+++ /dev/null
-// 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__
projects / apple / javascriptcore.git / blobdiff