[regexp] Properly flatten string during initialization

src/regexp/jsregexp.cc

 // 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.
 
 #include "src/regexp/jsregexp.h"
 
 #include <memory>
 
 #include "src/base/platform/platform.h"
 #include "src/compilation-cache.h"
@@ skipping 117 matching lines @@
   return true;
 }
 
 
 // Generic RegExp methods. Dispatches to implementation specific methods.
 
 
 MaybeHandle<Object> RegExpImpl::Compile(Handle<JSRegExp> re,
                                         Handle<String> pattern,
                                         JSRegExp::Flags flags) {
+  DCHECK(pattern->IsFlat());
+
   Isolate* isolate = re->GetIsolate();
   Zone zone(isolate->allocator(), ZONE_NAME);
   CompilationCache* compilation_cache = isolate->compilation_cache();
   MaybeHandle<FixedArray> maybe_cached =
       compilation_cache->LookupRegExp(pattern, flags);
   Handle<FixedArray> cached;
   if (maybe_cached.ToHandle(&cached)) {
     re->set_data(*cached);
     return re;
   }
-  pattern = String::Flatten(pattern);
+
   PostponeInterruptsScope postpone(isolate);
   RegExpCompileData parse_result;
   FlatStringReader reader(isolate, pattern);
   if (!RegExpParser::ParseRegExp(re->GetIsolate(), &zone, &reader, flags,
                                  &parse_result)) {
     // Throw an exception if we fail to parse the pattern.
     return ThrowRegExpException(re, pattern, parse_result.error);
   }
 
   bool has_been_compiled = false;
@@ skipping 6763 matching lines @@
 
 
 void RegExpResultsCache::Clear(FixedArray* cache) {
   for (int i = 0; i < kRegExpResultsCacheSize; i++) {
     cache->set(i, Smi::kZero);
   }
 }
 
 }  // namespace internal
 }  // namespace v8