#include "debug/LTage.hh"
LTAGE::LTAGE(const LTAGEParams *params)
- : BPredUnit(params),
- logRatioBiModalHystEntries(params->logRatioBiModalHystEntries),
+ : TAGE(params),
logSizeLoopPred(params->logSizeLoopPred),
- nHistoryTables(params->nHistoryTables),
- tagTableCounterBits(params->tagTableCounterBits),
- tagTableUBits(params->tagTableUBits),
- histBufferSize(params->histBufferSize),
- minHist(params->minHist),
- maxHist(params->maxHist),
- pathHistBits(params->pathHistBits),
loopTableAgeBits(params->loopTableAgeBits),
loopTableConfidenceBits(params->loopTableConfidenceBits),
loopTableTagBits(params->loopTableTagBits),
confidenceThreshold((1 << loopTableConfidenceBits) - 1),
loopTagMask((1 << loopTableTagBits) - 1),
loopNumIterMask((1 << loopTableIterBits) - 1),
- tagTableTagWidths(params->tagTableTagWidths),
- logTagTableSizes(params->logTagTableSizes),
- threadHistory(params->numThreads),
- logUResetPeriod(params->logUResetPeriod),
- useAltOnNaBits(params->useAltOnNaBits),
+ loopUseCounter(0),
withLoopBits(params->withLoopBits)
{
- // Current method for periodically resetting the u counter bits only
- // works for 1 or 2 bits
- // Also make sure that it is not 0
- assert(tagTableUBits <= 2 && (tagTableUBits > 0));
-
// we use uint16_t type for these vales, so they cannot be more than
// 16 bits
assert(loopTableTagBits <= 16);
assert(logSizeLoopPred >= logLoopTableAssoc);
- // we use int type for the path history, so it cannot be more than
- // its size
- assert(pathHistBits <= (sizeof(int)*8));
-
- // initialize the counter to half of the period
- assert(logUResetPeriod != 0);
- tCounter = ULL(1) << (logUResetPeriod - 1);
-
- assert(params->histBufferSize > params->maxHist * 2);
- useAltPredForNewlyAllocated = 0;
-
- for (auto& history : threadHistory) {
- history.pathHist = 0;
- history.globalHistory = new uint8_t[histBufferSize];
- history.gHist = history.globalHistory;
- memset(history.gHist, 0, histBufferSize);
- history.ptGhist = 0;
- }
-
- histLengths = new int [nHistoryTables+1];
- histLengths[1] = minHist;
- histLengths[nHistoryTables] = maxHist;
-
- for (int i = 2; i <= nHistoryTables; i++) {
- histLengths[i] = (int) (((double) minHist *
- pow ((double) (maxHist) / (double) minHist,
- (double) (i - 1) / (double) ((nHistoryTables- 1))))
- + 0.5);
- }
-
- assert(tagTableTagWidths.size() == (nHistoryTables+1));
- assert(logTagTableSizes.size() == (nHistoryTables+1));
-
- // First entry is for the Bimodal table and it is untagged in this
- // implementation
- assert(tagTableTagWidths[0] == 0);
-
- for (auto& history : threadHistory) {
- history.computeIndices = new FoldedHistory[nHistoryTables+1];
- history.computeTags[0] = new FoldedHistory[nHistoryTables+1];
- history.computeTags[1] = new FoldedHistory[nHistoryTables+1];
-
- for (int i = 1; i <= nHistoryTables; i++) {
- history.computeIndices[i].init(
- histLengths[i], (logTagTableSizes[i]));
- history.computeTags[0][i].init(
- history.computeIndices[i].origLength, tagTableTagWidths[i]);
- history.computeTags[1][i].init(
- history.computeIndices[i].origLength, tagTableTagWidths[i]-1);
- DPRINTF(LTage, "HistLength:%d, TTSize:%d, TTTWidth:%d\n",
- histLengths[i], logTagTableSizes[i], tagTableTagWidths[i]);
- }
- }
-
- const uint64_t bimodalTableSize = ULL(1) << logTagTableSizes[0];
- btablePrediction.resize(bimodalTableSize, false);
- btableHysteresis.resize(bimodalTableSize >> logRatioBiModalHystEntries,
- true);
-
ltable = new LoopEntry[ULL(1) << logSizeLoopPred];
- gtable = new TageEntry*[nHistoryTables + 1];
