1/* 2 * Copyright (c) 2018 Inria 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions are 7 * met: redistributions of source code must retain the above copyright 8 * notice, this list of conditions and the following disclaimer; 9 * redistributions in binary form must reproduce the above copyright 10 * notice, this list of conditions and the following disclaimer in the 11 * documentation and/or other materials provided with the distribution; 12 * neither the name of the copyright holders nor the names of its 13 * contributors may be used to endorse or promote products derived from 14 * this software without specific prior written permission. 15 * 16 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 17 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 18 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 19 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 20 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 21 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 22 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 26 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27 * 28 * Authors: Daniel Carvalho 29 */ 30 31/** 32 * @file 33 * Declaration of a skewed associative indexing policy. 34 */ 35 36#ifndef __MEM_CACHE_INDEXING_POLICIES_SKEWED_ASSOCIATIVE_HH__ 37#define __MEM_CACHE_INDEXING_POLICIES_SKEWED_ASSOCIATIVE_HH__ 38 39#include <vector> 40 41#include "mem/cache/tags/indexing_policies/base.hh" 42#include "params/SkewedAssociative.hh" 43 44class ReplaceableEntry; 45 46/** 47 * A skewed associative indexing policy. 48 * @sa \ref gem5MemorySystem "gem5 Memory System" 49 * 50 * The skewed indexing policy has a variable mapping based on a hash function, 51 * so a value x can be mapped to different sets, based on the way being used. 52 * 53 * For example, let's assume address A maps to set 3 on way 0. It will likely 54 * have a different set for every other way. Visually, the possible locations 55 * of A are, for a table with 4 ways and 8 sets (arbitrarily chosen sets; these 56 * locations depend on A and the hashing function used): 57 * Way 0 1 2 3 58 * Set _ _ _ _ 59 * 0 |_| |_| |X| |_| 60 * 1 |_| |_| |_| |X| 61 * 2 |_| |_| |_| |_| 62 * 3 |X| |_| |_| |_| 63 * 4 |_| |_| |_| |_| 64 * 5 |_| |X| |_| |_| 65 * 6 |_| |_| |_| |_| 66 * 7 |_| |_| |_| |_| 67 * 68 * If provided with an associativity higher than the number of skewing 69 * functions, the skewing functions of the extra ways might be sub-optimal. 70 */ 71class SkewedAssociative : public BaseIndexingPolicy 72{ 73 private: 74 /** 75 * The number of skewing functions implemented. Should be updated if more 76 * functions are added. If more than this number of skewing functions are 77 * needed (i.e., assoc > this value), we programatically generate new ones, 78 * which may be sub-optimal. 79 */ 80 const int NUM_SKEWING_FUNCTIONS = 8; 81 82 /** 83 * The amount to shift a set index to get its MSB. 84 */ 85 const int msbShift; 86 87 /** 88 * The hash function itself. Uses the hash function H, as described in 89 * "Skewed-Associative Caches", from Seznec et al. (section 3.3): It 90 * applies an XOR to the MSB and LSB, shifts all bits one bit to the right, 91 * and set the result of the XOR as the new MSB. 92 * 93 * This function is not bijective if the address has only 1 bit, as the MSB 94 * and LSB will be the same, and therefore the xor will always be 0. 95 * 96 * @param addr The address to be hashed. 97 * @param The hashed address. 98 */ 99 Addr hash(const Addr addr) const; 100 101 /** 102 * Inverse of the hash function. 103 * @sa hash(). 104 * 105 * @param addr The address to be dehashed. 106 * @param The dehashed address. 107 */ 108 Addr dehash(const Addr addr) const; 109 110 /** 111 * Address skewing function selection. It selects and applies one of the 112 * skewing functions functions based on the way provided. 113 * 114 * @param addr Address to be skewed. Should contain the set and tag bits. 115 * @param way The cache way, used to select a hash function. 116 * @return The skewed address. 117 */ 118 Addr skew(const Addr addr, const uint32_t way) const; 119 120 /** 121 * Address deskewing function (inverse of the skew function) of the given 122 * way. 123 * @sa skew() 124 * 125 * @param addr Address to be deskewed. Should contain the set and tag bits. 126 * @param way The cache way, used to select a hash function. 127 * @return The deskewed address. 128 */ 129 Addr deskew(const Addr addr, const uint32_t way) const; 130 131 /** 132 * Apply a skewing function to calculate address' set given a way. 133 * 134 * @param addr The address to calculate the set for. 135 * @param way The way to get the set from. 136 * @return The set index for given combination of address and way. 137 */ 138 uint32_t extractSet(const Addr addr, const uint32_t way) const; 139 140 public: 141 /** Convenience typedef. */ 142 typedef SkewedAssociativeParams Params; 143 144 /** 145 * Construct and initialize this policy. 146 */ 147 SkewedAssociative(const Params *p); 148 149 /** 150 * Destructor. 151 */ 152 ~SkewedAssociative() {}; 153 154 /** 155 * Find all possible entries for insertion and replacement of an address. 156 * Should be called immediately before ReplacementPolicy's findVictim() 157 * not to break cache resizing. 158 * 159 * @param addr The addr to a find possible entries for. 160 * @return The possible entries. 161 */ 162 std::vector<ReplaceableEntry*> getPossibleEntries(const Addr addr) const 163 override; 164 165 /** 166 * Regenerate an entry's address from its tag and assigned set and way. 167 * Uses the inverse of the skewing function. 168 * 169 * @param tag The tag bits. 170 * @param entry The entry. 171 * @return the entry's address. 172 */ 173 Addr regenerateAddr(const Addr tag, const ReplaceableEntry* entry) const 174 override; 175}; 176 177#endif //__MEM_CACHE_INDEXING_POLICIES_SKEWED_ASSOCIATIVE_HH__ 178