* Authors: Kevin Lim
*/
+#include "cpu/pred/tournament.hh"
+
#include "base/bitfield.hh"
#include "base/intmath.hh"
-#include "cpu/pred/tournament.hh"
TournamentBP::TournamentBP(const TournamentBPParams *params)
: BPredUnit(params),
assert(local_history_idx < localHistoryTableSize);
- // Commented code is for doing speculative update of counters and
- // all histories.
+ // Speculative update of the global history and the
+ // selected local history.
if (choice_prediction) {
if (global_prediction) {
updateGlobalHistTaken(tid);
TournamentBP::update(ThreadID tid, Addr branch_addr, bool taken,
void *bp_history, bool squashed)
{
- unsigned local_history_idx;
- unsigned local_predictor_idx M5_VAR_USED;
- unsigned local_predictor_hist;
+ assert(bp_history);
- // Get the local predictor's current prediction
- local_history_idx = calcLocHistIdx(branch_addr);
- local_predictor_hist = localHistoryTable[local_history_idx];
- local_predictor_idx = local_predictor_hist & localPredictorMask;
-
- if (bp_history) {
- BPHistory *history = static_cast<BPHistory *>(bp_history);
- // Update may also be called if the Branch target is incorrect even if
- // the prediction is correct. In that case do not update the counters.
- bool historyPred = false;
- unsigned old_local_pred_index = history->localHistory &
- localPredictorMask;
-
- bool old_local_pred_valid = history->localHistory !=
- invalidPredictorIndex;
+ BPHistory *history = static_cast<BPHistory *>(bp_history);
- assert(old_local_pred_index < localPredictorSize);
+ unsigned local_history_idx = calcLocHistIdx(branch_addr);
- if (history->globalUsed) {
- historyPred = history->globalPredTaken;
- } else {
- historyPred = history->localPredTaken;
- }
- if (historyPred != taken || !squashed) {
- // Update the choice predictor to tell it which one was correct if
- // there was a prediction.
- if (history->localPredTaken != history->globalPredTaken) {
- // If the local prediction matches the actual outcome,
- // decerement the counter. Otherwise increment the
- // counter.
- unsigned choice_predictor_idx =
- history->globalHistory & choiceHistoryMask;
- if (history->localPredTaken == taken) {
- choiceCtrs[choice_predictor_idx].decrement();
- } else if (history->globalPredTaken == taken) {
- choiceCtrs[choice_predictor_idx].increment();
- }
-
- }
-
- // Update the counters and local history with the proper
- // resolution of the branch. Global history is updated
- // speculatively and restored upon squash() calls, so it does not
- // need to be updated.
- unsigned global_predictor_idx =
- history->globalHistory & globalHistoryMask;
- if (taken) {
- globalCtrs[global_predictor_idx].increment();
- if (old_local_pred_valid) {
- localCtrs[old_local_pred_index].increment();
- }
- } else {
- globalCtrs[global_predictor_idx].decrement();
- if (old_local_pred_valid) {
- localCtrs[old_local_pred_index].decrement();
- }
- }
- }
- if (squashed) {
- if (taken) {
- globalHistory[tid] = (history->globalHistory << 1) | 1;
- globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
- if (old_local_pred_valid) {
- localHistoryTable[local_history_idx] =
- (history->localHistory << 1) | 1;
- }
- } else {
- globalHistory[tid] = (history->globalHistory << 1);
- globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
- if (old_local_pred_valid) {
- localHistoryTable[local_history_idx] =
- history->localHistory << 1;
- }
- }
+ assert(local_history_idx < localHistoryTableSize);
- } else {
- // We're done with this history, now delete it.
- delete history;
+ // Unconditional branches do not use local history.
+ bool old_local_pred_valid = history->localHistory !=
+ invalidPredictorIndex;
+
+ // If this is a misprediction, restore the speculatively
+ // updated state (global history register and local history)
+ // and update again.
+ if (squashed) {
+ // Global history restore and update
+ globalHistory[tid] = (history->globalHistory << 1) | taken;
+ globalHistory[tid] &= historyRegisterMask;
+
+ // Local history restore and update.
+ if (old_local_pred_valid) {
+ localHistoryTable[local_history_idx] =
+ (history->localHistory << 1) | taken;
}
- }
- assert(local_history_idx < localHistoryTableSize);
+ return;
+ }
+ unsigned old_local_pred_index = history->localHistory &
+ localPredictorMask;
+
+ assert(old_local_pred_index < localPredictorSize);
+
+ // Update the choice predictor to tell it which one was correct if
+ // there was a prediction.
+ if (history->localPredTaken != history->globalPredTaken &&
+ old_local_pred_valid)
+ {
+ // If the local prediction matches the actual outcome,
+ // decrement the counter. Otherwise increment the
+ // counter.
+ unsigned choice_predictor_idx =
+ history->globalHistory & choiceHistoryMask;
+ if (history->localPredTaken == taken) {
+ choiceCtrs[choice_predictor_idx].decrement();
+ } else if (history->globalPredTaken == taken) {
+ choiceCtrs[choice_predictor_idx].increment();
+ }
+ }
-}
+ // Update the counters with the proper
+ // resolution of the branch. Histories are updated
+ // speculatively, restored upon squash() calls, and
+ // recomputed upon update(squash = true) calls,
+ // so they do not need to be updated.
+ unsigned global_predictor_idx =
+ history->globalHistory & globalHistoryMask;
+ if (taken) {
+ globalCtrs[global_predictor_idx].increment();
+ if (old_local_pred_valid) {
+ localCtrs[old_local_pred_index].increment();
+ }
+ } else {
+ globalCtrs[global_predictor_idx].decrement();
+ if (old_local_pred_valid) {
+ localCtrs[old_local_pred_index].decrement();
+ }
+ }
-void
-TournamentBP::retireSquashed(ThreadID tid, void *bp_history)
-{
- BPHistory *history = static_cast<BPHistory *>(bp_history);
+ // We're done with this history, now delete it.
delete history;
}
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