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#ifndef PSEUDOLRUPOLICY_H
#define PSEUDOLRUPOLICY_H
#include <cmath>
#include "mem/ruby/system/AbstractReplacementPolicy.hh"
/**
* Implementation of tree-based pseudo-LRU replacement
*
* Works for any associativity between 1 and 128.
*
* Also implements associativities that are not a power of 2 by
* ignoring paths that lead to a larger index (i.e. truncating the
* tree). Note that when this occurs, the algorithm becomes less
* fair, as it will favor indicies in the larger (by index) half of
* the associative set. This is most unfair when the nearest power of
* 2 is one below the associativy, and most fair when it is one above.
*/
class PseudoLRUPolicy : public AbstractReplacementPolicy {
public:
PseudoLRUPolicy(Index num_sets, Index assoc);
~PseudoLRUPolicy();
void touch(Index set, Index way, Time time);
Index getVictim(Index set) const;
private:
unsigned int m_effective_assoc; /** nearest (to ceiling) power of 2 */
unsigned int m_num_levels; /** number of levels in the tree */
uint64* m_trees; /** bit representation of the trees, one for each set */
};
inline
PseudoLRUPolicy::PseudoLRUPolicy(Index num_sets, Index assoc)
: AbstractReplacementPolicy(num_sets, assoc)
{
int num_tree_nodes;
// associativity cannot exceed capacity of tree representation
assert(num_sets > 0 && assoc > 1 && assoc <= (Index) sizeof(uint64)*4);
m_trees = NULL;
m_num_levels = 0;
m_effective_assoc = 1;
while(m_effective_assoc < assoc){
m_effective_assoc <<= 1; // effective associativity is ceiling power of 2
}
assoc = m_effective_assoc;
while(true){
assoc /= 2;
if(!assoc) break;
m_num_levels++;
}
assert(m_num_levels < sizeof(unsigned int)*4);
num_tree_nodes = ((int)pow(2, m_num_levels))-1;
m_trees = new uint64[m_num_sets];
for(unsigned int i=0; i< m_num_sets; i++){
m_trees[i] = 0;
}
}
inline
PseudoLRUPolicy::~PseudoLRUPolicy()
{
if(m_trees != NULL)
delete[] m_trees;
}
inline
void PseudoLRUPolicy::touch(Index set, Index index, Time time){
assert(index >= 0 && index < m_assoc);
assert(set >= 0 && set < m_num_sets);
int tree_index = 0;
int node_val;
for(int i=m_num_levels -1; i>=0; i--){
node_val = (index >> i)&1;
if(node_val)
m_trees[set] |= node_val << tree_index;
else
m_trees[set] &= ~(1 << tree_index);
tree_index = node_val ? (tree_index*2)+2 : (tree_index*2)+1;
}
m_last_ref_ptr[set][index] = time;
}
inline
Index PseudoLRUPolicy::getVictim(Index set) const {
// assert(m_assoc != 0);
Index index = 0;
int tree_index = 0;
int node_val;
for(unsigned int i=0;i<m_num_levels;i++){
node_val = (m_trees[set]>>tree_index)&1;
index += node_val?0:(m_effective_assoc >> (i+1));
tree_index = node_val? (tree_index*2)+1 : (tree_index*2)+2;
}
assert(index >= 0 && index < m_effective_assoc);
/* return either the found index or the max possible index */
/* NOTE: this is not a fair replacement when assoc is not a power of 2 */
return (index > (m_assoc-1)) ? m_assoc-1:index;
}
#endif // PSEUDOLRUPOLICY_H
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