- for (int i = 1; i <= nHistoryTables; i++) {
- gtable[i] = new TageEntry[1<<(logTagTableSizes[i])];
- }
-
- tableIndices = new int [nHistoryTables+1];
- tableTags = new int [nHistoryTables+1];
-
- loopUseCounter = 0;
-}
-
-int
-LTAGE::bindex(Addr pc_in) const
-{
- return ((pc_in >> instShiftAmt) & ((ULL(1) << (logTagTableSizes[0])) - 1));
}
int
return (((pc_in >> instShiftAmt) & mask) << logLoopTableAssoc);
}
-int
-LTAGE::F(int A, int size, int bank) const
-{
- int A1, A2;
-
- A = A & ((ULL(1) << size) - 1);
- A1 = (A & ((ULL(1) << logTagTableSizes[bank]) - 1));
- A2 = (A >> logTagTableSizes[bank]);
- A2 = ((A2 << bank) & ((ULL(1) << logTagTableSizes[bank]) - 1))
- + (A2 >> (logTagTableSizes[bank] - bank));
- A = A1 ^ A2;
- A = ((A << bank) & ((ULL(1) << logTagTableSizes[bank]) - 1))
- + (A >> (logTagTableSizes[bank] - bank));
- return (A);
-}
-
-
-// gindex computes a full hash of pc, ghist and pathHist
-int
-LTAGE::gindex(ThreadID tid, Addr pc, int bank) const
-{
- int index;
- int hlen = (histLengths[bank] > pathHistBits) ? pathHistBits :
- histLengths[bank];
- const Addr shiftedPc = pc >> instShiftAmt;
- index =
- shiftedPc ^
- (shiftedPc >> ((int) abs(logTagTableSizes[bank] - bank) + 1)) ^
- threadHistory[tid].computeIndices[bank].comp ^
- F(threadHistory[tid].pathHist, hlen, bank);
-
- return (index & ((ULL(1) << (logTagTableSizes[bank])) - 1));
-}
-
-
-// Tag computation
-uint16_t
-LTAGE::gtag(ThreadID tid, Addr pc, int bank) const
-{
- int tag = (pc >> instShiftAmt) ^
- threadHistory[tid].computeTags[0][bank].comp ^
- (threadHistory[tid].computeTags[1][bank].comp << 1);
-
- return (tag & ((ULL(1) << tagTableTagWidths[bank]) - 1));
-}
-
-
-// Up-down saturating counter
-void
-LTAGE::ctrUpdate(int8_t & ctr, bool taken, int nbits)
-{
- assert(nbits <= sizeof(int8_t) << 3);
- if (taken) {
- if (ctr < ((1 << (nbits - 1)) - 1))
- ctr++;
- } else {
- if (ctr > -(1 << (nbits - 1)))
- ctr--;
- }
-}
-
-// Up-down unsigned saturating counter
-void
-LTAGE::unsignedCtrUpdate(uint8_t & ctr, bool up, unsigned nbits)
-{
- assert(nbits <= sizeof(uint8_t) << 3);
- if (up) {
- if (ctr < ((1 << nbits) - 1))
- ctr++;
- } else {
- if (ctr)
- ctr--;
- }
-}
-
-// Bimodal prediction
-bool
-LTAGE::getBimodePred(Addr pc, BranchInfo* bi) const
-{
- return btablePrediction[bi->bimodalIndex];
-}
-
-
-// Update the bimodal predictor: a hysteresis bit is shared among N prediction
-// bits (N = 2 ^ logRatioBiModalHystEntries)
-void
-LTAGE::baseUpdate(Addr pc, bool taken, BranchInfo* bi)
-{
- int inter = (btablePrediction[bi->bimodalIndex] << 1)
- + btableHysteresis[bi->bimodalIndex >> logRatioBiModalHystEntries];
- if (taken) {
- if (inter < 3)
- inter++;
- } else if (inter > 0) {
- inter--;
- }
- const bool pred = inter >> 1;
- const bool hyst = inter & 1;
- btablePrediction[bi->bimodalIndex] = pred;
- btableHysteresis[bi->bimodalIndex >> logRatioBiModalHystEntries] = hyst;
- DPRINTF(LTage, "Updating branch %lx, pred:%d, hyst:%d\n", pc, pred, hyst);
-}
-
-
//loop prediction: only used if high confidence
bool
-LTAGE::getLoop(Addr pc, BranchInfo* bi) const
+LTAGE::getLoop(Addr pc, LTageBranchInfo* bi) const
{
bi->loopHit = -1;
bi->loopPredValid = false;
}
void
-LTAGE::specLoopUpdate(Addr pc, bool taken, BranchInfo* bi)
+LTAGE::specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
{
if (bi->loopHit>=0) {
int index = lindex(pc);
}
void
-LTAGE::loopUpdate(Addr pc, bool taken, BranchInfo* bi)
+LTAGE::loopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
{
int idx = bi->loopIndex + bi->loopHit;
