* Implementation of a bi-mode branch predictor
*/
+#include "cpu/pred/bi_mode.hh"
+
#include "base/bitfield.hh"
#include "base/intmath.hh"
-#include "cpu/pred/bi_mode.hh"
-BiModeBP::BiModeBP(const Params *params)
- : BPredUnit(params), instShiftAmt(params->instShiftAmt),
- globalHistoryReg(0),
+BiModeBP::BiModeBP(const BiModeBPParams *params)
+ : BPredUnit(params),
+ globalHistoryReg(params->numThreads, 0),
globalHistoryBits(ceilLog2(params->globalPredictorSize)),
choicePredictorSize(params->choicePredictorSize),
choiceCtrBits(params->choiceCtrBits),
globalHistoryMask = globalPredictorSize - 1;
choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
- takenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
- notTakenThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
+ takenThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
+ notTakenThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
}
/*
* chooses the taken array and the taken array predicts taken.
*/
void
-BiModeBP::uncondBranch(void * &bpHistory)
+BiModeBP::uncondBranch(ThreadID tid, Addr pc, void * &bpHistory)
{
BPHistory *history = new BPHistory;
- history->globalHistoryReg = globalHistoryReg;
+ history->globalHistoryReg = globalHistoryReg[tid];
history->takenUsed = true;
history->takenPred = true;
history->notTakenPred = true;
history->finalPred = true;
bpHistory = static_cast<void*>(history);
- updateGlobalHistReg(true);
+ updateGlobalHistReg(tid, true);
}
void
-BiModeBP::squash(void *bpHistory)
+BiModeBP::squash(ThreadID tid, void *bpHistory)
{
BPHistory *history = static_cast<BPHistory*>(bpHistory);
- globalHistoryReg = history->globalHistoryReg;
+ globalHistoryReg[tid] = history->globalHistoryReg;
delete history;
}
* direction predictors for the final branch prediction.
*/
bool
-BiModeBP::lookup(Addr branchAddr, void * &bpHistory)
+BiModeBP::lookup(ThreadID tid, Addr branchAddr, void * &bpHistory)
{
unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
& choiceHistoryMask);
unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
- ^ globalHistoryReg)
+ ^ globalHistoryReg[tid])
& globalHistoryMask);
assert(choiceHistoryIdx < choicePredictorSize);
bool finalPrediction;
BPHistory *history = new BPHistory;
- history->globalHistoryReg = globalHistoryReg;
+ history->globalHistoryReg = globalHistoryReg[tid];
history->takenUsed = choicePrediction;
history->takenPred = takenGHBPrediction;
history->notTakenPred = notTakenGHBPrediction;
history->finalPred = finalPrediction;
bpHistory = static_cast<void*>(history);
- updateGlobalHistReg(finalPrediction);
+ updateGlobalHistReg(tid, finalPrediction);
return finalPrediction;
}
void
-BiModeBP::btbUpdate(Addr branchAddr, void * &bpHistory)
+BiModeBP::btbUpdate(ThreadID tid, Addr branchAddr, void * &bpHistory)
{
- globalHistoryReg &= (historyRegisterMask & ~ULL(1));
+ globalHistoryReg[tid] &= (historyRegisterMask & ~ULL(1));
}
/* Only the selected direction predictor will be updated with the final
* the direction predictors makes a correct final prediction.
*/
void
-BiModeBP::update(Addr branchAddr, bool taken, void *bpHistory, bool squashed)
+BiModeBP::update(ThreadID tid, Addr branchAddr, bool taken, void *bpHistory,
+ bool squashed)
{
- if (bpHistory) {
- BPHistory *history = static_cast<BPHistory*>(bpHistory);
+ assert(bpHistory);
- unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
- & choiceHistoryMask);
- unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
- ^ globalHistoryReg)
- & globalHistoryMask);
+ BPHistory *history = static_cast<BPHistory*>(bpHistory);
- assert(choiceHistoryIdx < choicePredictorSize);
- assert(globalHistoryIdx < globalPredictorSize);
+ // We do not update the counters speculatively on a squash.
+ // We just restore the global history register.
