Internalize strings in-place

src/heap/heap-inl.h

 // Copyright 2012 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_HEAP_HEAP_INL_H_
 #define V8_HEAP_HEAP_INL_H_
 
 #include <cmath>
 
 #include "src/base/platform/platform.h"
@@ skipping 207 matching lines @@
   if (IsOneByte(t, chars)) {
     return AllocateInternalizedStringImpl<true>(t, chars, hash_field);
   }
   return AllocateInternalizedStringImpl<false>(t, chars, hash_field);
 }
 
 
 AllocationResult Heap::AllocateOneByteInternalizedString(
     Vector<const uint8_t> str, uint32_t hash_field) {
   CHECK_GE(String::kMaxLength, str.length());
+  // The canonical empty_string is the only zero-length string we allow.
+  DCHECK_IMPLIES(str.length() == 0, roots_[kempty_stringRootIndex] == nullptr);
   // Compute map and object size.
   Map* map = one_byte_internalized_string_map();
   int size = SeqOneByteString::SizeFor(str.length());
 
   // Allocate string.
   HeapObject* result = nullptr;
   {
     AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
     if (!allocation.To(&result)) return allocation;
   }
@@ skipping 11 matching lines @@
   MemCopy(answer->address() + SeqOneByteString::kHeaderSize, str.start(),
           str.length());
 
   return answer;
 }
 
 
 AllocationResult Heap::AllocateTwoByteInternalizedString(Vector<const uc16> str,
                                                          uint32_t hash_field) {
   CHECK_GE(String::kMaxLength, str.length());
+  DCHECK_NE(0, str.length());  // Use Heap::empty_string() instead.
   // Compute map and object size.
   Map* map = internalized_string_map();
   int size = SeqTwoByteString::SizeFor(str.length());
 
   // Allocate string.
   HeapObject* result = nullptr;
   {
     AllocationResult allocation = AllocateRaw(size, OLD_SPACE);
     if (!allocation.To(&result)) return allocation;
   }
@@ skipping 590 matching lines @@
 
 void VerifySmisVisitor::VisitPointers(Object** start, Object** end) {
   for (Object** current = start; current < end; current++) {
     CHECK((*current)->IsSmi());
   }
 }
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_INL_H_