--- /dev/null
+/*
+ * Copyright (c) 2014 The University of Wisconsin
+ *
+ * Copyright (c) 2006 INRIA (Institut National de Recherche en
+ * Informatique et en Automatique / French National Research Institute
+ * for Computer Science and Applied Mathematics)
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Vignyan Reddy, Dibakar Gope and Arthur Perais,
+ * from André Seznec's code.
+ */
+
+/* @file
+ * Implementation of a L-TAGE branch predictor
+ */
+
+#include "cpu/pred/ltage.hh"
+
+#include "base/intmath.hh"
+#include "base/misc.hh"
+#include "base/random.hh"
+#include "base/trace.hh"
+#include "debug/Fetch.hh"
+#include "debug/LTage.hh"
+
+LTAGE::LTAGE(const LTAGEParams *params)
+ : BPredUnit(params),
+ logSizeBiMP(params->logSizeBiMP),
+ logSizeTagTables(params->logSizeTagTables),
+ logSizeLoopPred(params->logSizeLoopPred),
+ nHistoryTables(params->nHistoryTables),
+ tagTableCounterBits(params->tagTableCounterBits),
+ histBufferSize(params->histBufferSize),
+ minHist(params->minHist),
+ maxHist(params->maxHist),
+ minTagWidth(params->minTagWidth),
+ threadHistory(params->numThreads)
+{
+ assert(params->histBufferSize > params->maxHist * 2);
+ useAltPredForNewlyAllocated = 0;
+ logTick = 19;
+ tCounter = ULL(1) << (logTick - 1);
+
+ 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);
+ }
+
+ tagWidths[1] = minTagWidth;
+ tagWidths[2] = minTagWidth;
+ tagWidths[3] = minTagWidth + 1;
+ tagWidths[4] = minTagWidth + 1;
+ tagWidths[5] = minTagWidth + 2;
+ tagWidths[6] = minTagWidth + 3;
+ tagWidths[7] = minTagWidth + 4;
+ tagWidths[8] = minTagWidth + 5;
+ tagWidths[9] = minTagWidth + 5;
+ tagWidths[10] = minTagWidth + 6;
+ tagWidths[11] = minTagWidth + 7;
+ tagWidths[12] = minTagWidth + 8;
+
+ for (int i = 1; i <= 2; i++)
+ tagTableSizes[i] = logSizeTagTables - 1;
+ for (int i = 3; i <= 6; i++)
+ tagTableSizes[i] = logSizeTagTables;
+ for (int i = 7; i <= 10; i++)
+ tagTableSizes[i] = logSizeTagTables - 1;
+ for (int i = 11; i <= 12; i++)
+ tagTableSizes[i] = logSizeTagTables - 2;
+
+ 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], (tagTableSizes[i]));
+ history.computeTags[0][i].init(
+ history.computeIndices[i].origLength, tagWidths[i]);
+ history.computeTags[1][i].init(
+ history.computeIndices[i].origLength, tagWidths[i] - 1);
+ DPRINTF(LTage, "HistLength:%d, TTSize:%d, TTTWidth:%d\n",
+ histLengths[i], tagTableSizes[i], tagWidths[i]);
+ }
+ }
+
+ btable = new BimodalEntry[ULL(1) << logSizeBiMP];
+ ltable = new LoopEntry[ULL(1) << logSizeLoopPred];
+ gtable = new TageEntry*[nHistoryTables + 1];
+ for (int i = 1; i <= nHistoryTables; i++) {
+ gtable[i] = new TageEntry[1<<(tagTableSizes[i])];
+ }
+
+ tableIndices = new int [nHistoryTables+1];
+ tableTags = new int [nHistoryTables+1];
+
+ loopUseCounter = 0;
+}
+
+int
+LTAGE::bindex(Addr pc_in) const
