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#include "cpu/beta_cpu/bpred_unit.hh"
#include "base/traceflags.hh"
#include "base/trace.hh"
template<class Impl>
TwobitBPredUnit<Impl>::TwobitBPredUnit(Params ¶ms)
: BP(params.local_predictor_size,
params.local_ctr_bits,
params.instShiftAmt),
BTB(params.BTBEntries,
params.BTBTagSize,
params.instShiftAmt),
RAS(params.RASSize)
{
}
template <class Impl>
void
TwobitBPredUnit<Impl>::regStats()
{
lookups
.name(name() + ".BPredUnit.lookups")
.desc("Number of BP lookups")
;
condPredicted
.name(name() + ".BPredUnit.condPredicted")
.desc("Number of conditional branches predicted")
;
condIncorrect
.name(name() + ".BPredUnit.condIncorrect")
.desc("Number of conditional branches incorrect")
;
BTBLookups
.name(name() + ".BPredUnit.BTBLookups")
.desc("Number of BTB lookups")
;
BTBHits
.name(name() + ".BPredUnit.BTBHits")
.desc("Number of BTB hits")
;
BTBCorrect
.name(name() + ".BPredUnit.BTBCorrect")
.desc("Number of correct BTB predictions (this stat may not "
"work properly.")
;
usedRAS
.name(name() + ".BPredUnit.usedRAS")
.desc("Number of times the RAS was used.")
;
RASIncorrect
.name(name() + ".BPredUnit.RASInCorrect")
.desc("Number of incorrect RAS predictions.")
;
}
template <class Impl>
bool
TwobitBPredUnit<Impl>::predict(DynInstPtr &inst, Addr &PC)
{
// See if branch predictor predicts taken.
// If so, get its target addr either from the BTB or the RAS.
// Once that's done, speculatively update the predictor?
// Save off record of branch stuff so the RAS can be fixed
// up once it's done.
bool pred_taken = false;
Addr target;
++lookups;
if (inst->isUncondCtrl()) {
DPRINTF(Fetch, "BranchPred: Unconditional control.\n");
pred_taken = true;
} else {
++condPredicted;
pred_taken = BPLookup(PC);
DPRINTF(Fetch, "BranchPred: Branch predictor predicted %i for PC %#x"
"\n", pred_taken, inst->readPC());
}
PredictorHistory predict_record(inst->seqNum, PC, pred_taken);
// Now lookup in the BTB or RAS.
if (pred_taken) {
if (inst->isReturn()) {
++usedRAS;
// If it's a function return call, then look up the address
// in the RAS.
target = RAS.top();
// Record the top entry of the RAS, and its index.
predict_record.usedRAS = true;
predict_record.RASIndex = RAS.topIdx();
predict_record.RASTarget = target;
RAS.pop();
DPRINTF(Fetch, "BranchPred: Instruction %#x is a return, RAS "
"predicted target: %#x, RAS index: %i.\n",
inst->readPC(), target, predict_record.RASIndex);
} else {
++BTBLookups;
if (inst->isCall()) {
RAS.push(PC+sizeof(MachInst));
// Record that it was a call so that the top RAS entry can
// be popped off if the speculation is incorrect.
predict_record.wasCall = true;
DPRINTF(Fetch, "BranchPred: Instruction %#x was a call, "
"adding %#x to the RAS.\n",
inst->readPC(), PC+sizeof(MachInst));
}
if (BTB.valid(PC)) {
++BTBHits;
//If it's anything else, use the BTB to get the target addr.
target = BTB.lookup(PC);
DPRINTF(Fetch, "BranchPred: Instruction %#x predicted target "
"is %#x.\n", inst->readPC(), target);
} else {
DPRINTF(Fetch, "BranchPred: BTB doesn't have a valid entry."
"\n");
pred_taken = false;
}
}
}
if (pred_taken) {
// Set the PC and the instruction's predicted target.
PC = target;
inst->setPredTarg(target);
} else {
PC = PC + sizeof(MachInst);
inst->setPredTarg(PC);
}
predHist.push_front(predict_record);
assert(!predHist.empty());
return pred_taken;
}
template <class Impl>
void
TwobitBPredUnit<Impl>::update(const InstSeqNum &done_sn)
{
DPRINTF(Fetch, "BranchPred: Commiting branches until sequence number "
"%i.\n", done_sn);
while (!predHist.empty() && predHist.back().seqNum <= done_sn) {
assert(!predHist.empty());
// Update the branch predictor with the correct results of branches.
BP.update(predHist.back().PC, predHist.back().predTaken);
predHist.pop_back();
}
}
template <class Impl>
void
TwobitBPredUnit<Impl>::squash(const InstSeqNum &squashed_sn)
{
while (!predHist.empty() && predHist.front().seqNum > squashed_sn) {
if (predHist.front().usedRAS) {
DPRINTF(Fetch, "BranchPred: Restoring top of RAS to: %i, "
"target: %#x.\n",
predHist.front().RASIndex,
predHist.front().RASTarget);
RAS.restore(predHist.front().RASIndex,
predHist.front().RASTarget);
} else if (predHist.front().wasCall) {
DPRINTF(Fetch, "BranchPred: Removing speculative entry added "
"to the RAS.\n");
RAS.pop();
}
predHist.pop_front();
}
}
template <class Impl>
void
TwobitBPredUnit<Impl>::squash(const InstSeqNum &squashed_sn,
const Addr &corr_target,
const bool actually_taken)
{
// Now that we know that a branch was mispredicted, we need to undo
// all the branches that have been seen up until this branch and
// fix up everything.
++condIncorrect;
DPRINTF(Fetch, "BranchPred: Squashing from sequence number %i, "
"setting target to %#x.\n",
squashed_sn, corr_target);
while (!predHist.empty() && predHist.front().seqNum > squashed_sn) {
if (predHist.front().usedRAS) {
DPRINTF(Fetch, "BranchPred: Restoring top of RAS to: %i, "
"target: %#x.\n",
predHist.front().RASIndex,
predHist.front().RASTarget);
RAS.restore(predHist.front().RASIndex,
predHist.front().RASTarget);
} else if (predHist.front().wasCall) {
DPRINTF(Fetch, "BranchPred: Removing speculative entry added "
"to the RAS.\n");
RAS.pop();
}
predHist.pop_front();
}
predHist.front().predTaken = actually_taken;
if (predHist.front().usedRAS) {
++RASIncorrect;
}
BP.update(predHist.front().PC, actually_taken);
BTB.update(predHist.front().PC, corr_target);
}
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