Implement efficient element copying in ElementsAccessors.

src/elements.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 11 matching lines @@
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #ifndef V8_ELEMENTS_H_
 #define V8_ELEMENTS_H_
 
 #include "objects.h"
+#include "heap.h"
+#include "isolate.h"
 
 namespace v8 {
 namespace internal {
 
 // Abstract base class for handles that can operate on objects with differing
 // ElementsKinds.
 class ElementsAccessor {
  public:
   explicit ElementsAccessor(const char* name) : name_(name) { }
   virtual ~ElementsAccessor() { }
 
-  virtual const char* name() const { return name_; }
+  virtual ElementsKind kind() const = 0;
+  const char* name() const { return name_; }
 
   // Returns true if a holder contains an element with the specified key
   // without iterating up the prototype chain.  The caller can optionally pass
   // in the backing store to use for the check, which must be compatible with
   // the ElementsKind of the ElementsAccessor. If backing_store is NULL, the
   // holder->elements() is used as the backing store.
   virtual bool HasElement(Object* receiver,
                           JSObject* holder,
                           uint32_t key,
                           FixedArrayBase* backing_store = NULL) = 0;
@@ skipping 24 matching lines @@
   // EcmaScript 5.1 semantics.
   virtual MaybeObject* SetCapacityAndLength(JSArray* array,
                                             int capacity,
                                             int length) = 0;
 
   // Deletes an element in an object, returning a new elements backing store.
   virtual MaybeObject* Delete(JSObject* holder,
                               uint32_t key,
                               JSReceiver::DeleteMode mode) = 0;
 
+  virtual MaybeObject* CopyElements(JSObject* from_holder,
+                                    uint32_t from_start,
+                                    FixedArrayBase* to,
+                                    ElementsKind to_kind,
+                                    uint32_t to_start,
+                                    int copy_size,
+                                    FixedArrayBase* from = NULL) = 0;

[Sven Panne, 2012/03/09 08:15:18]

Reading only this header, it is a bit confusing why there is "from" in addition
to "from_holder". A comment for a method with more than half a dozen parameter
wouldn't hurt... ;-)

[danno, 2012/03/09 12:12:35]

Done.

+
+  MaybeObject* CopyElements(JSObject* from_holder,
+                            FixedArrayBase* to,
+                            ElementsKind to_kind,
+                            FixedArrayBase* from = NULL) {
+    return CopyElements(from_holder, 0, to, to_kind, 0, -1, from);
+  }
+
   virtual MaybeObject* AddElementsToFixedArray(Object* receiver,
                                                JSObject* holder,
                                                FixedArray* to,
                                                FixedArrayBase* from = NULL) = 0;
 
   // Returns a shared ElementsAccessor for the specified ElementsKind.
   static ElementsAccessor* ForKind(ElementsKind elements_kind) {
     ASSERT(elements_kind < kElementsKindCount);
     return elements_accessors_[elements_kind];
   }
@@ skipping 18 matching lines @@
   virtual uint32_t GetKeyForIndex(FixedArrayBase* backing_store,
                                   uint32_t index) = 0;
 
  private:
   static ElementsAccessor** elements_accessors_;
   const char* name_;
 
   DISALLOW_COPY_AND_ASSIGN(ElementsAccessor);
 };
 
+
+void CopyObjectToObjectElements(AssertNoAllocation* no_gc,

[Sven Panne, 2012/03/09 08:15:18]

Hmmm, the first parameter is actually a phantom type, I guess. We have one other
such type in v8 I know of (WhitenessWitness), but I have the gut feeling that
passing an instance of such phantom types around at runtime is very inelegant.
What we really want is a compile time check with no runtime overhead. I am sure
one can solve this via template tricks^H^H^H technology, but I don't have a
solution at hand. Not a blocker for the CL, but just something to keep in mind.

[danno, 2012/03/09 12:12:35]

Done.

+                                FixedArray* from_obj,
+                                ElementsKind from_kind,
+                                uint32_t from_start,
+                                FixedArray* to_obj,
+                                ElementsKind to_kind,
+                                uint32_t to_start,
+                                int copy_size);
+
+
 } }  // namespace v8::internal
 
 #endif  // V8_ELEMENTS_H_