+{
+ return ((pc_in) & ((ULL(1) << (logSizeBiMP)) - 1));
+}
+
+int
+LTAGE::lindex(Addr pc_in) const
+{
+ return (((pc_in) & ((ULL(1) << (logSizeLoopPred - 2)) - 1)) << 2);
+}
+
+int
+LTAGE::F(int A, int size, int bank) const
+{
+ int A1, A2;
+
+ A = A & ((ULL(1) << size) - 1);
+ A1 = (A & ((ULL(1) << tagTableSizes[bank]) - 1));
+ A2 = (A >> tagTableSizes[bank]);
+ A2 = ((A2 << bank) & ((ULL(1) << tagTableSizes[bank]) - 1))
+ + (A2 >> (tagTableSizes[bank] - bank));
+ A = A1 ^ A2;
+ A = ((A << bank) & ((ULL(1) << tagTableSizes[bank]) - 1))
+ + (A >> (tagTableSizes[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] > 16) ? 16 : histLengths[bank];
+ index =
+ (pc) ^ ((pc) >> ((int) abs(tagTableSizes[bank] - bank) + 1)) ^
+ threadHistory[tid].computeIndices[bank].comp ^
+ F(threadHistory[tid].pathHist, hlen, bank);
+
+ return (index & ((ULL(1) << (tagTableSizes[bank])) - 1));
+}
+
+
+// Tag computation
+uint16_t
+LTAGE::gtag(ThreadID tid, Addr pc, int bank) const
+{
+ int tag = (pc) ^ threadHistory[tid].computeTags[0][bank].comp
+ ^ (threadHistory[tid].computeTags[1][bank].comp << 1);
+
+ return (tag & ((ULL(1) << tagWidths[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--;
+ }
+}
+
+// Bimodal prediction
+bool
+LTAGE::getBimodePred(Addr pc, BranchInfo* bi) const
+{
+ return (btable[bi->bimodalIndex].pred > 0);
+}
+
+
+// Update the bimodal predictor: a hysteresis bit is shared among 4 prediction
+// bits
+void
+LTAGE::baseUpdate(Addr pc, bool taken, BranchInfo* bi)
+{
+ int inter = (btable[bi->bimodalIndex].pred << 1)
+ + btable[bi->bimodalIndex ].hyst;
+ if (taken) {
+ if (inter < 3)
+ inter++;
+ } else if (inter > 0) {
+ inter--;
+ }
+ btable[bi->bimodalIndex].pred = inter >> 1;
+ btable[bi->bimodalIndex].hyst = (inter & 1);
+ DPRINTF(LTage, "Updating branch %lx, pred:%d, hyst:%d\n",
+ pc, btable[bi->bimodalIndex].pred,btable[bi->bimodalIndex].hyst);
+}
+
+
+//loop prediction: only used if high confidence
+bool
+LTAGE::getLoop(Addr pc, BranchInfo* bi) const
+{
+ bi->loopHit = -1;
+ bi->loopPredValid = false;
+ bi->loopIndex = lindex(pc);
+ bi->loopTag = ((pc) >> (logSizeLoopPred - 2));
+
+ for (int i = 0; i < 4; i++) {
+ if (ltable[bi->loopIndex + i].tag == bi->loopTag) {
+ bi->loopHit = i;
+ bi->loopPredValid = (ltable[bi->loopIndex + i].confidence >= 3);
+ bi->currentIter = ltable[bi->loopIndex + i].currentIterSpec;
+ if (ltable[bi->loopIndex + i].currentIterSpec + 1 ==
+ ltable[bi->loopIndex + i].numIter) {
+ return !(ltable[bi->loopIndex + i].dir);
+ }else {
+ return (ltable[bi->loopIndex + i].dir);
+ }
+ }
+ }
+ return false;
+}
+
+void
+LTAGE::specLoopUpdate(Addr pc, bool taken, BranchInfo* bi)
+{
+ if (bi->loopHit>=0) {
+ int index = lindex(pc);
+ if (taken != ltable[index].dir) {
+ ltable[index].currentIterSpec = 0;
+ } else {
+ ltable[index].currentIterSpec++;
