[regexp] Properly flatten string during initialization

src/factory.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 "src/factory.h"
 
 #include "src/accessors.h"
 #include "src/allocation-site-scopes.h"
 #include "src/ast/ast.h"
 #include "src/base/bits.h"
@@ skipping 693 matching lines @@
       return result;
     }
 
     return (is_one_byte_data_in_two_byte_string)
         ? ConcatStringContent<uint8_t>(
             NewRawOneByteString(length).ToHandleChecked(), left, right)
         : ConcatStringContent<uc16>(
             NewRawTwoByteString(length).ToHandleChecked(), left, right);
   }
 
+  bool one_byte = (is_one_byte || is_one_byte_data_in_two_byte_string);
+  return NewConsString(left, right, length, one_byte);
+}
+
+Handle<String> Factory::NewConsString(Handle<String> left, Handle<String> right,
+                                      int length, bool one_byte) {
+  DCHECK(!left->IsThinString());
+  DCHECK(!right->IsThinString());
+  DCHECK_GE(length, ConsString::kMinLength);
+  DCHECK_LE(length, String::kMaxLength);
+
   Handle<ConsString> result =
-      (is_one_byte || is_one_byte_data_in_two_byte_string)
-          ? New<ConsString>(cons_one_byte_string_map(), NEW_SPACE)
-          : New<ConsString>(cons_string_map(), NEW_SPACE);
+      one_byte ? New<ConsString>(cons_one_byte_string_map(), NEW_SPACE)
+               : New<ConsString>(cons_string_map(), NEW_SPACE);
 
   DisallowHeapAllocation no_gc;
   WriteBarrierMode mode = result->GetWriteBarrierMode(no_gc);
 
   result->set_hash_field(String::kEmptyHashField);
   result->set_length(length);
   result->set_first(*left, mode);
   result->set_second(*right, mode);
   return result;
 }
@@ skipping 2136 matching lines @@
     Handle<AccessorInfo> prototype =
         Accessors::FunctionPrototypeInfo(isolate(), rw_attribs);
     Descriptor d = Descriptor::AccessorConstant(
         Handle<Name>(Name::cast(prototype->name())), prototype, rw_attribs);
     map->AppendDescriptor(&d);
   }
 }
 
 }  // namespace internal
 }  // namespace v8