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