+ }
+ }
+}
+
+void
+LTAGE::loopUpdate(Addr pc, bool taken, BranchInfo* bi)
+{
+ int idx = bi->loopIndex + bi->loopHit;
+ if (bi->loopHit >= 0) {
+ //already a hit
+ if (bi->loopPredValid) {
+ if (taken != bi->loopPred) {
+ // free the entry
+ ltable[idx].numIter = 0;
+ ltable[idx].age = 0;
+ ltable[idx].confidence = 0;
+ ltable[idx].currentIter = 0;
+ return;
+ } else if (bi->loopPred != bi->tagePred) {
+ DPRINTF(LTage, "Loop Prediction success:%lx\n",pc);
+ if (ltable[idx].age < 7)
+ ltable[idx].age++;
+ }
+ }
+
+ ltable[idx].currentIter++;
+ if (ltable[idx].currentIter > ltable[idx].numIter) {
+ ltable[idx].confidence = 0;
+ if (ltable[idx].numIter != 0) {
+ // free the entry
+ ltable[idx].numIter = 0;
+ ltable[idx].age = 0;
+ ltable[idx].confidence = 0;
+ }
+ }
+
+ if (taken != ltable[idx].dir) {
+ if (ltable[idx].currentIter == ltable[idx].numIter) {
+ DPRINTF(LTage, "Loop End predicted successfully:%lx\n", pc);
+
+ if (ltable[idx].confidence < 7) {
+ ltable[idx].confidence++;
+ }
+ //just do not predict when the loop count is 1 or 2
+ if (ltable[idx].numIter < 3) {
+ // free the entry
+ ltable[idx].dir = taken;
+ ltable[idx].numIter = 0;
+ ltable[idx].age = 0;
+ ltable[idx].confidence = 0;
+ }
+ } else {
+ DPRINTF(LTage, "Loop End predicted incorrectly:%lx\n", pc);
+ if (ltable[idx].numIter == 0) {
+ // first complete nest;
+ ltable[idx].confidence = 0;
+ ltable[idx].numIter = ltable[idx].currentIter;
+ } else {
+ //not the same number of iterations as last time: free the
+ //entry
+ ltable[idx].numIter = 0;
+ ltable[idx].age = 0;
+ ltable[idx].confidence = 0;
+ }
+ }
+ ltable[idx].currentIter = 0;
+ }
+
+ } else if (taken) {
+ //try to allocate an entry on taken branch
+ int nrand = random_mt.random<int>();
+ for (int i = 0; i < 4; i++) {
+ int loop_hit = (nrand + i) & 3;
+ idx = bi->loopIndex + loop_hit;
+ if (ltable[idx].age == 0) {
+ DPRINTF(LTage, "Allocating loop pred entry for branch %lx\n",
+ pc);
+ ltable[idx].dir = !taken;
+ ltable[idx].tag = bi->loopTag;
+ ltable[idx].numIter = 0;
+ ltable[idx].age = 7;
+ ltable[idx].confidence = 0;
+ ltable[idx].currentIter = 1;
+ break;
+
+ }
+ else
+ ltable[idx].age--;
+ }
+ }
+
+}
+
+// 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);
+ 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];
+ }
+
+ 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);
+ }
+
+ 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;
+ }
+ //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)
+{
+ 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), 7);
+ }
+ }
+
+ // 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, 4);
+ }
+ }
+ }
+
+ if (alloc) {
+ // is there some "unuseful" entry to allocate
+ int8_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;
+ gtable[i][bi->tableIndices[i]].u = 0; //?
