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+// Copyright (c) 2007, Google Inc.
+// 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 with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// ---
+// Author: Geoff Pike
+//
+// This file provides a minimal cache that can hold a <key, value> pair
+// with little if any wasted space.  The types of the key and value
+// must be unsigned integral types or at least have unsigned semantics
+// for >>, casting, and similar operations.
+//
+// Synchronization is not provided.  However, the cache is implemented
+// as an array of cache entries whose type is chosen at compile time.
+// If a[i] is atomic on your hardware for the chosen array type then
+// raciness will not necessarily lead to bugginess.  The cache entries
+// must be large enough to hold a partial key and a value packed
+// together.  The partial keys are bit strings of length
+// kKeybits - kHashbits, and the values are bit strings of length kValuebits.
+//
+// In an effort to use minimal space, every cache entry represents
+// some <key, value> pair; the class provides no way to mark a cache
+// entry as empty or uninitialized.  In practice, you may want to have
+// reserved keys or values to get around this limitation.  For example, in
+// tcmalloc's PageID-to-sizeclass cache, a value of 0 is used as
+// "unknown sizeclass."
+//
+// Usage Considerations
+// --------------------
+//
+// kHashbits controls the size of the cache.  The best value for
+// kHashbits will of course depend on the application.  Perhaps try
+// tuning the value of kHashbits by measuring different values on your
+// favorite benchmark.  Also remember not to be a pig; other
+// programs that need resources may suffer if you are.
+//
+// The main uses for this class will be when performance is
+// critical and there's a convenient type to hold the cache's
+// entries.  As described above, the number of bits required
+// for a cache entry is (kKeybits - kHashbits) + kValuebits.  Suppose
+// kKeybits + kValuebits is 43.  Then it probably makes sense to
+// chose kHashbits >= 11 so that cache entries fit in a uint32.
+//
+// On the other hand, suppose kKeybits = kValuebits = 64.  Then
+// using this class may be less worthwhile.  You'll probably
+// be using 128 bits for each entry anyway, so maybe just pick
+// a hash function, H, and use an array indexed by H(key):
+//    void Put(K key, V value) { a_[H(key)] = pair<K, V>(key, value); }
+//    V GetOrDefault(K key, V default) { const pair<K, V> &p = a_[H(key)]; ... }
+//    etc.
+//
+// Further Details
+// ---------------
+//
+// For caches used only by one thread, the following is true:
+// 1. For a cache c,
+//      (c.Put(key, value), c.GetOrDefault(key, 0)) == value
+//    and
+//      (c.Put(key, value), <...>, c.GetOrDefault(key, 0)) == value
+//    if the elided code contains no c.Put calls.
+//
+// 2. Has(key) will return false if no <key, value> pair with that key
+//    has ever been Put.  However, a newly initialized cache will have
+//    some <key, value> pairs already present.  When you create a new
+//    cache, you must specify an "initial value."  The initialization
+//    procedure is equivalent to Clear(initial_value), which is
+//    equivalent to Put(k, initial_value) for all keys k from 0 to
+//    2^kHashbits - 1.
+//
+// 3. If key and key' differ then the only way Put(key, value) may
+//    cause Has(key') to change is that Has(key') may change from true to
+//    false. Furthermore, a Put() call that doesn't change Has(key')
+//    doesn't change GetOrDefault(key', ...) either.
+//
+// Implementation details:
+//
+// This is a direct-mapped cache with 2^kHashbits entries;
+// the hash function simply takes the low bits of the key.
+// So, we don't have to store the low bits of the key in the entries.
+// Instead, an entry is the high bits of a key and a value, packed
+// together.  E.g., a 20 bit key and a 7 bit value only require
+// a uint16 for each entry if kHashbits >= 11.
+//
+// Alternatives to this scheme will be added as needed.
+
+#ifndef TCMALLOC_PACKED_CACHE_INL_H__
+#define TCMALLOC_PACKED_CACHE_INL_H__
+
+#ifndef WTF_CHANGES
+#include "base/basictypes.h"  // for COMPILE_ASSERT
+#include "base/logging.h"     // for DCHECK
+#endif
+
+#ifndef DCHECK_EQ
+#define DCHECK_EQ(val1, val2) ASSERT((val1) == (val2))
+#endif
+
+// A safe way of doing "(1 << n) - 1" -- without worrying about overflow
+// Note this will all be resolved to a constant expression at compile-time
+#define N_ONES_(IntType, N)                                     \
+  ( (N) == 0 ? 0 : ((static_cast<IntType>(1) << ((N)-1))-1 +    \
+                    (static_cast<IntType>(1) << ((N)-1))) )
+
+// The types K and V provide upper bounds on the number of valid keys
+// and values, but we explicitly require the keys to be less than
+// 2^kKeybits and the values to be less than 2^kValuebits.  The size of
+// the table is controlled by kHashbits, and the type of each entry in
+// the cache is T.  See also the big comment at the top of the file.
