Layout descriptor must be trimmed when corresponding descriptors array is trimmed to stay in sync.

src/layout-descriptor.h

 // Copyright 2014 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.
 
 #ifndef V8_LAYOUT_DESCRIPTOR_H_
 #define V8_LAYOUT_DESCRIPTOR_H_
 
 #include <iosfwd>
 
 #include "src/objects.h"
@@ skipping 52 matching lines @@
   // Otherwise the |full_layout_descriptor| is returned.
   static Handle<LayoutDescriptor> AppendIfFastOrUseFull(
       Handle<Map> map, PropertyDetails details,
       Handle<LayoutDescriptor> full_layout_descriptor);
 
   // Layout descriptor that corresponds to an object all fields of which are
   // tagged (FastPointerLayout).
   V8_INLINE static LayoutDescriptor* FastPointerLayout();
 
   // Check that this layout descriptor corresponds to given map.
-  bool IsConsistentWithMap(Map* map);
+  bool IsConsistentWithMap(Map* map, bool check_tail = false);
+
+  // Trims this layout descriptor to given number of descriptors. This happens
+  // only when corresponding descriptors array is trimmed.
+  // The layout descriptor could be trimmed if it was slow or it could
+  // become fast.
+  LayoutDescriptor* Trim(Heap* heap, Map* map, DescriptorArray* descriptors,
+                         int num_descriptors);
 
 #ifdef OBJECT_PRINT
   // For our gdb macros, we should perhaps change these in the future.
   void Print();
 
   void Print(std::ostream& os);  // NOLINT
 #endif
 
   // Capacity of layout descriptors in bits.
   V8_INLINE int capacity();
 
   static Handle<LayoutDescriptor> NewForTesting(Isolate* isolate, int length);
   LayoutDescriptor* SetTaggedForTesting(int field_index, bool tagged);
 
  private:
   static const int kNumberOfBits = 32;
 
   V8_INLINE static Handle<LayoutDescriptor> New(Isolate* isolate, int length);
   V8_INLINE static LayoutDescriptor* FromSmi(Smi* smi);
 
   V8_INLINE static bool InobjectUnboxedField(int inobject_properties,
                                              PropertyDetails details);
 
+  // Calculates minimal layout descriptor capacity required for given
+  // |map|, |descriptors| and |num_descriptors|.
+  V8_INLINE static int CalculateCapacity(Map* map, DescriptorArray* descriptors,
+                                         int num_descriptors);
+
+  // Calculates the length of the slow-mode backing store array by given layout
+  // descriptor length.
+  V8_INLINE static int GetSlowModeBackingStoreLength(int length);
+
+  // Fills in clean |layout_descriptor| according to given |map|, |descriptors|
+  // and |num_descriptors|.
+  V8_INLINE static LayoutDescriptor* Initialize(
+      LayoutDescriptor* layout_descriptor, Map* map,
+      DescriptorArray* descriptors, int num_descriptors);
+
   static Handle<LayoutDescriptor> EnsureCapacity(
       Isolate* isolate, Handle<LayoutDescriptor> layout_descriptor,
       int new_capacity);
 
   // Returns false if requested field_index is out of bounds.
   V8_INLINE bool GetIndexes(int field_index, int* layout_word_index,
                             int* layout_bit_index);
 
   V8_INLINE MUST_USE_RESULT LayoutDescriptor* SetRawData(int field_index) {
     return SetTagged(field_index, false);
@@ skipping 23 matching lines @@
 
  private:
   bool all_fields_tagged_;
   int header_size_;
   LayoutDescriptor* layout_descriptor_;
 };
 }
 }  // namespace v8::internal
 
 #endif  // V8_LAYOUT_DESCRIPTOR_H_