text: /*
   * Copyright (c) 2018 Inria
   * 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 the copyright holders 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.
   *
   * Authors: Daniel Carvalho
   */
  
  /**
   * @file
   * Declaration of a skewed associative indexing policy.
   */
  
  #ifndef __MEM_CACHE_INDEXING_POLICIES_SKEWED_ASSOCIATIVE_HH__
  #define __MEM_CACHE_INDEXING_POLICIES_SKEWED_ASSOCIATIVE_HH__
  
  #include <vector>
  
  #include "mem/cache/tags/indexing_policies/base.hh"
  #include "params/SkewedAssociative.hh"
  
  class ReplaceableEntry;
  
  /**
   * A skewed associative indexing policy.
   * @sa  \ref gem5MemorySystem "gem5 Memory System"
   *
   * The skewed indexing policy has a variable mapping based on a hash function,
   * so a value x can be mapped to different sets, based on the way being used.
   *
   * For example, let's assume address A maps to set 3 on way 0. It will likely
   * have a different set for every other way. Visually, the possible locations
   * of A are, for a table with 4 ways and 8 sets (arbitrarily chosen sets; these
   * locations depend on A and the hashing function used):
   *    Way 0   1   2   3
   *  Set   _   _   _   _
   *    0  |_| |_| |X| |_|
   *    1  |_| |_| |_| |X|
   *    2  |_| |_| |_| |_|
   *    3  |X| |_| |_| |_|
   *    4  |_| |_| |_| |_|
   *    5  |_| |X| |_| |_|
   *    6  |_| |_| |_| |_|
   *    7  |_| |_| |_| |_|
   *
   * If provided with an associativity higher than the number of skewing
   * functions, the skewing functions of the extra ways might be sub-optimal.
   */
  class SkewedAssociative : public BaseIndexingPolicy
  {
    private:
      /**
       * The number of skewing functions implemented. Should be updated if more
       * functions are added. If more than this number of skewing functions are
       * needed (i.e., assoc > this value), we programatically generate new ones,
       * which may be sub-optimal.
       */
      const int NUM_SKEWING_FUNCTIONS = 8;
  
      /**
       * The amount to shift a set index to get its MSB.
       */
      const int msbShift;
  
      /**
       * The hash function itself. Uses the hash function H, as described in
       * "Skewed-Associative Caches", from Seznec et al. (section 3.3): It
       * applies an XOR to the MSB and LSB, shifts all bits one bit to the right,
       * and set the result of the XOR as the new MSB.
       *
       * This function is not bijective if the address has only 1 bit, as the MSB
       * and LSB will be the same, and therefore the xor will always be 0.
       *
       * @param addr The address to be hashed.
       * @param The hashed address.
       */
      Addr hash(const Addr addr) const;
  
      /**
       * Inverse of the hash function.
       * @sa hash().
       *
       * @param addr The address to be dehashed.
       * @param The dehashed address.
       */
      Addr dehash(const Addr addr) const;
  
      /**
       * Address skewing function selection. It selects and applies one of the
       * skewing functions functions based on the way provided.
       *
       * @param addr Address to be skewed. Should contain the set and tag bits.
       * @param way The cache way, used to select a hash function.
       * @return The skewed address.
       */
      Addr skew(const Addr addr, const uint32_t way) const;
  
      /**
       * Address deskewing function (inverse of the skew function) of the given
       * way.
       * @sa skew()
       *
       * @param addr Address to be deskewed. Should contain the set and tag bits.
       * @param way The cache way, used to select a hash function.
       * @return The deskewed address.
       */
      Addr deskew(const Addr addr, const uint32_t way) const;
  
      /**
       * Apply a skewing function to calculate address' set given a way.
       *
       * @param addr The address to calculate the set for.
       * @param way The way to get the set from.
       * @return The set index for given combination of address and way.
       */
      uint32_t extractSet(const Addr addr, const uint32_t way) const;
  
    public:
      /** Convenience typedef. */
       typedef SkewedAssociativeParams Params;
  
      /**
       * Construct and initialize this policy.
       */
      SkewedAssociative(const Params *p);
  
      /**
       * Destructor.
       */
      ~SkewedAssociative() {};
  
      /**
       * Find all possible entries for insertion and replacement of an address.
       * Should be called immediately before ReplacementPolicy's findVictim()
       * not to break cache resizing.
       *
       * @param addr The addr to a find possible entries for.
       * @return The possible entries.
       */
      std::vector<ReplaceableEntry*> getPossibleEntries(const Addr addr, uint32_t securityDomain)
                                                                     override;
  
      /**
       * Regenerate an entry's address from its tag and assigned set and way.
       * Uses the inverse of the skewing function.
       *
       * @param tag The tag bits.
       * @param entry The entry.
       * @return the entry's address.
       */
      Addr regenerateAddr(const Addr tag, const ReplaceableEntry* entry) const
                                                                     override;
  };
  
  #endif //__MEM_CACHE_INDEXING_POLICIES_SKEWED_ASSOCIATIVE_HH__
  