+ }
+ }
+ }
+ //periodic reset of u: reset is not complete but bit by bit
+ tCounter++;
+ if ((tCounter & ((ULL(1) << logTick) - 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) << tagTableSizes[i]); j++) {
+ gtable[i][j].u = gtable[i][j].u >> 1;
+ }
+ }
+ }
+
+ 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);
+ }
+ }
+
+ // update the u counter
+ if (longest_match_pred != bi->altTaken) {
+ if (longest_match_pred == taken) {
+ if (gtable[bi->hitBank][bi->hitBankIndex].u < 1) {
+ gtable[bi->hitBank][bi->hitBankIndex].u++;
+ }
+ }
+ }
+ } else {
+ baseUpdate(pc, taken, bi);
+ }
+
+ //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) & 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) << 16) - 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);
+}
+
+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);
+ }
+
+ if (bi->condBranch) {
+ if (bi->loopHit >= 0) {
+ int idx = bi->loopIndex + bi->loopHit;
+ 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);
+ if (bi->condBranch) {
+ if (bi->loopHit >= 0) {
+ int idx = bi->loopIndex + bi->loopHit;
+ ltable[idx].currentIterSpec = bi->currentIter;
+ }
+ }
+
+ 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]);
+}
+
+LTAGE*
+LTAGEParams::create()
+{
+ return new LTAGE(this);
+}
--- /dev/null
+/*
+ * Copyright (c) 2014 The University of Wisconsin
+ *
+ * Copyright (c) 2006 INRIA (Institut National de Recherche en
+ * Informatique et en Automatique / French National Research Institute
+ * for Computer Science and Applied Mathematics)
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Vignyan Reddy, Dibakar Gope and Arthur Perais,
+ * from André Seznec's code.
+ */
+
+/* @file
+ * Implementation of a L-TAGE branch predictor. TAGE is a global-history based
+ * branch predictor. It features a PC-indexed bimodal predictor and N
+ * partially tagged tables, indexed with a hash of the PC and the global
+ * branch history. The different lengths of global branch history used to
+ * index the partially tagged tables grow geometrically. A small path history
+ * is also used in the hash. L-TAGE also features a loop predictor that records
+ * iteration count of loops and predicts accordingly.
+ *
+ * All TAGE tables are accessed in parallel, and the one using the longest
+ * history that matches provides the prediction (some exceptions apply).
+ * Entries are allocated in components using a longer history than the
+ * one that predicted when the prediction is incorrect.
+ */
+
+#ifndef __CPU_PRED_LTAGE
+#define __CPU_PRED_LTAGE
+
+#include <vector>
+
+#include "base/types.hh"
+#include "cpu/pred/bpred_unit.hh"
+#include "params/LTAGE.hh"
+
+class LTAGE: public BPredUnit
+{
+ public:
+ LTAGE(const LTAGEParams *params);
+
+ // Base class methods.
+ void uncondBranch(ThreadID tid, Addr br_pc, void* &bp_history) override;
+ bool lookup(ThreadID tid, Addr branch_addr, void* &bp_history) override;
+ void btbUpdate(ThreadID tid, Addr branch_addr, void* &bp_history) override;
+ void update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
+ bool squashed) override;
+ void squash(ThreadID tid, void *bp_history) override;
+ unsigned getGHR(ThreadID tid, void *bp_history) const override;
+
+ private:
+ // Prediction Structures
+ // Loop Predictor Entry
+ struct LoopEntry
+ {
+ uint16_t numIter;
+ uint16_t currentIter;
+ uint16_t currentIterSpec;
+ uint8_t confidence;
+ uint16_t tag;
+ uint8_t age;
+ bool dir;
+
+ LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0),
+ confidence(0), tag(0), age(0), dir(0) { }
+ };
+
+ // Bimodal Predictor Entry
+ struct BimodalEntry
+ {
+ uint8_t pred;
+ uint8_t hyst;
+
+ BimodalEntry() : pred(0), hyst(1) { }
+ };
+
+ // Tage Entry
+ struct TageEntry
+ {
+ int8_t ctr;
+ uint16_t tag;
+ int8_t u;
+ TageEntry() : ctr(0), tag(0), u(0) { }
+ };
+
+ // Folded History Table - compressed history
+ // to mix with instruction PC to index partially
+ // tagged tables.