+template <int kKeybits, typename T>
+class PackedCache {
+ public:
+  typedef uintptr_t K;
+  typedef size_t V;
+  static const size_t kHashbits = 12;
+  static const size_t kValuebits = 8;
+
+  explicit PackedCache(V initial_value) {
+    COMPILE_ASSERT(kKeybits <= sizeof(K) * 8, key_size);
+    COMPILE_ASSERT(kValuebits <= sizeof(V) * 8, value_size);
+    COMPILE_ASSERT(kHashbits <= kKeybits, hash_function);
+    COMPILE_ASSERT(kKeybits - kHashbits + kValuebits <= kTbits,
+                   entry_size_must_be_big_enough);
+    Clear(initial_value);
+  }
+
+  void Put(K key, V value) {
+    DCHECK_EQ(key, key & kKeyMask);
+    DCHECK_EQ(value, value & kValueMask);
+    array_[Hash(key)] = static_cast<T>(KeyToUpper(key) | value);
+  }
+
+  bool Has(K key) const {
+    DCHECK_EQ(key, key & kKeyMask);
+    return KeyMatch(array_[Hash(key)], key);
+  }
+
+  V GetOrDefault(K key, V default_value) const {
+    // As with other code in this class, we touch array_ as few times
+    // as we can.  Assuming entries are read atomically (e.g., their
+    // type is uintptr_t on most hardware) then certain races are
+    // harmless.
+    DCHECK_EQ(key, key & kKeyMask);
+    T entry = array_[Hash(key)];
+    return KeyMatch(entry, key) ? EntryToValue(entry) : default_value;
+  }
+
+  void Clear(V value) {
+    DCHECK_EQ(value, value & kValueMask);
+    for (int i = 0; i < 1 << kHashbits; i++) {
+      array_[i] = static_cast<T>(value);
+    }
+  }
+
+ private:
+  // We are going to pack a value and the upper part of a key into
+  // an entry of type T.  The UPPER type is for the upper part of a key,
+  // after the key has been masked and shifted for inclusion in an entry.
+  typedef T UPPER;
+
+  static V EntryToValue(T t) { return t & kValueMask; }
+
+  static UPPER EntryToUpper(T t) { return t & kUpperMask; }
+
+  // If v is a V and u is an UPPER then you can create an entry by
+  // doing u | v.  kHashbits determines where in a K to find the upper
+  // part of the key, and kValuebits determines where in the entry to put
+  // it.
+  static UPPER KeyToUpper(K k) {
+    const int shift = kHashbits - kValuebits;
+    // Assume kHashbits >= kValuebits. It would be easy to lift this assumption.
+    return static_cast<T>(k >> shift) & kUpperMask;
+  }
+
+  // This is roughly the inverse of KeyToUpper().  Some of the key has been
+  // thrown away, since KeyToUpper() masks off the low bits of the key.
+  static K UpperToPartialKey(UPPER u) {
+    DCHECK_EQ(u, u & kUpperMask);
+    const int shift = kHashbits - kValuebits;
+    // Assume kHashbits >= kValuebits. It would be easy to lift this assumption.
+    return static_cast<K>(u) << shift;
+  }
+
+  static size_t Hash(K key) {
+    return static_cast<size_t>(key) & N_ONES_(size_t, kHashbits);
+  }
+
+  // Does the entry's partial key match the relevant part of the given key?
+  static bool KeyMatch(T entry, K key) {
+    return ((KeyToUpper(key) ^ entry) & kUpperMask) == 0;
+  }
+
+  static const size_t kTbits = 8 * sizeof(T);
+  static const int kUpperbits = kKeybits - kHashbits;
+
+  // For masking a K.
+  static const K kKeyMask = N_ONES_(K, kKeybits);
+
+  // For masking a T.
+  static const T kUpperMask = N_ONES_(T, kUpperbits) << kValuebits;
+
+  // For masking a V or a T.
+  static const V kValueMask = N_ONES_(V, kValuebits);
+
+  T array_[1 << kHashbits];
+};
+
+#undef N_ONES_
+
+#endif  // TCMALLOC_PACKED_CACHE_INL_H__