if (bi->loopHit >= 0) {
}
-// shifting the global history: we manage the history in a big table in order
-// to reduce simulation time
-void
-LTAGE::updateGHist(uint8_t * &h, bool dir, uint8_t * tab, int &pt)
-{
- if (pt == 0) {
- DPRINTF(LTage, "Rolling over the histories\n");
- // Copy beginning of globalHistoryBuffer to end, such that
- // the last maxHist outcomes are still reachable
- // through pt[0 .. maxHist - 1].
- for (int i = 0; i < maxHist; i++)
- tab[histBufferSize - maxHist + i] = tab[i];
- pt = histBufferSize - maxHist;
- h = &tab[pt];
- }
- pt--;
- h--;
- h[0] = (dir) ? 1 : 0;
-}
-
-// Get GHR for hashing indirect predictor
-// Build history backwards from pointer in
-// bp_history.
-unsigned
-LTAGE::getGHR(ThreadID tid, void *bp_history) const
-{
- BranchInfo* bi = static_cast<BranchInfo*>(bp_history);
- unsigned val = 0;
- for (unsigned i = 0; i < 32; i++) {
- // Make sure we don't go out of bounds
- int gh_offset = bi->ptGhist + i;
- assert(&(threadHistory[tid].globalHistory[gh_offset]) <
- threadHistory[tid].globalHistory + histBufferSize);
- val |= ((threadHistory[tid].globalHistory[gh_offset] & 0x1) << i);
- }
-
- return val;
-}
-
//prediction
bool
LTAGE::predict(ThreadID tid, Addr branch_pc, bool cond_branch, void* &b)
{
- BranchInfo *bi = new BranchInfo(nHistoryTables+1);
+ LTageBranchInfo *bi = new LTageBranchInfo(nHistoryTables+1);
b = (void*)(bi);
- Addr pc = branch_pc;
- bool pred_taken = true;
- bi->loopHit = -1;
- if (cond_branch) {
- // TAGE prediction
-
- // computes the table addresses and the partial tags
- for (int i = 1; i <= nHistoryTables; i++) {
- tableIndices[i] = gindex(tid, pc, i);
- bi->tableIndices[i] = tableIndices[i];
- tableTags[i] = gtag(tid, pc, i);
- bi->tableTags[i] = tableTags[i];
- }
+ bool pred_taken = tagePredict(tid, branch_pc, cond_branch, bi);
- bi->bimodalIndex = bindex(pc);
-
- bi->hitBank = 0;
- bi->altBank = 0;
- //Look for the bank with longest matching history
- for (int i = nHistoryTables; i > 0; i--) {
- if (gtable[i][tableIndices[i]].tag == tableTags[i]) {
- bi->hitBank = i;
- bi->hitBankIndex = tableIndices[bi->hitBank];
- break;
- }
- }
- //Look for the alternate bank
- for (int i = bi->hitBank - 1; i > 0; i--) {
- if (gtable[i][tableIndices[i]].tag == tableTags[i]) {
- bi->altBank = i;
- bi->altBankIndex = tableIndices[bi->altBank];
- break;
- }
- }
- //computes the prediction and the alternate prediction
- if (bi->hitBank > 0) {
- if (bi->altBank > 0) {
- bi->altTaken =
- gtable[bi->altBank][tableIndices[bi->altBank]].ctr >= 0;
- }else {
- bi->altTaken = getBimodePred(pc, bi);
- }
+ if (cond_branch) {
+ bi->loopPred = getLoop(branch_pc, bi); // loop prediction
- bi->longestMatchPred =
- gtable[bi->hitBank][tableIndices[bi->hitBank]].ctr >= 0;
- bi->pseudoNewAlloc =
- abs(2 * gtable[bi->hitBank][bi->hitBankIndex].ctr + 1) <= 1;
-
- //if the entry is recognized as a newly allocated entry and
- //useAltPredForNewlyAllocated is positive use the alternate
- //prediction
- if ((useAltPredForNewlyAllocated < 0)
- || abs(2 *
- gtable[bi->hitBank][tableIndices[bi->hitBank]].ctr + 1) > 1)
- bi->tagePred = bi->longestMatchPred;
- else
- bi->tagePred = bi->altTaken;
- } else {
- bi->altTaken = getBimodePred(pc, bi);
- bi->tagePred = bi->altTaken;
- bi->longestMatchPred = bi->altTaken;
+ if ((loopUseCounter >= 0) && bi->loopPredValid) {
+ pred_taken = bi->loopPred;
}
- //end TAGE prediction
-
- bi->loopPred = getLoop(pc, bi); // loop prediction
-
- pred_taken = (((loopUseCounter >= 0) && bi->loopPredValid)) ?
