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#include "cpu/beta_cpu/tournament_pred.hh"
TournamentBP::TournamentBP(unsigned _local_predictor_size,
unsigned _local_ctr_bits,
unsigned _local_history_table_size,
unsigned _local_history_bits,
unsigned _global_predictor_size,
unsigned _global_ctr_bits,
unsigned _global_history_bits,
unsigned _choice_predictor_size,
unsigned _choice_ctr_bits,
unsigned _instShiftAmt)
: local_predictor_size(_local_predictor_size),
local_ctr_bits(_local_ctr_bits),
local_history_table_size(_local_history_table_size),
local_history_bits(_local_history_bits),
global_predictor_size(_global_predictor_size),
global_ctr_bits(_global_ctr_bits),
global_history_bits(_global_history_bits),
choice_predictor_size(_global_predictor_size),
choice_ctr_bits(_choice_ctr_bits),
instShiftAmt(_instShiftAmt)
{
//Should do checks here to make sure sizes are correct (powers of 2)
//Setup the array of counters for the local predictor
local_ctrs = new SatCounter[local_predictor_size];
for (int i = 0; i < local_predictor_size; ++i)
local_ctrs[i].setBits(local_ctr_bits);
//Setup the history table for the local table
local_history_table = new unsigned[local_history_table_size];
for (int i = 0; i < local_history_table_size; ++i)
local_history_table[i] = 0;
// Setup the local history mask
localHistoryMask = (1 << local_history_bits) - 1;
//Setup the array of counters for the global predictor
global_ctrs = new SatCounter[global_predictor_size];
for (int i = 0; i < global_predictor_size; ++i)
global_ctrs[i].setBits(global_ctr_bits);
//Clear the global history
global_history = 0;
// Setup the global history mask
globalHistoryMask = (1 << global_history_bits) - 1;
//Setup the array of counters for the choice predictor
choice_ctrs = new SatCounter[choice_predictor_size];
for (int i = 0; i < choice_predictor_size; ++i)
choice_ctrs[i].setBits(choice_ctr_bits);
threshold = (1 << (local_ctr_bits - 1)) - 1;
threshold = threshold / 2;
}
inline
unsigned
TournamentBP::calcLocHistIdx(Addr &branch_addr)
{
return (branch_addr >> instShiftAmt) & (local_history_table_size - 1);
}
inline
void
TournamentBP::updateHistoriesTaken(unsigned local_history_idx)
{
global_history = (global_history << 1) | 1;
global_history = global_history & globalHistoryMask;
local_history_table[local_history_idx] =
(local_history_table[local_history_idx] << 1) | 1;
}
inline
void
TournamentBP::updateHistoriesNotTaken(unsigned local_history_idx)
{
global_history = (global_history << 1);
global_history = global_history & globalHistoryMask;
local_history_table[local_history_idx] =
(local_history_table[local_history_idx] << 1);
}
bool
TournamentBP::lookup(Addr &branch_addr)
{
uint8_t local_prediction;
unsigned local_history_idx;
unsigned local_predictor_idx;
uint8_t global_prediction;
uint8_t choice_prediction;
//Lookup in the local predictor to get its branch prediction
local_history_idx = calcLocHistIdx(branch_addr);
local_predictor_idx = local_history_table[local_history_idx]
& localHistoryMask;
local_prediction = local_ctrs[local_predictor_idx].read();
//Lookup in the global predictor to get its branch prediction
global_prediction = global_ctrs[global_history].read();
//Lookup in the choice predictor to see which one to use
choice_prediction = choice_ctrs[global_history].read();
//@todo Put a threshold value in for the three predictors that can
// be set through the constructor (so this isn't hard coded).
//Also should put some of this code into functions.
if (choice_prediction > threshold) {
if (global_prediction > threshold) {
updateHistoriesTaken(local_history_idx);
assert(global_history < global_predictor_size &&
local_history_idx < local_predictor_size);
global_ctrs[global_history].increment();
local_ctrs[local_history_idx].increment();
return true;
} else {
updateHistoriesNotTaken(local_history_idx);
assert(global_history < global_predictor_size &&
local_history_idx < local_predictor_size);
global_ctrs[global_history].decrement();
local_ctrs[local_history_idx].decrement();
return false;
}
} else {
if (local_prediction > threshold) {
updateHistoriesTaken(local_history_idx);
assert(global_history < global_predictor_size &&
local_history_idx < local_predictor_size);
global_ctrs[global_history].increment();
local_ctrs[local_history_idx].increment();
return true;
} else {
updateHistoriesNotTaken(local_history_idx);
assert(global_history < global_predictor_size &&
local_history_idx < local_predictor_size);
global_ctrs[global_history].decrement();
local_ctrs[local_history_idx].decrement();
return false;
}
}
}
// Update the branch predictor if it predicted a branch wrong.
void
TournamentBP::update(Addr &branch_addr, unsigned correct_gh, bool taken)
{
uint8_t local_prediction;
unsigned local_history_idx;
unsigned local_predictor_idx;
bool local_pred_taken;
uint8_t global_prediction;
bool global_pred_taken;
// Load the correct global history into the register.
global_history = correct_gh;
// Get the local predictor's current prediction, remove the incorrect
// update, and update the local predictor
local_history_idx = calcLocHistIdx(branch_addr);
local_predictor_idx = local_history_table[local_history_idx];
local_predictor_idx = (local_predictor_idx >> 1) & localHistoryMask;
local_prediction = local_ctrs[local_predictor_idx].read();
local_pred_taken = local_prediction > threshold;
//Get the global predictor's current prediction, and update the
//global predictor
global_prediction = global_ctrs[global_history].read();
global_pred_taken = global_prediction > threshold;
//Update the choice predictor to tell it which one was correct
if (local_pred_taken != global_pred_taken) {
//If the local prediction matches the actual outcome, decerement
//the counter. Otherwise increment the counter.
if (local_pred_taken == taken) {
choice_ctrs[global_history].decrement();
} else {
choice_ctrs[global_history].increment();
}
}
if (taken) {
assert(global_history < global_predictor_size &&
local_predictor_idx < local_predictor_size);
local_ctrs[local_predictor_idx].increment();
global_ctrs[global_history].increment();
global_history = (global_history << 1) | 1;
global_history = global_history & globalHistoryMask;
local_history_table[local_history_idx] |= 1;
}
else {
assert(global_history < global_predictor_size &&
local_predictor_idx < local_predictor_size);
local_ctrs[local_predictor_idx].decrement();
global_ctrs[global_history].decrement();
global_history = (global_history << 1);
global_history = global_history & globalHistoryMask;
local_history_table[local_history_idx] &= ~1;
}
}
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