[elements] Enable left-trimming again

src/elements.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/elements.h"
 
 #include "src/arguments.h"
 #include "src/conversions.h"
 #include "src/factory.h"
 #include "src/messages.h"
@@ skipping 659 matching lines @@
     return ElementsAccessorSubclass::ShiftImpl(receiver, backing_store);
   }
 
   static Handle<Object> ShiftImpl(Handle<JSArray> receiver,
                                   Handle<FixedArrayBase> backing_store) {
     UNREACHABLE();
     return Handle<Object>();
   }
 
   void SetLength(Handle<JSArray> array, uint32_t length) final {
-    ElementsAccessorSubclass::SetLengthImpl(array, length,
+    ElementsAccessorSubclass::SetLengthImpl(array->GetIsolate(), array, length,
                                             handle(array->elements()));
   }
 
-  static void SetLengthImpl(Handle<JSArray> array, uint32_t length,
+  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
+                            uint32_t length,
                             Handle<FixedArrayBase> backing_store) {
     DCHECK(!array->SetLengthWouldNormalize(length));
     DCHECK(IsFastElementsKind(array->GetElementsKind()));
     uint32_t old_length = 0;
     CHECK(array->length()->ToArrayIndex(&old_length));
 
     if (old_length < length) {
       ElementsKind kind = array->GetElementsKind();
       if (!IsFastHoleyElementsKind(kind)) {
         kind = GetHoleyElementsKind(kind);
         JSObject::TransitionElementsKind(array, kind);
       }
     }
 
     // Check whether the backing store should be shrunk.
     uint32_t capacity = backing_store->length();
+    old_length = Min(old_length, capacity);
     if (length == 0) {
       array->initialize_elements();
     } else if (length <= capacity) {
       if (array->HasFastSmiOrObjectElements()) {
         backing_store = JSObject::EnsureWritableFastElements(array);
       }
       if (2 * length <= capacity) {
         // If more than half the elements won't be used, trim the array.
-        array->GetHeap()->RightTrimFixedArray<Heap::CONCURRENT_TO_SWEEPER>(
+        isolate->heap()->RightTrimFixedArray<Heap::CONCURRENT_TO_SWEEPER>(
             *backing_store, capacity - length);
       } else {
         // Otherwise, fill the unused tail with holes.
         for (uint32_t i = length; i < old_length; i++) {
           BackingStore::cast(*backing_store)->set_the_hole(i);
         }
       }
     } else {
       // Check whether the backing store should be expanded.
       capacity = Max(length, JSObject::NewElementsCapacity(capacity));
@@ skipping 245 matching lines @@
 
 
 class DictionaryElementsAccessor
     : public ElementsAccessorBase<DictionaryElementsAccessor,
                                   ElementsKindTraits<DICTIONARY_ELEMENTS> > {
  public:
   explicit DictionaryElementsAccessor(const char* name)
       : ElementsAccessorBase<DictionaryElementsAccessor,
                              ElementsKindTraits<DICTIONARY_ELEMENTS> >(name) {}
 
-  static void SetLengthImpl(Handle<JSArray> array, uint32_t length,
+  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
+                            uint32_t length,
                             Handle<FixedArrayBase> backing_store) {
     Handle<SeededNumberDictionary> dict =
         Handle<SeededNumberDictionary>::cast(backing_store);
-    Isolate* isolate = array->GetIsolate();
     int capacity = dict->Capacity();
     uint32_t old_length = 0;
     CHECK(array->length()->ToArrayLength(&old_length));
     if (length < old_length) {
       if (dict->requires_slow_elements()) {
         // Find last non-deletable element in range of elements to be
         // deleted and adjust range accordingly.
         for (int entry = 0; entry < capacity; entry++) {
           DisallowHeapAllocation no_gc;
           Object* index = dict->KeyAt(entry);
@@ skipping 355 matching lines @@
                                                       args, push_size, AT_END);
   }
 
   static uint32_t UnshiftImpl(Handle<JSArray> receiver,
                               Handle<FixedArrayBase> backing_store,
                               Arguments* args, uint32_t unshift_size) {
     return FastElementsAccessorSubclass::AddArguments(
         receiver, backing_store, args, unshift_size, AT_START);
   }
 
