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/*
* 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) const
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__
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