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

src/layout-descriptor-inl.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_INL_H_
 #define V8_LAYOUT_DESCRIPTOR_INL_H_
 
 #include "src/layout-descriptor.h"
 
 namespace v8 {
 namespace internal {
 
 LayoutDescriptor* LayoutDescriptor::FromSmi(Smi* smi) {
   return LayoutDescriptor::cast(smi);
 }
 
 
 Handle<LayoutDescriptor> LayoutDescriptor::New(Isolate* isolate, int length) {
   if (length <= kSmiValueSize) {
     // The whole bit vector fits into a smi.
     return handle(LayoutDescriptor::FromSmi(Smi::FromInt(0)), isolate);
   }
-
-  length = (length + kNumberOfBits - 1) / kNumberOfBits;
-  DCHECK(length > 0);
-
-  if (SmiValuesAre32Bits() && (length & 1)) {
-    // On 64-bit systems if the length is odd then the half-word space would be
-    // lost anyway (due to alignment and the fact that we are allocating
-    // uint32-typed array), so we increase the length of allocated array
-    // to utilize that "lost" space which could also help to avoid layout
-    // descriptor reallocations.
-    ++length;
-  }
+  length = GetSlowModeBackingStoreLength(length);
   return Handle<LayoutDescriptor>::cast(
       isolate->factory()->NewFixedTypedArray(length, kExternalUint32Array));
 }
 
 
 bool LayoutDescriptor::InobjectUnboxedField(int inobject_properties,
                                             PropertyDetails details) {
   if (details.type() != DATA || !details.representation().IsDouble()) {
     return false;
   }
@@ skipping 102 matching lines @@
   // This is a mixed descriptor which is a fixed typed array.
   MapWord map_word = reinterpret_cast<HeapObject*>(object)->map_word();
   if (map_word.IsForwardingAddress()) {
     // Mark-compact has already moved layout descriptor.
     object = map_word.ToForwardingAddress();
   }
   return LayoutDescriptor::cast(object);
 }
 
 
+int LayoutDescriptor::GetSlowModeBackingStoreLength(int length) {
+  length = (length + kNumberOfBits - 1) / kNumberOfBits;
+  DCHECK_LT(0, length);
+
+  if (SmiValuesAre32Bits() && (length & 1)) {
+    // On 64-bit systems if the length is odd then the half-word space would be
+    // lost anyway (due to alignment and the fact that we are allocating
+    // uint32-typed array), so we increase the length of allocated array
+    // to utilize that "lost" space which could also help to avoid layout
+    // descriptor reallocations.
+    ++length;
+  }
+  return length;
+}
+
+
+int LayoutDescriptor::CalculateCapacity(Map* map, DescriptorArray* descriptors,
+                                        int num_descriptors) {
+  int inobject_properties = map->inobject_properties();
+  if (inobject_properties == 0) return 0;
+
+  DCHECK_LE(num_descriptors, descriptors->number_of_descriptors());
+
+  int layout_descriptor_length;
+  const int kMaxWordsPerField = kDoubleSize / kPointerSize;
+
+  if (num_descriptors <= kSmiValueSize / kMaxWordsPerField) {
+    // Even in the "worst" case (all fields are doubles) it would fit into
+    // a Smi, so no need to calculate length.
+    layout_descriptor_length = kSmiValueSize;
+
+  } else {
+    layout_descriptor_length = 0;
+
+    for (int i = 0; i < num_descriptors; i++) {
+      PropertyDetails details = descriptors->GetDetails(i);
+      if (!InobjectUnboxedField(inobject_properties, details)) continue;
+      int field_index = details.field_index();
+      int field_width_in_words = details.field_width_in_words();
+      layout_descriptor_length =
+          Max(layout_descriptor_length, field_index + field_width_in_words);
+    }
+  }
+  layout_descriptor_length = Min(layout_descriptor_length, inobject_properties);
+  return layout_descriptor_length;
+}
+
+
+LayoutDescriptor* LayoutDescriptor::Initialize(
+    LayoutDescriptor* layout_descriptor, Map* map, DescriptorArray* descriptors,
+    int num_descriptors) {
+  DisallowHeapAllocation no_allocation;
+  int inobject_properties = map->inobject_properties();
+
+  for (int i = 0; i < num_descriptors; i++) {
+    PropertyDetails details = descriptors->GetDetails(i);
+    if (!InobjectUnboxedField(inobject_properties, details)) {
+      DCHECK(details.location() != kField ||
+             layout_descriptor->IsTagged(details.field_index()));
+      continue;
+    }
+    int field_index = details.field_index();
+    layout_descriptor = layout_descriptor->SetRawData(field_index);
+    if (details.field_width_in_words() > 1) {
+      layout_descriptor = layout_descriptor->SetRawData(field_index + 1);
+    }
+  }
+  return layout_descriptor;
+}
+
+
 // InobjectPropertiesHelper is a helper class for querying whether inobject
 // property at offset is Double or not.
 LayoutDescriptorHelper::LayoutDescriptorHelper(Map* map)
     : all_fields_tagged_(true),
       header_size_(0),
       layout_descriptor_(LayoutDescriptor::FastPointerLayout()) {
   if (!FLAG_unbox_double_fields) return;
 
   layout_descriptor_ = map->layout_descriptor_gc_safe();
   if (layout_descriptor_->IsFastPointerLayout()) {
@@ skipping 15 matching lines @@
   // Object headers do not contain non-tagged fields.
   if (offset_in_bytes < header_size_) return true;
   int field_index = (offset_in_bytes - header_size_) / kPointerSize;
 
   return layout_descriptor_->IsTagged(field_index);
 }
 }
 }  // namespace v8::internal
 
 #endif  // V8_LAYOUT_DESCRIPTOR_INL_H_