-  static void MoveElements(Heap* heap, Handle<FixedArrayBase> backing_store,
-                           int dst_index, int src_index, int len,
-                           int hole_start, int hole_end) {
+  static void MoveElements(Isolate* isolate, Handle<JSArray> receiver,
+                           Handle<FixedArrayBase>& backing_store, int dst_index,
+                           int src_index, int len, int hole_start,
+                           int hole_end) {
     UNREACHABLE();
   }
 
   static Handle<JSArray> SliceImpl(Handle<JSObject> receiver,
                                    Handle<FixedArrayBase> backing_store,
                                    uint32_t start, uint32_t end) {
     DCHECK(start < end);
     Isolate* isolate = receiver->GetIsolate();
     int result_len = end - start;
     Handle<JSArray> result_array = isolate->factory()->NewJSArray(
@@ skipping 28 matching lines @@
         KindTraits::Kind, delete_count, delete_count);
     if (delete_count > 0) {
       DisallowHeapAllocation no_gc;
       FastElementsAccessorSubclass::CopyElementsImpl(
           *backing_store, start, deleted_elements->elements(), KindTraits::Kind,
           0, kPackedSizeNotKnown, delete_count);
     }
 
     // Delete and move elements to make space for add_count new elements.
     if (add_count < delete_count) {
-      FastElementsAccessorSubclass::SpliceShrinkStep(backing_store, heap, start,
-                                                     delete_count, add_count,
-                                                     length, new_length);
+      FastElementsAccessorSubclass::SpliceShrinkStep(
+          isolate, receiver, backing_store, start, delete_count, add_count,
+          length, new_length);
     } else if (add_count > delete_count) {
-      backing_store = FastElementsAccessorSubclass::SpliceGrowStep(
-          receiver, backing_store, isolate, heap, start, delete_count,
-          add_count, length, new_length);
+      FastElementsAccessorSubclass::SpliceGrowStep(
+          isolate, receiver, backing_store, start, delete_count, add_count,
+          length, new_length);
     }
 
     // Copy over the arguments.
     FastElementsAccessorSubclass::CopyArguments(args, backing_store, add_count,
                                                 3, start);
 
     receiver->set_length(Smi::FromInt(new_length));
     FastElementsAccessorSubclass::TryTransitionResultArrayToPacked(
         deleted_elements);
     return deleted_elements;
   }
 
  private:
-  static void SpliceShrinkStep(Handle<FixedArrayBase> backing_store, Heap* heap,
+  // SpliceShrinkStep might modify the backing_store.
+  static void SpliceShrinkStep(Isolate* isolate, Handle<JSArray> receiver,
+                               Handle<FixedArrayBase>& backing_store,
                                uint32_t start, uint32_t delete_count,
                                uint32_t add_count, uint32_t len,
                                uint32_t new_length) {
     const int move_left_count = len - delete_count - start;
     const int move_left_dst_index = start + add_count;
     FastElementsAccessorSubclass::MoveElements(
-        heap, backing_store, move_left_dst_index, start + delete_count,
-        move_left_count, new_length, len);
+        isolate, receiver, backing_store, move_left_dst_index,
+        start + delete_count, move_left_count, new_length, len);
   }
 
-
-  static Handle<FixedArrayBase> SpliceGrowStep(
-      Handle<JSArray> receiver, Handle<FixedArrayBase> backing_store,
-      Isolate* isolate, Heap* heap, uint32_t start, uint32_t delete_count,
-      uint32_t add_count, uint32_t length, uint32_t new_length) {
+  // SpliceGrowStep might modify the backing_store.
+  static void SpliceGrowStep(Isolate* isolate, Handle<JSArray> receiver,
+                             Handle<FixedArrayBase>& backing_store,
+                             uint32_t start, uint32_t delete_count,
+                             uint32_t add_count, uint32_t length,
+                             uint32_t new_length) {
     // Check we do not overflow the new_length.
     DCHECK((add_count - delete_count) <= (Smi::kMaxValue - length));
     // Check if backing_store is big enough.
     if (new_length <= static_cast<uint32_t>(backing_store->length())) {
       FastElementsAccessorSubclass::MoveElements(
-          heap, backing_store, start + add_count, start + delete_count,
-          (length - delete_count - start), 0, 0);
-      return backing_store;
+          isolate, receiver, backing_store, start + add_count,
+          start + delete_count, (length - delete_count - start), 0, 0);
+      return;
     }
     // New backing storage is needed.
     int capacity = JSObject::NewElementsCapacity(new_length);
     // Partially copy all elements up to start.
     Handle<FixedArrayBase> new_elms =
         FastElementsAccessorSubclass::ConvertElementsWithCapacity(
             receiver, backing_store, KindTraits::Kind, capacity, start);
     // Copy the trailing elements after start + delete_count
     FastElementsAccessorSubclass::CopyElementsImpl(
         *backing_store, start + delete_count, *new_elms, KindTraits::Kind,
         start + add_count, kPackedSizeNotKnown,
         ElementsAccessor::kCopyToEndAndInitializeToHole);
     receiver->set_elements(*new_elms);
-    return new_elms;
+    backing_store = new_elms;
   }
 
