gem5 / public / gem5 / 1c233ee9d23babe8964d24f386e82f64c30594da / . / src / mem / cache / tags / indexing_policies / skewed_associative.hh

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/** | |

* @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" | |

namespace gem5 | |

{ | |

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; | |

}; | |

} // namespace gem5 | |

#endif //__MEM_CACHE_INDEXING_POLICIES_SKEWED_ASSOCIATIVE_HH__ |