Fast-path in SlowFlatten for ConsStrings with empty first part

src/objects.cc

 // Copyright 2015 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/objects.h"
 
 #include <cmath>
 #include <iomanip>
 #include <memory>
 #include <sstream>
@@ skipping 2197 matching lines @@
            (c0 == 'F' || c0 == 'H' || c0 == 'M' || c0 == 'N' || c0 == 'R' ||
             c0 == 'S' || c0 == 'X')) {
     return true;    // An MP3File, an M.
   }
   return false;
 }
 
 
 Handle<String> String::SlowFlatten(Handle<ConsString> cons,
                                    PretenureFlag pretenure) {
+  DCHECK(cons->second()->length() != 0);
+
+  // TurboFan can create cons strings with empty first parts.
+  if (cons->first()->length() == 0) return handle(cons->second());
+
   DCHECK(AllowHeapAllocation::IsAllowed());
-  DCHECK(cons->second()->length() != 0);
   Isolate* isolate = cons->GetIsolate();
   int length = cons->length();
   PretenureFlag tenure = isolate->heap()->InNewSpace(*cons) ? pretenure
                                                             : TENURED;
   Handle<SeqString> result;
   if (cons->IsOneByteRepresentation()) {
     Handle<SeqOneByteString> flat = isolate->factory()->NewRawOneByteString(
         length, tenure).ToHandleChecked();
     DisallowHeapAllocation no_gc;
     WriteToFlat(*cons, flat->GetChars(), 0, length);
@@ skipping 18073 matching lines @@
     // Check if the accessor uses a cached property.
     if (!fti->cached_property_name()->IsTheHole(isolate)) {
       return handle(Name::cast(fti->cached_property_name()));
     }
   }
   return MaybeHandle<Name>();
 }
 
 }  // namespace internal
 }  // namespace v8