   static Handle<Object> RemoveElement(Handle<JSArray> receiver,
                                       Handle<FixedArrayBase> backing_store,
                                       Where remove_position) {
+    Isolate* isolate = receiver->GetIsolate();
     uint32_t length =
         static_cast<uint32_t>(Smi::cast(receiver->length())->value());
-    Isolate* isolate = receiver->GetIsolate();
     DCHECK(length > 0);
     int new_length = length - 1;
     int remove_index = remove_position == AT_START ? 0 : new_length;
     Handle<Object> result =
         FastElementsAccessorSubclass::GetImpl(backing_store, remove_index);
     if (remove_position == AT_START) {
-      Heap* heap = isolate->heap();
-      FastElementsAccessorSubclass::MoveElements(heap, backing_store, 0, 1,
-                                                 new_length, 0, 0);
+      FastElementsAccessorSubclass::MoveElements(
+          isolate, receiver, backing_store, 0, 1, new_length, 0, 0);
     }
-    FastElementsAccessorSubclass::SetLengthImpl(receiver, new_length,
+    FastElementsAccessorSubclass::SetLengthImpl(isolate, receiver, new_length,
                                                 backing_store);
 
     if (IsHoleyElementsKind(KindTraits::Kind) && result->IsTheHole()) {
       return receiver->GetIsolate()->factory()->undefined_value();
     }
     return result;
   }
 
   static uint32_t AddArguments(Handle<JSArray> receiver,
                                Handle<FixedArrayBase> backing_store,
@@ skipping 13 matching lines @@
       int copy_dst_index = remove_position == AT_START ? add_size : 0;
       // Copy over all objects to a new backing_store.
       backing_store = FastElementsAccessorSubclass::ConvertElementsWithCapacity(
           receiver, backing_store, KindTraits::Kind, capacity, 0,
           copy_dst_index, ElementsAccessor::kCopyToEndAndInitializeToHole);
       receiver->set_elements(*backing_store);
     } else if (remove_position == AT_START) {
       // If the backing store has enough capacity and we add elements to the
       // start we have to shift the existing objects.
       Isolate* isolate = receiver->GetIsolate();
-      FastElementsAccessorSubclass::MoveElements(isolate->heap(), backing_store,
-                                                 add_size, 0, length, 0, 0);
+      FastElementsAccessorSubclass::MoveElements(
+          isolate, receiver, backing_store, add_size, 0, length, 0, 0);
     }
 
     int insertion_index = remove_position == AT_START ? 0 : length;
     // Copy the arguments to the start.
     FastElementsAccessorSubclass::CopyArguments(args, backing_store, add_size,
                                                 1, insertion_index);
     // Set the length.
     receiver->set_length(Smi::FromInt(new_length));
     return new_length;
   }
@@ skipping 32 matching lines @@
                              Object* value, WriteBarrierMode mode) {
     FixedArray::cast(backing_store)->set(entry, value, mode);
   }
 
   static Object* GetRaw(FixedArray* backing_store, uint32_t entry) {
     uint32_t index = FastElementsAccessorSubclass::GetIndexForEntryImpl(
         backing_store, entry);
     return backing_store->get(index);
   }
 
-  static void MoveElements(Heap* heap, Handle<FixedArrayBase> backing_store,
-                           int dst_index, int src_index, int len,
-                           int hole_start, int hole_end) {
+  static void MoveElements(Isolate* isolate, Handle<JSArray> receiver,
+                           Handle<FixedArrayBase>& backing_store, int dst_index,
+                           int src_index, int len, int hole_start,
+                           int hole_end) {
+    Heap* heap = isolate->heap();
     Handle<FixedArray> dst_elms = Handle<FixedArray>::cast(backing_store);
-    if (len != 0) {
+    if (heap->CanMoveObjectStart(*dst_elms) && dst_index == 0) {
+      dst_elms = Handle<FixedArray>(

[Igor Sheludko, 2015/12/22 17:15:01]

Sorry for confusion. I actually meant to do this:
  *dst_elms.location() = FixedArray::cast(heap->Left...);

This way you don't even need to touch the backing_store handle because they
share location. And all the other copies would be updated automatically and you
don't have to pass backing_store by reference.

+          FixedArray::cast(heap->LeftTrimFixedArray(*dst_elms, src_index)),
+          isolate);
+      backing_store = dst_elms;

[Igor Sheludko, 2015/12/22 17:15:01]

Not needed once the above comment is addressed.

