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Diffstat (limited to 'Source/JavaScriptCore/wtf/TCPackedCache.h')
| -rw-r--r-- | Source/JavaScriptCore/wtf/TCPackedCache.h | 234 |
1 files changed, 0 insertions, 234 deletions
diff --git a/Source/JavaScriptCore/wtf/TCPackedCache.h b/Source/JavaScriptCore/wtf/TCPackedCache.h deleted file mode 100644 index 0464f8fdc..000000000 --- a/Source/JavaScriptCore/wtf/TCPackedCache.h +++ /dev/null @@ -1,234 +0,0 @@ -// 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__ |