHoles(capacity);
+    Object* obj = Heap::AllocateRawFixedArray(capacity);
     if (obj->IsFailure()) return obj;
 
-    AssertNoAllocation no_gc;
+    reinterpret_cast<Array*>(obj)->set_map(Heap::fixed_array_map());
     FixedArray* new_elms = FixedArray::cast(obj);
-    WriteBarrierMode mode = new_elms->GetWriteBarrierMode(no_gc);
-    // Fill out the new array with old elements.
-    for (int i = 0; i < len; i++)
-      new_elms->set(to_add + i,
-                    GetElementToMove(i, elms, prototype),
-                    mode);
+    new_elms->set_length(capacity);
+
+    Object** new_elms_data = new_elms->data_start();
+    memcpy(new_elms_data + to_add, elms->data_start(), len * kPointerSize);
+
+    // Let's hope that compiler can easily optimize this filling code.
+    Object* hole = Heap::the_hole_value();
+    for (int i = new_length; i < capacity; i++) {
+      new_elms_data[i] = hole;
+    }
 
     elms = new_elms;
     array->set_elements(elms);
   } else {
-    AssertNoAllocation no_gc;
-    WriteBarrierMode mode = elms->GetWriteBarrierMode(no_gc);
-
-    // Move elements to the right
-    for (int i = 0; i < len; i++) {
-      elms->set(new_length - i - 1,
-                GetElementToMove(len - i - 1, elms, prototype),
-                mode);
-    }
+    memmove(elms->data_start() + to_add,
+            elms->data_start(),
+            len * kPointerSize);
   }
 
   // Add the provided values.
   AssertNoAllocation no_gc;
-  WriteBarrierMode mode = elms->GetWriteBarrierMode(no_gc);
   for (int i = 0; i < to_add; i++) {
-    elms->set(i, args[i + 1], mode);
+    elms->set(i, args[i + 1], SKIP_WRITE_BARRIER);
+  }
+  WriteBarrierMode mode = elms->GetWriteBarrierMode(no_gc);
+  if (mode == UPDATE_WRITE_BARRIER) {
+    Address address = elms->address();
+    for (int i = 0; i < new_length; i++) {
+      Heap::RecordWrite(address, FixedArray::kHeaderSize + i * kPointerSize);
+    }
   }
 
   // Set the length.
@@ -426,31 +474,6 @@ BUILTIN(ArrayUnshift) {
 }
 
 
-static Object* CallJsBuiltin(const char* name,
-                             BuiltinArguments<NO_EXTRA_ARGUMENTS> args) {
-  HandleScope handleScope;
-
-  Handle<Object> js_builtin =
-      GetProperty(Handle<JSObject>(Top::global_context()->builtins()),
-                  name);
-  ASSERT(js_builtin->IsJSFunction());
-  Handle<JSFunction> function(Handle<JSFunction>::cast(js_builtin));
-  Vector<Object**> argv(Vector<Object**>::New(args.length() - 1));
-  int n_args = args.length() - 1;
-  for (int i = 0; i < n_args; i++) {
-    argv[i] = &args[i + 1];
-  }
-  bool pending_exception = false;
-  Handle<Object> result = Execution::Call(function,
-                                          args.receiver(),
-                                          n_args,
-                                          argv.start(),
-                                          &pending_exception);
-  if (pending_exception) return Failure::Exception();
-  return *result;
-}
-
-
 BUILTIN(ArraySlice) {
   JSArray* array = JSArray::cast(*args.receiver());
   ASSERT(array->HasFastElements());