1 // Copyright (c) 2007, Google Inc. 2 // All rights reserved. 3 // 4 // Redistribution and use in source and binary forms, with or without 5 // modification, are permitted provided that the following conditions are 6 // met: 7 // 8 // * Redistributions of source code must retain the above copyright 9 // notice, this list of conditions and the following disclaimer. 10 // * Redistributions in binary form must reproduce the above 11 // copyright notice, this list of conditions and the following disclaimer 12 // in the documentation and/or other materials provided with the 13 // distribution. 14 // * Neither the name of Google Inc. nor the names of its 15 // contributors may be used to endorse or promote products derived from 16 // this software without specific prior written permission. 17 // 18 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 19 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 20 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 21 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 22 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 23 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 24 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 25 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 26 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 27 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 28 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 29 30 // --- 31 // Author: Geoff Pike 32 // 33 // This file provides a minimal cache that can hold a <key, value> pair 34 // with little if any wasted space. The types of the key and value 35 // must be unsigned integral types or at least have unsigned semantics 36 // for >>, casting, and similar operations. 37 // 38 // Synchronization is not provided. However, the cache is implemented 39 // as an array of cache entries whose type is chosen at compile time. 40 // If a[i] is atomic on your hardware for the chosen array type then 41 // raciness will not necessarily lead to bugginess. The cache entries 42 // must be large enough to hold a partial key and a value packed 43 // together. The partial keys are bit strings of length 44 // kKeybits - kHashbits, and the values are bit strings of length kValuebits. 45 // 46 // In an effort to use minimal space, every cache entry represents 47 // some <key, value> pair; the class provides no way to mark a cache 48 // entry as empty or uninitialized. In practice, you may want to have 49 // reserved keys or values to get around this limitation. For example, in 50 // tcmalloc's PageID-to-sizeclass cache, a value of 0 is used as 51 // "unknown sizeclass." 52 // 53 // Usage Considerations 54 // -------------------- 55 // 56 // kHashbits controls the size of the cache. The best value for 57 // kHashbits will of course depend on the application. Perhaps try 58 // tuning the value of kHashbits by measuring different values on your 59 // favorite benchmark. Also remember not to be a pig; other 60 // programs that need resources may suffer if you are. 61 // 62 // The main uses for this class will be when performance is 63 // critical and there's a convenient type to hold the cache's 64 // entries. As described above, the number of bits required 65 // for a cache entry is (kKeybits - kHashbits) + kValuebits. Suppose 66 // kKeybits + kValuebits is 43. Then it probably makes sense to 67 // chose kHashbits >= 11 so that cache entries fit in a uint32. 68 // 69 // On the other hand, suppose kKeybits = kValuebits = 64. Then 70 // using this class may be less worthwhile. You'll probably 71 // be using 128 bits for each entry anyway, so maybe just pick 72 // a hash function, H, and use an array indexed by H(key): 73 // void Put(K key, V value) { a_[H(key)] = pair<K, V>(key, value); } 74 // V GetOrDefault(K key, V default) { const pair<K, V> &p = a_[H(key)]; ... } 75 // etc. 76 // 77 // Further Details 78 // --------------- 79 // 80 // For caches used only by one thread, the following is true: 81 // 1. For a cache c, 82 // (c.Put(key, value), c.GetOrDefault(key, 0)) == value 83 // and 84 // (c.Put(key, value), <...>, c.GetOrDefault(key, 0)) == value 85 // if the elided code contains no c.Put calls. 86 // 87 // 2. Has(key) will return false if no <key, value> pair with that key 88 // has ever been Put. However, a newly initialized cache will have 89 // some <key, value> pairs already present. When you create a new 90 // cache, you must specify an "initial value." The initialization 91 // procedure is equivalent to Clear(initial_value), which is 92 // equivalent to Put(k, initial_value) for all keys k from 0 to 93 // 2^kHashbits - 1. 94 // 95 // 3. If key and key' differ then the only way Put(key, value) may 96 // cause Has(key') to change is that Has(key') may change from true to 97 // false. Furthermore, a Put() call that doesn't change Has(key') 98 // doesn't change GetOrDefault(key', ...) either. 99 // 100 // Implementation details: 101 // 102 // This is a direct-mapped cache with 2^kHashbits entries; 103 // the hash function simply takes the low bits of the key. 104 // So, we don't have to store the low bits of the key in the entries. 