+      receiver->set_elements(*dst_elms);
+      // Adjust the hole offset as the array has been shrunk.
+      hole_end -= src_index;
+      DCHECK_LE(hole_start, backing_store->length());
+      DCHECK_LE(hole_end, backing_store->length());
+    } else if (len != 0) {
       DisallowHeapAllocation no_gc;
       heap->MoveElements(*dst_elms, dst_index, src_index, len);
     }
     if (hole_start != hole_end) {
       dst_elms->FillWithHoles(hole_start, hole_end);
     }
   }
 
   // NOTE: this method violates the handlified function signature convention:
   // raw pointer parameters in the function that allocates.
@@ skipping 98 matching lines @@
   static inline void SetImpl(FixedArrayBase* backing_store, uint32_t entry,
                              Object* value) {
     FixedDoubleArray::cast(backing_store)->set(entry, value->Number());
   }
 
   static inline void SetImpl(FixedArrayBase* backing_store, uint32_t entry,
                              Object* value, WriteBarrierMode mode) {
     FixedDoubleArray::cast(backing_store)->set(entry, value->Number());
   }
 
-  static void MoveElements(Heap* heap, Handle<FixedArrayBase> backing_store,
-                           int dst_index, int src_index, int len,
-                           int hole_start, int hole_end) {
+  static void MoveElements(Isolate* isolate, Handle<JSArray> receiver,
+                           Handle<FixedArrayBase>& backing_store, int dst_index,
+                           int src_index, int len, int hole_start,
+                           int hole_end) {
+    Heap* heap = isolate->heap();
     Handle<FixedDoubleArray> dst_elms =
         Handle<FixedDoubleArray>::cast(backing_store);
-    if (len != 0) {
+    if (heap->CanMoveObjectStart(*dst_elms) && dst_index == 0) {
+      dst_elms = Handle<FixedDoubleArray>(

[Igor Sheludko, 2015/12/22 17:15:01]

Ditto.

+          FixedDoubleArray::cast(
+              heap->LeftTrimFixedArray(*dst_elms, src_index)),
+          isolate);
+      backing_store = dst_elms;

[Igor Sheludko, 2015/12/22 17:15:01]

Ditto.

+      receiver->set_elements(*dst_elms);
+      // Adjust the hole offset as the array has been shrunk.
+      hole_end -= src_index;
+      DCHECK_LE(hole_start, backing_store->length());
+      DCHECK_LE(hole_end, backing_store->length());
+    } else if (len != 0) {
       MemMove(dst_elms->data_start() + dst_index,
               dst_elms->data_start() + src_index, len * kDoubleSize);
     }
     if (hole_start != hole_end) {
       dst_elms->FillWithHoles(hole_start, hole_end);
     }
   }
 
   static void CopyElementsImpl(FixedArrayBase* from, uint32_t from_start,
                                FixedArrayBase* to, ElementsKind from_kind,
@@ skipping 85 matching lines @@
                                 uint32_t entry) {
     uint32_t index = GetIndexForEntryImpl(*backing_store, entry);
     return BackingStore::get(Handle<BackingStore>::cast(backing_store), index);
   }
 
   static PropertyDetails GetDetailsImpl(FixedArrayBase* backing_store,
                                         uint32_t entry) {
     return PropertyDetails(DONT_DELETE, DATA, 0, PropertyCellType::kNoCell);
   }
 
-  static void SetLengthImpl(Handle<JSArray> array, uint32_t length,
+  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
+                            uint32_t length,
                             Handle<FixedArrayBase> backing_store) {
     // External arrays do not support changing their length.
     UNREACHABLE();
   }
 
   static void DeleteImpl(Handle<JSObject> obj, uint32_t entry) {
     UNREACHABLE();
   }
 
   static uint32_t GetIndexForEntryImpl(FixedArrayBase* backing_store,
@@ skipping 93 matching lines @@
         Context* context = Context::cast(parameter_map->get(0));
         int context_entry = alias->aliased_context_slot();
         DCHECK(!context->get(context_entry)->IsTheHole());
         context->set(context_entry, value);
       } else {
         ArgumentsAccessor::SetImpl(arguments, entry - length, value);
       }
     }
   }
 
-  static void SetLengthImpl(Handle<JSArray> array, uint32_t length,
+  static void SetLengthImpl(Isolate* isolate, Handle<JSArray> array,
+                            uint32_t length,
                             Handle<FixedArrayBase> parameter_map) {
     // Sloppy arguments objects are not arrays.
     UNREACHABLE();
   }
 
   static uint32_t GetCapacityImpl(JSObject* holder,
                                   FixedArrayBase* backing_store) {
     FixedArray* parameter_map = FixedArray::cast(backing_store);
     FixedArrayBase* arguments = FixedArrayBase::cast(parameter_map->get(1));
     return parameter_map->length() - 2 +
@@ skipping 456 matching lines @@
     }
   }
 
   DCHECK(j == result_len);
   return result_array;
 }
 
 ElementsAccessor** ElementsAccessor::elements_accessors_ = NULL;
 }  // namespace internal
 }  // namespace v8