Backport hash collision workaround to 3.6.

src/objects.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 5716 matching lines @@
   DISALLOW_IMPLICIT_CONSTRUCTORS(PolymorphicCodeCacheHashTable);
 };
 
 
 enum AllowNullsFlag {ALLOW_NULLS, DISALLOW_NULLS};
 enum RobustnessFlag {ROBUST_STRING_TRAVERSAL, FAST_STRING_TRAVERSAL};
 
 
 class StringHasher {
  public:
-  explicit inline StringHasher(int length);
+  explicit inline StringHasher(int length, uint32_t seed);
 
   // Returns true if the hash of this string can be computed without
   // looking at the contents.
   inline bool has_trivial_hash();
 
   // Add a character to the hash and update the array index calculation.
   inline void AddCharacter(uc32 c);
 
   // Adds a character to the hash but does not update the array index
   // calculation.  This can only be called when it has been verified
@@ skipping 30 matching lines @@
   uint32_t array_index_;
   bool is_array_index_;
   bool is_first_char_;
   bool is_valid_;
   friend class TwoCharHashTableKey;
 };
 
 
 // Calculates string hash.
 template <typename schar>
-inline uint32_t HashSequentialString(const schar* chars, int length);
+inline uint32_t HashSequentialString(const schar* chars,
+                                     int length,
+                                     uint32_t seed);
 
 
 // The characteristics of a string are stored in its map.  Retrieving these
 // few bits of information is moderately expensive, involving two memory
 // loads where the second is dependent on the first.  To improve efficiency
 // the shape of the string is given its own class so that it can be retrieved
 // once and used for several string operations.  A StringShape is small enough
 // to be passed by value and is immutable, but be aware that flattening a
 // string can potentially alter its shape.  Also be aware that a GC caused by
 // something else can alter the shape of a string due to ConsString
@@ skipping 201 matching lines @@
   SmartArrayPointer<uc16> ToWideCString(
       RobustnessFlag robustness_flag = FAST_STRING_TRAVERSAL);
 
   // Tells whether the hash code has been computed.
   inline bool HasHashCode();
 
   // Returns a hash value used for the property table
   inline uint32_t Hash();
 
   static uint32_t ComputeHashField(unibrow::CharacterStream* buffer,
-                                   int length);
+                                   int length,
+                                   uint32_t seed);
 
   static bool ComputeArrayIndex(unibrow::CharacterStream* buffer,
                                 uint32_t* index,
                                 int length);
 
   // Externalization.
   bool MakeExternal(v8::String::ExternalStringResource* resource);
   bool MakeExternal(v8::String::ExternalAsciiStringResource* resource);
 
   // Conversion.
@@ skipping 44 matching lines @@
   // whether a hash code has been computed.  If the hash code has been
   // computed the 2nd bit tells whether the string can be used as an
   // array index.
   static const int kHashNotComputedMask = 1;
   static const int kIsNotArrayIndexMask = 1 << 1;
   static const int kNofHashBitFields = 2;
 
   // Shift constant retrieving hash code from hash field.
   static const int kHashShift = kNofHashBitFields;
 
+  // Only these bits are relevant in the hash, since the top two are shifted
+  // out.
+  static const uint32_t kHashBitMask = 0xffffffffu >> kHashShift;

[Vyacheslav Egorov (Chromium), 2012/01/09 19:09:33]

this constant is duplicated in every code generator.

[Erik Corry, 2012/01/10 00:18:44]

Yes, this should be fixed in bleeding edge.

+
   // Array index strings this short can keep their index in the hash
   // field.
   static const int kMaxCachedArrayIndexLength = 7;
 
   // For strings which are array indexes the hash value has the string length
   // mixed into the hash, mainly to avoid a hash value of zero which would be
   // the case for the string '0'. 24 bits are used for the array index value.
   static const int kArrayIndexValueBits = 24;
   static const int kArrayIndexLengthBits =
       kBitsPerInt - kArrayIndexValueBits - kNofHashBitFields;
@@ skipping 1401 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_