Allow externalizing strings in old pointer space.

src/objects.cc

 // Copyright 2013 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 1255 matching lines @@
   }
 #endif  // DEBUG
   Heap* heap = GetHeap();
   int size = this->Size();  // Byte size of the original string.
   if (size < ExternalString::kShortSize) {
     return false;
   }
   bool is_ascii = this->IsOneByteRepresentation();
   bool is_internalized = this->IsInternalizedString();
 
-  // Morph the object to an external string by adjusting the map and
-  // reinitializing the fields.
-  if (size >= ExternalString::kSize) {
+  // Morph the string to an external string by replacing the map and
+  // reinitializing the fields.  This won't work if
+  // - the space the existing string occupies is too small for a regular
+  //   external string.
+  // - the existing string is in old pointer space and the backing store of
+  //   the external string is not aligned.  The GC cannot deal with fields
+  //   containing an unaligned address that points to outside of V8's heap.
+  // In either case we resort to a short external string instead, omitting
+  // the field caching the address of the backing store.  When we encounter
+  // short external strings in generated code, we need to bailout to runtime.
+  if (size < ExternalString::kSize ||
+      (!IsAligned(reinterpret_cast<intptr_t>(resource->data()), kPointerSize) &&
+       heap->old_pointer_space()->Contains(this))) {
     this->set_map_no_write_barrier(
         is_internalized
             ? (is_ascii
-                   ? heap->external_internalized_string_with_one_byte_data_map()
-                   : heap->external_internalized_string_map())
+                ? heap->
+                    short_external_internalized_string_with_one_byte_data_map()
+                : heap->short_external_internalized_string_map())
             : (is_ascii
-                   ? heap->external_string_with_one_byte_data_map()
-                   : heap->external_string_map()));
+                ? heap->short_external_string_with_one_byte_data_map()
+                : heap->short_external_string_map()));
   } else {
     this->set_map_no_write_barrier(
         is_internalized
-          ? (is_ascii
-               ? heap->
-                   short_external_internalized_string_with_one_byte_data_map()
-               : heap->short_external_internalized_string_map())
-          : (is_ascii
-                 ? heap->short_external_string_with_one_byte_data_map()
-                 : heap->short_external_string_map()));
+            ? (is_ascii
+                ? heap->external_internalized_string_with_one_byte_data_map()
+                : heap->external_internalized_string_map())
+            : (is_ascii
+                ? heap->external_string_with_one_byte_data_map()
+                : heap->external_string_map()));
   }
   ExternalTwoByteString* self = ExternalTwoByteString::cast(this);
   self->set_resource(resource);
   if (is_internalized) self->Hash();  // Force regeneration of the hash value.
 
   // Fill the remainder of the string with dead wood.
   int new_size = this->Size();  // Byte size of the external String object.
   heap->CreateFillerObjectAt(this->address() + new_size, size - new_size);
   if (Marking::IsBlack(Marking::MarkBitFrom(this))) {
     MemoryChunk::IncrementLiveBytesFromMutator(this->address(),
@@ skipping 20 matching lines @@
                   resource->length() * sizeof(smart_chars[0])) == 0);
   }
 #endif  // DEBUG
   Heap* heap = GetHeap();
   int size = this->Size();  // Byte size of the original string.
   if (size < ExternalString::kShortSize) {
     return false;
   }
   bool is_internalized = this->IsInternalizedString();
 
-  // Morph the object to an external string by adjusting the map and
-  // reinitializing the fields.  Use short version if space is limited.
-  if (size >= ExternalString::kSize) {
+  // Morph the string to an external string by replacing the map and
+  // reinitializing the fields.  This won't work if
+  // - the space the existing string occupies is too small for a regular
+  //   external string.
+  // - the existing string is in old pointer space and the backing store of
+  //   the external string is not aligned.  The GC cannot deal with fields
+  //   containing an unaligned address that points to outside of V8's heap.
+  // In either case we resort to a short external string instead, omitting
+  // the field caching the address of the backing store.  When we encounter
+  // short external strings in generated code, we need to bailout to runtime.
+  if (size < ExternalString::kSize ||
+      (!IsAligned(reinterpret_cast<intptr_t>(resource->data()), kPointerSize) &&
+       heap->old_pointer_space()->Contains(this))) {
+    this->set_map_no_write_barrier(
+        is_internalized ? heap->short_external_ascii_internalized_string_map()
+                        : heap->short_external_ascii_string_map());
+  } else {
     this->set_map_no_write_barrier(
         is_internalized ? heap->external_ascii_internalized_string_map()
                         : heap->external_ascii_string_map());
-  } else {
-    this->set_map_no_write_barrier(
-        is_internalized ? heap->short_external_ascii_internalized_string_map()
-                        : heap->short_external_ascii_string_map());
   }
   ExternalAsciiString* self = ExternalAsciiString::cast(this);
   self->set_resource(resource);
   if (is_internalized) self->Hash();  // Force regeneration of the hash value.
 
   // Fill the remainder of the string with dead wood.
   int new_size = this->Size();  // Byte size of the external String object.
   heap->CreateFillerObjectAt(this->address() + new_size, size - new_size);
   if (Marking::IsBlack(Marking::MarkBitFrom(this))) {
     MemoryChunk::IncrementLiveBytesFromMutator(this->address(),
@@ skipping 15129 matching lines @@
 #define ERROR_MESSAGES_TEXTS(C, T) T,
   static const char* error_messages_[] = {
       ERROR_MESSAGES_LIST(ERROR_MESSAGES_TEXTS)
   };
 #undef ERROR_MESSAGES_TEXTS
   return error_messages_[reason];
 }
 
 
 } }  // namespace v8::internal