- (bi->loopPred): (bi->tagePred);
DPRINTF(LTage, "Predict for %lx: taken?:%d, loopTaken?:%d, "
"loopValid?:%d, loopUseCounter:%d, tagePred:%d, altPred:%d\n",
branch_pc, pred_taken, bi->loopPred, bi->loopPredValid,
loopUseCounter, bi->tagePred, bi->altTaken);
}
- bi->branchPC = branch_pc;
- bi->condBranch = cond_branch;
+
specLoopUpdate(branch_pc, pred_taken, bi);
return pred_taken;
}
-// PREDICTOR UPDATE
void
-LTAGE::update(ThreadID tid, Addr branch_pc, bool taken, void* bp_history,
- bool squashed)
+LTAGE::condBranchUpdate(Addr branch_pc, bool taken,
+ TageBranchInfo* tage_bi, int nrand)
{
- assert(bp_history);
-
- BranchInfo *bi = static_cast<BranchInfo*>(bp_history);
-
- if (squashed) {
- // This restores the global history, then update it
- // and recomputes the folded histories.
- squash(tid, taken, bp_history);
- return;
- }
-
- int nrand = random_mt.random<int>(0,3);
- Addr pc = branch_pc;
- if (bi->condBranch) {
- DPRINTF(LTage, "Updating tables for branch:%lx; taken?:%d\n",
- branch_pc, taken);
- // first update the loop predictor
- loopUpdate(pc, taken, bi);
-
- if (bi->loopPredValid) {
- if (bi->tagePred != bi->loopPred) {
- ctrUpdate(loopUseCounter,
- (bi->loopPred == taken),
- withLoopBits);
- }
- }
-
- // TAGE UPDATE
- // try to allocate a new entries only if prediction was wrong
- bool longest_match_pred = false;
- bool alloc = (bi->tagePred != taken) && (bi->hitBank < nHistoryTables);
- if (bi->hitBank > 0) {
- // Manage the selection between longest matching and alternate
- // matching for "pseudo"-newly allocated longest matching entry
- longest_match_pred = bi->longestMatchPred;
- bool PseudoNewAlloc = bi->pseudoNewAlloc;
- // an entry is considered as newly allocated if its prediction
- // counter is weak
- if (PseudoNewAlloc) {
- if (longest_match_pred == taken) {
- alloc = false;
- }
- // if it was delivering the correct prediction, no need to
- // allocate new entry even if the overall prediction was false
- if (longest_match_pred != bi->altTaken) {
- ctrUpdate(useAltPredForNewlyAllocated,
- bi->altTaken == taken, useAltOnNaBits);
- }
- }
- }
-
- if (alloc) {
- // is there some "unuseful" entry to allocate
- uint8_t min = 1;
- for (int i = nHistoryTables; i > bi->hitBank; i--) {
- if (gtable[i][bi->tableIndices[i]].u < min) {
- min = gtable[i][bi->tableIndices[i]].u;
- }
- }
-
- // we allocate an entry with a longer history
- // to avoid ping-pong, we do not choose systematically the next
