The “Hybrid” predictor is actually a tournament predictor. You now need to look

Question

The “Hybrid” predictor is actually a tournament predictor. You now need to look at its source code (which is in BPred.h and BPRed.cpp files in the ~/sesc/src/libcore/ directory) and determine which of the parameters in the configuration file controls which aspect of the predictor. Hint: the “Hybrid” predictor is implemented in the BPHybrid class, so its constructor and predict method will tell you most of what you need to find out.

The meta-predictor in this hybrid predictor is a table that has__________ entries and each entry is a_________ –bit counter. This meta-predictor decides, based on the PC address (i.e. the address from which we fetched the branch instruction), whether to make the prediction using a simple (no history) array of counters, or to use a (is it local or global?)_________history predictor. The simpler (non-history) predictor uses__________-bit counters, and has______of them (this number is specified using a parameter label_______ in the BPredIssueX section of the configuration file). The history-based predictor has________bits of history, which are combined with the PC address to index into an array that has________entries (this number of entries is specified in the configuration file using parameter label_________), and each entry is a_________-bit counter.

BPred.h

class BPHybrid:public BPred {
private:
BPBTB btb;

const ushort historySize;
const HistoryType historyMask;

SCTable globalTable;

HistoryType ghr; // Global History Register

SCTable localTable;
SCTable metaTable;

protected:
public:
BPHybrid(int32_t i, int32_t fetchWidth, const char *section);
~BPHybrid();

BPRed.cpp

BPHybrid::BPHybrid(int32_t i, int32_t fetchWidth, const char *section)
:BPred(i, fetchWidth, section,"Hybrid")
,btb( i, fetchWidth, section)
,historySize(SescConf->getInt(section,"historySize"))
,historyMask((1 << historySize) - 1)
,globalTable(i,section
,SescConf->getInt(section,"l2Size")
,SescConf->getInt(section,"l2Bits"))
,ghr(0)
,localTable(i,section
,SescConf->getInt(section,"localSize")
,SescConf->getInt(section,"localBits"))
,metaTable(i,section
,SescConf->getInt(section,"MetaSize")
,SescConf->getInt(section,"MetaBits"))

{
// Constraints
SescConf->isInt(section, "localSize");
SescConf->isPower2(section, "localSize");
SescConf->isBetween(section, "localBits", 1, 7);

SescConf->isInt(section , "MetaSize");
SescConf->isPower2(section , "MetaSize");
SescConf->isBetween(section , "MetaBits", 1, 7);

SescConf->isInt(section, "historySize");
SescConf->isBetween(section, "historySize", 1, 63);

SescConf->isInt(section, "l2Size");
SescConf->isPower2(section, "l2Size");
SescConf->isBetween(section,"l2Bits", 1, 7);
}

BPHybrid::~BPHybrid()
{
}

PredType BPHybrid::predict(const Instruction *inst, InstID oracleID, bool doUpdate)
{
bpredEnergy->inc();

if( inst->isBranchTaken() )
return btb.predict(inst, oracleID, doUpdate);

bool taken = (inst->calcNextInstID() != oracleID);
HistoryType iID = calcInstID(inst);
HistoryType l2Index = ghr;

// update historyTable statistics
if (doUpdate) {
ghr = ((ghr << 1) | ((iID>>2 & 1)^(taken?1:0))) & historyMask;
}

// calculate Table possition
l2Index = ((l2Index ^ iID) & historyMask ) | (iID<<historySize);

bool globalTaken;
bool localTaken;
if (doUpdate) {
globalTaken = globalTable.predict(l2Index, taken);
localTaken = localTable.predict(iID, taken);
} else {
globalTaken = globalTable.predict(l2Index);
localTaken = localTable.predict(iID);
}

bool metaOut;
if (!doUpdate) {
metaOut = metaTable.predict(l2Index); // do not update meta
} else if( globalTaken == taken && localTaken != taken) {
// global is correct, local incorrect
metaOut = metaTable.predict(l2Index, false);
} else if( globalTaken != taken && localTaken == taken) {
// global is incorrect, local correct
metaOut = metaTable.predict(l2Index, true);
} else {
metaOut = metaTable.predict(l2Index); // do not update meta
}

bool ptaken = metaOut ? localTaken : globalTaken;

if (taken != ptaken) {
if (doUpdate)
btb.updateOnly(inst,oracleID);
return MissPrediction;
}

return ptaken ? btb.predict(inst, oracleID, doUpdate) : CorrectPrediction;
}

void BPHybrid::switchIn(Pid_t pid)
{
}

void BPHybrid::switchOut(Pid_t pid)

Explanation / Answer

The meta-predictor in this hybrid predictor is a table that has_____4_____ entries and each entry is a_____32____ –bit counter. This meta-predictor decides, based on the PC address (i.e. the address from which we fetched the branch instruction), whether to make the prediction using a simple (no history) array of counters, or to use a (is it local or global?)____global_____history predictor. The simpler (non-history) predictor uses____32______-bit counters, and has___1___of them (this number is specified using a parameter label_______ in the BPredIssueX section of the configuration file). The history-based predictor has___32_____bits of history, which are combined with the PC address to index into an array that has____2____entries (this number of entries is specified in the configuration file using parameter label_________), and each entry is a____32_____-bit counter.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.