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

src/layout-descriptor.cc

 // 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.
 
 #include <sstream>
 
 #include "src/v8.h"
 
 #include "src/base/bits.h"
 #include "src/layout-descriptor.h"
 
 using v8::base::bits::CountTrailingZeros32;
 
 namespace v8 {
 namespace internal {
 
 Handle<LayoutDescriptor> LayoutDescriptor::New(
     Handle<Map> map, Handle<DescriptorArray> descriptors, int num_descriptors) {
   Isolate* isolate = descriptors->GetIsolate();
   if (!FLAG_unbox_double_fields) return handle(FastPointerLayout(), isolate);
 
-  int inobject_properties = map->inobject_properties();
-  if (inobject_properties == 0) return handle(FastPointerLayout(), isolate);
+  int layout_descriptor_length =
+      CalculateCapacity(*map, *descriptors, num_descriptors);
 
-  DCHECK(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);
-    }
-
-    if (layout_descriptor_length == 0) {
-      // No double fields were found, use fast pointer layout.
-      return handle(FastPointerLayout(), isolate);
-    }
+  if (layout_descriptor_length == 0) {
+    // No double fields were found, use fast pointer layout.
+    return handle(FastPointerLayout(), isolate);
   }
-  layout_descriptor_length = Min(layout_descriptor_length, inobject_properties);
 
   // Initially, layout descriptor corresponds to an object with all fields
   // tagged.
   Handle<LayoutDescriptor> layout_descriptor_handle =
       LayoutDescriptor::New(isolate, layout_descriptor_length);
 
-  DisallowHeapAllocation no_allocation;
-  LayoutDescriptor* layout_descriptor = *layout_descriptor_handle;
+  LayoutDescriptor* layout_descriptor = Initialize(
+      *layout_descriptor_handle, *map, *descriptors, num_descriptors);
 
-  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();
-    layout_descriptor = layout_descriptor->SetRawData(field_index);
-    if (details.field_width_in_words() > 1) {
-      layout_descriptor = layout_descriptor->SetRawData(field_index + 1);
-    }
-  }
   return handle(layout_descriptor, isolate);
 }
 
 
 Handle<LayoutDescriptor> LayoutDescriptor::ShareAppend(
     Handle<Map> map, PropertyDetails details) {
   DCHECK(map->owns_descriptors());
   Isolate* isolate = map->GetIsolate();
   Handle<LayoutDescriptor> layout_descriptor(map->GetLayoutDescriptor(),
                                              isolate);
@@ skipping 170 matching lines @@
     DCHECK(offset_in_bytes < *out_end_of_contiguous_region_offset);
     return true;
   }
   *out_end_of_contiguous_region_offset =
       offset_in_bytes + sequence_length * kPointerSize;
   DCHECK(offset_in_bytes < *out_end_of_contiguous_region_offset);
   return tagged;
 }
 
 
-bool LayoutDescriptor::IsConsistentWithMap(Map* map) {
+LayoutDescriptor* LayoutDescriptor::Trim(Heap* heap, Map* map,
+                                         DescriptorArray* descriptors,
+                                         int num_descriptors) {
+  DisallowHeapAllocation no_allocation;
+  // Fast mode descriptors are never shared and therefore always fully
+  // correspond to their map.
+  if (!IsSlowLayout()) return this;
+
+  int layout_descriptor_length =
+      CalculateCapacity(map, descriptors, num_descriptors);
+  // It must not become fast-mode descriptor here, because otherwise it has to
+  // be fast pointer layout descriptor already but it's is slow mode now.
+  DCHECK_LT(kSmiValueSize, layout_descriptor_length);
+
+  // Trim, clean and reinitialize this slow-mode layout descriptor.
+  int array_length = GetSlowModeBackingStoreLength(layout_descriptor_length);
+  int current_length = length();
+  if (current_length != array_length) {
+    DCHECK_LT(array_length, current_length);
+    int delta = current_length - array_length;
+    heap->RightTrimFixedArray<Heap::FROM_GC>(this, delta);

[Toon Verwaest, 2015/03/30 16:10:21]

I hope DataSize() is updated here? Otherwise you'll overwrite the filler object.

[Igor Sheludko, 2015/03/30 16:33:27]

Yes, sure! It's length() * elements_size, essentially.

+  }
+  memset(DataPtr(), 0, DataSize());
+  LayoutDescriptor* layout_descriptor =
+      Initialize(this, map, descriptors, num_descriptors);
+  DCHECK_EQ(this, layout_descriptor);
+  return layout_descriptor;
+}
+
+
+bool LayoutDescriptor::IsConsistentWithMap(Map* map, bool check_tail) {
   if (FLAG_unbox_double_fields) {
     DescriptorArray* descriptors = map->instance_descriptors();
     int nof_descriptors = map->NumberOfOwnDescriptors();
+    int last_field_index = 0;
     for (int i = 0; i < nof_descriptors; i++) {
       PropertyDetails details = descriptors->GetDetails(i);
-      if (details.type() != DATA) continue;
+      if (details.location() != kField) continue;
       FieldIndex field_index = FieldIndex::ForDescriptor(map, i);
       bool tagged_expected =
           !field_index.is_inobject() || !details.representation().IsDouble();
       for (int bit = 0; bit < details.field_width_in_words(); bit++) {
         bool tagged_actual = IsTagged(details.field_index() + bit);
         DCHECK_EQ(tagged_expected, tagged_actual);
         if (tagged_actual != tagged_expected) return false;
       }
+      last_field_index =
+          Max(last_field_index,
+              details.field_index() + details.field_width_in_words());
+    }
+    if (check_tail) {
+      int n = capacity();
+      for (int i = last_field_index; i < n; i++) {
+        DCHECK(IsTagged(i));
+      }
     }
   }
   return true;
 }
 }
 }  // namespace v8::internal