- // entry, but among the 3 next entries
- int Y = nrand &
- ((ULL(1) << (nHistoryTables - bi->hitBank - 1)) - 1);
- int X = bi->hitBank + 1;
- if (Y & 1) {
- X++;
- if (Y & 2)
- X++;
- }
- // No entry available, forces one to be available
- if (min > 0) {
- gtable[X][bi->tableIndices[X]].u = 0;
- }
-
-
- //Allocate only one entry
- for (int i = X; i <= nHistoryTables; i++) {
- if ((gtable[i][bi->tableIndices[i]].u == 0)) {
- gtable[i][bi->tableIndices[i]].tag = bi->tableTags[i];
- gtable[i][bi->tableIndices[i]].ctr = (taken) ? 0 : -1;
- break;
- }
- }
- }
- //periodic reset of u: reset is not complete but bit by bit
- tCounter++;
- if ((tCounter & ((ULL(1) << logUResetPeriod) - 1)) == 0) {
- // reset least significant bit
- // most significant bit becomes least significant bit
- for (int i = 1; i <= nHistoryTables; i++) {
- for (int j = 0; j < (ULL(1) << logTagTableSizes[i]); j++) {
- gtable[i][j].u = gtable[i][j].u >> 1;
- }
- }
- }
+ LTageBranchInfo* bi = static_cast<LTageBranchInfo*>(tage_bi);
- if (bi->hitBank > 0) {
- DPRINTF(LTage, "Updating tag table entry (%d,%d) for branch %lx\n",
- bi->hitBank, bi->hitBankIndex, branch_pc);
- ctrUpdate(gtable[bi->hitBank][bi->hitBankIndex].ctr, taken,
- tagTableCounterBits);
- // if the provider entry is not certified to be useful also update
- // the alternate prediction
- if (gtable[bi->hitBank][bi->hitBankIndex].u == 0) {
- if (bi->altBank > 0) {
- ctrUpdate(gtable[bi->altBank][bi->altBankIndex].ctr, taken,
- tagTableCounterBits);
- DPRINTF(LTage, "Updating tag table entry (%d,%d) for"
- " branch %lx\n", bi->hitBank, bi->hitBankIndex,
- branch_pc);
- }
- if (bi->altBank == 0) {
- baseUpdate(pc, taken, bi);
- }
- }
+ // first update the loop predictor
+ loopUpdate(branch_pc, taken, bi);
- // update the u counter
- if (bi->tagePred != bi->altTaken) {
- unsignedCtrUpdate(gtable[bi->hitBank][bi->hitBankIndex].u,
- bi->tagePred == taken, tagTableUBits);
- }
- } else {
- baseUpdate(pc, taken, bi);
+ if (bi->loopPredValid) {
+ if (bi->tagePred != bi->loopPred) {
+ ctrUpdate(loopUseCounter,
+ (bi->loopPred == taken),
+ withLoopBits);
}
-
- //END PREDICTOR UPDATE
}
- if (!squashed) {
- delete bi;
- }
-}
-void
-LTAGE::updateHistories(ThreadID tid, Addr branch_pc, bool taken, void* b)
-{
- BranchInfo* bi = (BranchInfo*)(b);
- ThreadHistory& tHist = threadHistory[tid];
- // UPDATE HISTORIES
- bool pathbit = ((branch_pc >> instShiftAmt) & 1);
- //on a squash, return pointers to this and recompute indices.
