add fast path for hashing small cons strings

src/objects.cc

 // Copyright 2013 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 <sstream>
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/allocation-site-scopes.h"
@@ skipping 9280 matching lines @@
     }
   }
   *utf16_length_out = static_cast<int>(utf16_length);
   // Must set length here so that hash computation is correct.
   hasher.length_ = utf16_length;
   return hasher.GetHashField();
 }
 
 
 void IteratingStringHasher::VisitConsString(ConsString* cons_string) {
+  // Run small ConsStrings through ConsStringIterator.
+  if (cons_string->length() < 64) {
+    ConsStringIterator iter(cons_string);
+    int offset;
+    String* string;
+    while (nullptr != (string = iter.Next(&offset))) {
+      DCHECK_EQ(0, offset);
+      String::VisitFlat(this, string, 0);
+    }
+    return;
+  }
+  // Slow case.
   const int max_length = String::kMaxHashCalcLength;
   int length = std::min(cons_string->length(), max_length);
   if (cons_string->HasOnlyOneByteChars()) {
     uint8_t* buffer = new uint8_t[length];
     String::WriteToFlat(cons_string, buffer, 0, length);
     AddCharacters(buffer, length);
     delete[] buffer;
   } else {
     uint16_t* buffer = new uint16_t[length];
     String::WriteToFlat(cons_string, buffer, 0, length);
@@ skipping 7526 matching lines @@
   Handle<DependentCode> codes =
       DependentCode::Insert(handle(cell->dependent_code(), info->isolate()),
                             DependentCode::kPropertyCellChangedGroup,
                             info->object_wrapper());
   if (*codes != cell->dependent_code()) cell->set_dependent_code(*codes);
   info->dependencies(DependentCode::kPropertyCellChangedGroup)->Add(
       cell, info->zone());
 }
 
 } }  // namespace v8::internal