+ if (squashed) {
+ globalHistoryReg[tid] = (history->globalHistoryReg << 1) | taken;
+ return;
+ }
- if (history->takenUsed) {
- // if the taken array's prediction was used, update it
- if (taken) {
- takenCounters[globalHistoryIdx].increment();
- } else {
- takenCounters[globalHistoryIdx].decrement();
- }
+ unsigned choiceHistoryIdx = ((branchAddr >> instShiftAmt)
+ & choiceHistoryMask);
+ unsigned globalHistoryIdx = (((branchAddr >> instShiftAmt)
+ ^ history->globalHistoryReg)
+ & globalHistoryMask);
+
+ assert(choiceHistoryIdx < choicePredictorSize);
+ assert(globalHistoryIdx < globalPredictorSize);
+
+ if (history->takenUsed) {
+ // if the taken array's prediction was used, update it
+ if (taken) {
+ takenCounters[globalHistoryIdx].increment();
} else {
- // if the not-taken array's prediction was used, update it
- if (taken) {
- notTakenCounters[globalHistoryIdx].increment();
- } else {
- notTakenCounters[globalHistoryIdx].decrement();
- }
+ takenCounters[globalHistoryIdx].decrement();
}
-
- if (history->finalPred == taken) {
- /* If the final prediction matches the actual branch's
- * outcome and the choice predictor matches the final
- * outcome, we update the choice predictor, otherwise it
- * is not updated. While the designers of the bi-mode
- * predictor don't explicity say why this is done, one
- * can infer that it is to preserve the choice predictor's
- * bias with respect to the branch being predicted; afterall,
- * the whole point of the bi-mode predictor is to identify the
- * atypical case when a branch deviates from its bias.
- */
- if (history->finalPred == history->takenUsed) {
- if (taken) {
- choiceCounters[choiceHistoryIdx].increment();
- } else {
- choiceCounters[choiceHistoryIdx].decrement();
- }
- }
+ } else {
+ // if the not-taken array's prediction was used, update it
+ if (taken) {
+ notTakenCounters[globalHistoryIdx].increment();
} else {
- // always update the choice predictor on an incorrect prediction
+ notTakenCounters[globalHistoryIdx].decrement();
+ }
+ }
+
+ if (history->finalPred == taken) {
+ /* If the final prediction matches the actual branch's
+ * outcome and the choice predictor matches the final
+ * outcome, we update the choice predictor, otherwise it
+ * is not updated. While the designers of the bi-mode
+ * predictor don't explicity say why this is done, one
+ * can infer that it is to preserve the choice predictor's
+ * bias with respect to the branch being predicted; afterall,
+ * the whole point of the bi-mode predictor is to identify the
+ * atypical case when a branch deviates from its bias.
+ */
+ if (history->finalPred == history->takenUsed) {
if (taken) {
choiceCounters[choiceHistoryIdx].increment();
} else {
choiceCounters[choiceHistoryIdx].decrement();
}
}
-
- if (squashed) {
- if (taken) {
- globalHistoryReg = (history->globalHistoryReg << 1) | 1;
- } else {
- globalHistoryReg = (history->globalHistoryReg << 1);
- }
- globalHistoryReg &= historyRegisterMask;
+ } else {
+ // always update the choice predictor on an incorrect prediction
+ if (taken) {
+ choiceCounters[choiceHistoryIdx].increment();
} else {
- delete history;
+ choiceCounters[choiceHistoryIdx].decrement();
}
}
+
+ delete history;
}
-void
-BiModeBP::retireSquashed(void *bp_history)
+unsigned
+BiModeBP::getGHR(ThreadID tid, void *bp_history) const
{
- BPHistory *history = static_cast<BPHistory*>(bp_history);
- delete history;
+ return static_cast<BPHistory*>(bp_history)->globalHistoryReg;
}
void
-BiModeBP::updateGlobalHistReg(bool taken)
+BiModeBP::updateGlobalHistReg(ThreadID tid, bool taken)
+{
+ globalHistoryReg[tid] = taken ? (globalHistoryReg[tid] << 1) | 1 :
+ (globalHistoryReg[tid] << 1);
+ globalHistoryReg[tid] &= historyRegisterMask;
+}
+
+BiModeBP*
+BiModeBPParams::create()
{
- globalHistoryReg = taken ? (globalHistoryReg << 1) | 1 :
- (globalHistoryReg << 1);
- globalHistoryReg &= historyRegisterMask;
+ return new BiModeBP(this);
}
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