105 // Instead, an entry is the high bits of a key and a value, packed 106 // together. E.g., a 20 bit key and a 7 bit value only require 107 // a uint16 for each entry if kHashbits >= 11. 108 // 109 // Alternatives to this scheme will be added as needed. 110 111 #ifndef TCMALLOC_PACKED_CACHE_INL_H__ 112 #define TCMALLOC_PACKED_CACHE_INL_H__ 113 114 #ifndef WTF_CHANGES 115 #include "base/basictypes.h" // for COMPILE_ASSERT 116 #include "base/logging.h" // for DCHECK 117 #endif 118 119 #ifndef DCHECK_EQ 120 #define DCHECK_EQ(val1, val2) ASSERT((val1) == (val2)) 121 #endif 122 123 // A safe way of doing "(1 << n) - 1" -- without worrying about overflow 124 // Note this will all be resolved to a constant expression at compile-time 125 #define N_ONES_(IntType, N) \ 126 ( (N) == 0 ? 0 : ((static_cast<IntType>(1) << ((N)-1))-1 + \ 127 (static_cast<IntType>(1) << ((N)-1))) ) 128 129 // The types K and V provide upper bounds on the number of valid keys 130 // and values, but we explicitly require the keys to be less than 131 // 2^kKeybits and the values to be less than 2^kValuebits. The size of 132 // the table is controlled by kHashbits, and the type of each entry in 133 // the cache is T. See also the big comment at the top of the file. 134 template <int kKeybits, typename T> 135 class PackedCache { 136 public: 137 typedef uintptr_t K; 138 typedef size_t V; 139 static const size_t kHashbits = 12; 140 static const size_t kValuebits = 8; 141 PackedCache(V initial_value)142 explicit PackedCache(V initial_value) { 143 COMPILE_ASSERT(kKeybits <= sizeof(K) * 8, key_size); 144 COMPILE_ASSERT(kValuebits <= sizeof(V) * 8, value_size); 145 COMPILE_ASSERT(kHashbits <= kKeybits, hash_function); 146 COMPILE_ASSERT(kKeybits - kHashbits + kValuebits <= kTbits, 147 entry_size_must_be_big_enough); 148 Clear(initial_value); 149 } 150 Put(K key,V value)151 void Put(K key, V value) { 152 DCHECK_EQ(key, key & kKeyMask); 153 DCHECK_EQ(value, value & kValueMask); 154 array_[Hash(key)] = static_cast<T>(KeyToUpper(key) | value); 155 } 156 Has(K key)157 bool Has(K key) const { 158 DCHECK_EQ(key, key & kKeyMask); 159 return KeyMatch(array_[Hash(key)], key); 160 } 161 GetOrDefault(K key,V default_value)162 V GetOrDefault(K key, V default_value) const { 163 // As with other code in this class, we touch array_ as few times 164 // as we can. Assuming entries are read atomically (e.g., their 165 // type is uintptr_t on most hardware) then certain races are 166 // harmless. 167 DCHECK_EQ(key, key & kKeyMask); 168 T entry = array_[Hash(key)]; 169 return KeyMatch(entry, key) ? EntryToValue(entry) : default_value; 170 } 171 Clear(V value)172 void Clear(V value) { 173 DCHECK_EQ(value, value & kValueMask); 174 for (int i = 0; i < 1 << kHashbits; i++) { 175 array_[i] = static_cast<T>(value); 176 } 177 } 178 179 private: 180 // We are going to pack a value and the upper part of a key into 181 // an entry of type T. The UPPER type is for the upper part of a key, 182 // after the key has been masked and shifted for inclusion in an entry. 183 typedef T UPPER; 184 EntryToValue(T t)185 static V EntryToValue(T t) { return t & kValueMask; } 186 EntryToUpper(T t)187 static UPPER EntryToUpper(T t) { return t & kUpperMask; } 188 189 // If v is a V and u is an UPPER then you can create an entry by 190 // doing u | v. kHashbits determines where in a K to find the upper 191 // part of the key, and kValuebits determines where in the entry to put 192 // it. KeyToUpper(K k)193 static UPPER KeyToUpper(K k) { 194 const int shift = kHashbits - kValuebits; 195 // Assume kHashbits >= kValuebits. It would be easy to lift this assumption. 196 return static_cast<T>(k >> shift) & kUpperMask; 197 } 198 199 // This is roughly the inverse of KeyToUpper(). Some of the key has been 200 // thrown away, since KeyToUpper() masks off the low bits of the key. UpperToPartialKey(UPPER u)201 static K UpperToPartialKey(UPPER u) { 202 DCHECK_EQ(u, u & kUpperMask); 203 const int shift = kHashbits - kValuebits; 204 // Assume kHashbits >= kValuebits. It would be easy to lift this assumption. 205 return static_cast<K>(u) << shift; 206 } 207 Hash(K key)208 static size_t Hash(K key) { 209 return static_cast<size_t>(key) & N_ONES_(size_t, kHashbits); 210 } 211 212 // Does the entry's partial key match the relevant part of the given key? KeyMatch(T entry,K key)213 static bool KeyMatch(T entry, K key) { 214 return ((KeyToUpper(key) ^ entry) & kUpperMask) == 0; 215 } 216 217 static const size_t kTbits = 8 * sizeof(T); 218 static const int kUpperbits = kKeybits - kHashbits; 219 220 // For masking a K. 221 static const K kKeyMask = N_ONES_(K, kKeybits); 222 223 // For masking a T. 224 static const T kUpperMask = N_ONES_(T, kUpperbits) << kValuebits; 225 226 // For masking a V or a T. 227 static const V kValueMask = N_ONES_(V, kValuebits); 228 229 T array_[1 << kHashbits]; 230 }; 231 232 #undef N_ONES_ 233 234 #endif // TCMALLOC_PACKED_CACHE_INL_H__ 235