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Branch predictor: Fixes the tournament branch predictor.

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Branch predictor could not predict a branch in a nested loop because:
 1. The global history was not updated after a mispredict squash.
 2. The global history was updated in the fetch stage. The choice predictors
    that were updated  used the changed global history. This is incorrect, as
    it incorporates the state of global history after the branch in
    encountered. Fixed update to choice predictor using the global history
    state before the branch happened.
 3. The global predictor table was also updated using the global history state
    before the branch happened as above.

Additionally, parameters to initialize ctr and history size were reversed.
This commit is contained in:
Mrinmoy Ghosh 2011-07-10 12:56:08 -05:00
parent c7e7b89058
commit 3396fd9e84
1 changed files with 38 additions and 17 deletions
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55
src/cpu/pred/tournament.cc
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@ -36,8 +36,8 @@ TournamentBP::TournamentBP(unsigned _localPredictorSize,
unsigned _localHistoryTableSize,
unsigned _localHistoryBits,
unsigned _globalPredictorSize,
unsigned _globalCtrBits,
unsigned _globalHistoryBits,
unsigned _globalCtrBits,
unsigned _choicePredictorSize,
unsigned _choiceCtrBits,
unsigned _instShiftAmt)
@ -247,12 +247,47 @@ TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history)
// decerement the counter. Otherwise increment the
// counter.
if (history->localPredTaken == taken) {
choiceCtrs[globalHistory].decrement();
choiceCtrs[history->globalHistory].decrement();
} else if (history->globalPredTaken == taken){
choiceCtrs[globalHistory].increment();
choiceCtrs[history->globalHistory].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.
if (taken) {
localCtrs[local_predictor_idx].increment();
globalCtrs[history->globalHistory].increment();
updateLocalHistTaken(local_history_idx);
} else {
localCtrs[local_predictor_idx].decrement();
globalCtrs[history->globalHistory].decrement();
updateLocalHistNotTaken(local_history_idx);
}
bool mispredict = false;
//global predictor used and mispredicted
if (history->globalUsed && history->globalPredTaken != taken)
mispredict = true;
//local predictor used and mispredicted
else if (!history->globalUsed && history->localPredTaken != taken)
mispredict = true;
if (mispredict) {
if (taken) {
globalHistory = globalHistory | 1;
} else {
unsigned mask = globalHistoryMask - 1;
globalHistory = globalHistory & mask;
}
}
// We're done with this history, now delete it.
delete history;
}
@ -261,21 +296,7 @@ TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history)
local_history_idx < localHistoryTableSize &&
local_predictor_idx < localPredictorSize);
// 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.
if (taken) {
localCtrs[local_predictor_idx].increment();
globalCtrs[globalHistory].increment();
updateLocalHistTaken(local_history_idx);
} else {
localCtrs[local_predictor_idx].decrement();
globalCtrs[globalHistory].decrement();
updateLocalHistNotTaken(local_history_idx);
}
}
void