Avoid double initialization of arrays.

src/elements.cc

 // 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 135 matching lines @@
 }
 
 
 static void CopyObjectToObjectElements(FixedArray* from,
                                        ElementsKind from_kind,
                                        uint32_t from_start,
                                        FixedArray* to,
                                        ElementsKind to_kind,
                                        uint32_t to_start,
                                        int raw_copy_size) {
   ASSERT(to->map() != HEAP->fixed_cow_array_map());

[Michael Starzinger, 2012/11/27 11:53:56]

We are assuming that this function does not allocate, so we should add a
no-allocation scope to the top of the function.

AssertNoAllocation no_allocation;

[Toon Verwaest, 2012/11/27 11:59:30]

Done.

   int copy_size = raw_copy_size;
   if (raw_copy_size < 0) {
     ASSERT(raw_copy_size == ElementsAccessor::kCopyToEnd ||
            raw_copy_size == ElementsAccessor::kCopyToEndAndInitializeToHole);
     copy_size = Min(from->length() - from_start,
                     to->length() - to_start);
-#ifdef DEBUG
-    // FAST_*_ELEMENTS arrays cannot be uninitialized. Ensure they are already
-    // marked with the hole.
     if (raw_copy_size == ElementsAccessor::kCopyToEndAndInitializeToHole) {
-      for (int i = to_start + copy_size; i < to->length(); ++i) {
-        ASSERT(to->get(i)->IsTheHole());
+      int start = to_start + copy_size;
+      int length = to->length() - start;
+      if (length > 0) {
+        Heap* heap = from->GetHeap();
+        MemsetPointer(to->data_start() + start, heap->the_hole_value(), length);
       }
     }
-#endif
   }
   ASSERT((copy_size + static_cast<int>(to_start)) <= to->length() &&
          (copy_size + static_cast<int>(from_start)) <= from->length());
   if (copy_size == 0) return;
   ASSERT(IsFastSmiOrObjectElementsKind(from_kind));
   ASSERT(IsFastSmiOrObjectElementsKind(to_kind));
   Address to_address = to->address() + FixedArray::kHeaderSize;
   Address from_address = from->address() + FixedArray::kHeaderSize;
   CopyWords(reinterpret_cast<Object**>(to_address) + to_start,
             reinterpret_cast<Object**>(from_address) + from_start,
@@ skipping 10 matching lines @@
   }
 }
 
 
 static void CopyDictionaryToObjectElements(SeededNumberDictionary* from,
                                            uint32_t from_start,
                                            FixedArray* to,
                                            ElementsKind to_kind,
                                            uint32_t to_start,
                                            int raw_copy_size) {
   int copy_size = raw_copy_size;

[Michael Starzinger, 2012/11/27 11:53:56]

Likewise.

[Toon Verwaest, 2012/11/27 11:59:30]

Done.

   Heap* heap = from->GetHeap();
   if (raw_copy_size < 0) {
     ASSERT(raw_copy_size == ElementsAccessor::kCopyToEnd ||
            raw_copy_size == ElementsAccessor::kCopyToEndAndInitializeToHole);
     copy_size = from->max_number_key() + 1 - from_start;
-#ifdef DEBUG
-    // Fast object arrays cannot be uninitialized. Ensure they are already
-    // marked with the hole.
     if (raw_copy_size == ElementsAccessor::kCopyToEndAndInitializeToHole) {
-      for (int i = to_start + copy_size; i < to->length(); ++i) {
-        ASSERT(to->get(i)->IsTheHole());
+      int start = to_start + copy_size;
+      int length = to->length() - start;
+      if (length > 0) {
+        Heap* heap = from->GetHeap();
+        MemsetPointer(to->data_start() + start, heap->the_hole_value(), length);
       }
     }
-#endif
   }
   ASSERT(to != from);
   ASSERT(IsFastSmiOrObjectElementsKind(to_kind));
   if (copy_size == 0) return;
   uint32_t to_length = to->length();
   if (to_start + copy_size > to_length) {
     copy_size = to_length - to_start;
   }
   for (int i = 0; i < copy_size; i++) {
     int entry = from->FindEntry(i + from_start);
@@ skipping 23 matching lines @@
     ElementsKind to_kind,
     uint32_t to_start,
     int raw_copy_size) {
   ASSERT(IsFastSmiOrObjectElementsKind(to_kind));
   int copy_size = raw_copy_size;
   if (raw_copy_size < 0) {
     ASSERT(raw_copy_size == ElementsAccessor::kCopyToEnd ||
            raw_copy_size == ElementsAccessor::kCopyToEndAndInitializeToHole);
     copy_size = Min(from->length() - from_start,
                     to->length() - to_start);
-#ifdef DEBUG
-    // FAST_*_ELEMENTS arrays cannot be uninitialized. Ensure they are already
-    // marked with the hole.
     if (raw_copy_size == ElementsAccessor::kCopyToEndAndInitializeToHole) {
-      for (int i = to_start + copy_size; i < to->length(); ++i) {
-        ASSERT(to->get(i)->IsTheHole());
+      // Also initialize the area that will be copied over since HeapNumber
+      // allocation below can cause an incremental marking step, requiring all
+      // existing heap objects to be propertly initialized.
+      int start = to_start;
+      int length = to->length() - start;
+      if (length > 0) {
+        Heap* heap = from->GetHeap();
+        MemsetPointer(to->data_start() + start, heap->the_hole_value(), length);
       }
     }
-#endif
   }
   ASSERT((copy_size + static_cast<int>(to_start)) <= to->length() &&
          (copy_size + static_cast<int>(from_start)) <= from->length());
   if (copy_size == 0) return from;
   for (int i = 0; i < copy_size; ++i) {
     if (IsFastSmiElementsKind(to_kind)) {
       UNIMPLEMENTED();
       return Failure::Exception();
     } else {
       MaybeObject* maybe_value = from->get(i + from_start);
@@ skipping 1576 matching lines @@
   if (!maybe_obj->To(&new_backing_store)) return maybe_obj;
   new_backing_store->set(0, length);
   { MaybeObject* result = array->SetContent(new_backing_store);
     if (result->IsFailure()) return result;
   }
   return array;
 }
 
 
 } }  // namespace v8::internal