- //update user history
- updateGHist(tHist.gHist, taken, tHist.globalHistory, tHist.ptGhist);
- tHist.pathHist = (tHist.pathHist << 1) + pathbit;
- tHist.pathHist = (tHist.pathHist & ((ULL(1) << pathHistBits) - 1));
-
- bi->ptGhist = tHist.ptGhist;
- bi->pathHist = tHist.pathHist;
- //prepare next index and tag computations for user branchs
- for (int i = 1; i <= nHistoryTables; i++)
- {
- bi->ci[i] = tHist.computeIndices[i].comp;
- bi->ct0[i] = tHist.computeTags[0][i].comp;
- bi->ct1[i] = tHist.computeTags[1][i].comp;
- tHist.computeIndices[i].update(tHist.gHist);
- tHist.computeTags[0][i].update(tHist.gHist);
- tHist.computeTags[1][i].update(tHist.gHist);
- }
- DPRINTF(LTage, "Updating global histories with branch:%lx; taken?:%d, "
- "path Hist: %x; pointer:%d\n", branch_pc, taken, tHist.pathHist,
- tHist.ptGhist);
+ TAGE::condBranchUpdate(branch_pc, taken, bi, nrand);
}
void
LTAGE::squash(ThreadID tid, bool taken, void *bp_history)
{
- BranchInfo* bi = (BranchInfo*)(bp_history);
- ThreadHistory& tHist = threadHistory[tid];
- DPRINTF(LTage, "Restoring branch info: %lx; taken? %d; PathHistory:%x, "
- "pointer:%d\n", bi->branchPC,taken, bi->pathHist, bi->ptGhist);
- tHist.pathHist = bi->pathHist;
- tHist.ptGhist = bi->ptGhist;
- tHist.gHist = &(tHist.globalHistory[tHist.ptGhist]);
- tHist.gHist[0] = (taken ? 1 : 0);
- for (int i = 1; i <= nHistoryTables; i++) {
- tHist.computeIndices[i].comp = bi->ci[i];
- tHist.computeTags[0][i].comp = bi->ct0[i];
- tHist.computeTags[1][i].comp = bi->ct1[i];
- tHist.computeIndices[i].update(tHist.gHist);
- tHist.computeTags[0][i].update(tHist.gHist);
- tHist.computeTags[1][i].update(tHist.gHist);
- }
+ TAGE::squash(tid, taken, bp_history);
+
+ LTageBranchInfo* bi = (LTageBranchInfo*)(bp_history);
if (bi->condBranch) {
if (bi->loopHit >= 0) {
ltable[idx].currentIterSpec = bi->currentIter;
}
}
-
}
void
LTAGE::squash(ThreadID tid, void *bp_history)
{
- BranchInfo* bi = (BranchInfo*)(bp_history);
- DPRINTF(LTage, "Deleting branch info: %lx\n", bi->branchPC);
+ LTageBranchInfo* bi = (LTageBranchInfo*)(bp_history);
if (bi->condBranch) {
if (bi->loopHit >= 0) {
int idx = bi->loopIndex + bi->loopHit;
}
}
- delete bi;
-}
-
-bool
-LTAGE::lookup(ThreadID tid, Addr branch_pc, void* &bp_history)
-{
- bool retval = predict(tid, branch_pc, true, bp_history);
-
- DPRINTF(LTage, "Lookup branch: %lx; predict:%d\n", branch_pc, retval);
- updateHistories(tid, branch_pc, retval, bp_history);
- assert(threadHistory[tid].gHist ==
- &threadHistory[tid].globalHistory[threadHistory[tid].ptGhist]);
-
- return retval;
-}
-
-void
-LTAGE::btbUpdate(ThreadID tid, Addr branch_pc, void* &bp_history)
-{
- BranchInfo* bi = (BranchInfo*) bp_history;
- ThreadHistory& tHist = threadHistory[tid];
- DPRINTF(LTage, "BTB miss resets prediction: %lx\n", branch_pc);
- assert(tHist.gHist == &tHist.globalHistory[tHist.ptGhist]);
- tHist.gHist[0] = 0;
- for (int i = 1; i <= nHistoryTables; i++) {
- tHist.computeIndices[i].comp = bi->ci[i];
- tHist.computeTags[0][i].comp = bi->ct0[i];
- tHist.computeTags[1][i].comp = bi->ct1[i];
- tHist.computeIndices[i].update(tHist.gHist);
- tHist.computeTags[0][i].update(tHist.gHist);
- tHist.computeTags[1][i].update(tHist.gHist);
- }
-}
-
-void
-LTAGE::uncondBranch(ThreadID tid, Addr br_pc, void* &bp_history)
-{
- DPRINTF(LTage, "UnConditionalBranch: %lx\n", br_pc);
- predict(tid, br_pc, false, bp_history);
- updateHistories(tid, br_pc, true, bp_history);
- assert(threadHistory[tid].gHist ==
- &threadHistory[tid].globalHistory[threadHistory[tid].ptGhist]);
+ TAGE::squash(tid, bp_history);
}
LTAGE*
projects / gem5.git / blobdiff