+ struct FoldedHistory
+ {
+ unsigned comp;
+ int compLength;
+ int origLength;
+ int outpoint;
+
+ void init(int original_length, int compressed_length)
+ {
+ comp = 0;
+ origLength = original_length;
+ compLength = compressed_length;
+ outpoint = original_length % compressed_length;
+ }
+
+ void update(uint8_t * h)
+ {
+ comp = (comp << 1) | h[0];
+ comp ^= h[origLength] << outpoint;
+ comp ^= (comp >> compLength);
+ comp &= (ULL(1) << compLength) - 1;
+ }
+ };
+
+ // Primary branch history entry
+ struct BranchInfo
+ {
+ int pathHist;
+ int ptGhist;
+ int hitBank;
+ int hitBankIndex;
+ int altBank;
+ int altBankIndex;
+ int bimodalIndex;
+ int loopTag;
+ uint16_t currentIter;
+
+ bool tagePred;
+ bool altTaken;
+ bool loopPred;
+ bool loopPredValid;
+ int loopIndex;
+ int loopHit;
+ bool condBranch;
+ bool longestMatchPred;
+ bool pseudoNewAlloc;
+ Addr branchPC;
+
+ // Pointer to dynamically allocated storage
+ // to save table indices and folded histories.
+ // To do one call to new instead of five.
+ int *storage;
+
+ // Pointers to actual saved array within the dynamically
+ // allocated storage.
+ int *tableIndices;
+ int *tableTags;
+ int *ci;
+ int *ct0;
+ int *ct1;
+
+ BranchInfo(int sz)
+ : pathHist(0), ptGhist(0),
+ hitBank(0), hitBankIndex(0),
+ altBank(0), altBankIndex(0),
+ bimodalIndex(0), loopTag(0), currentIter(0),
+ tagePred(false), altTaken(false), loopPred(false),
+ loopPredValid(false), loopIndex(0), loopHit(0),
+ condBranch(false), longestMatchPred(false),
+ pseudoNewAlloc(false), branchPC(0)
+ {
+ storage = new int [sz * 5];
+ tableIndices = storage;
+ tableTags = storage + sz;
+ ci = tableTags + sz;
+ ct0 = ci + sz;
+ ct1 = ct0 + sz;
+ }
+
+ ~BranchInfo()
+ {
+ delete[] storage;
+ }
+ };
+
+ /**
+ * Computes the index used to access the
+ * bimodal table.
+ * @param pc_in The unshifted branch PC.
+ */
+ int bindex(Addr pc_in) const;
+
+ /**
+ * Computes the index used to access the
+ * loop predictor.
+ * @param pc_in The unshifted branch PC.
+ */
+ int lindex(Addr pc_in) const;
+
+ /**
+ * Computes the index used to access a
+ * partially tagged table.
+ * @param tid The thread ID used to select the
+ * global histories to use.
+ * @param pc The unshifted branch PC.
+ * @param bank The partially tagged table to access.
+ */
+ inline int gindex(ThreadID tid, Addr pc, int bank) const;
+
+ /**
+ * Utility function to shuffle the path history
+ * depending on which tagged table we are accessing.
+ * @param phist The path history.
+ * @param size Number of path history bits to use.
+ * @param bank The partially tagged table to access.
+ */
+ int F(int phist, int size, int bank) const;
+
+ /**
+ * Computes the partial tag of a tagged table.
+ * @param tid the thread ID used to select the
+ * global histories to use.
+ * @param pc The unshifted branch PC.
+ * @param bank The partially tagged table to access.
+ */
+ inline uint16_t gtag(ThreadID tid, Addr pc, int bank) const;
+
+ /**
+ * Updates a direction counter based on the actual
+ * branch outcome.
+ * @param ctr Reference to counter to update.
+ * @param taken Actual branch outcome.
+ * @param nbits Counter width.
+ */
+ void ctrUpdate(int8_t & ctr, bool taken, int nbits);
+
+ /**
+ * Get a branch prediction from the bimodal
+ * predictor.
+ * @param pc The unshifted branch PC.
+ * @param bi Pointer to information on the
+ * prediction.
+ */
+ bool getBimodePred(Addr pc, BranchInfo* bi) const;
+
+ /**
+ * Updates the bimodal predictor.
+ * @param pc The unshifted branch PC.
+ * @param taken The actual branch outcome.
+ * @param bi Pointer to information on the prediction
+ * recorded at prediction time.
+ */
+ void baseUpdate(Addr pc, bool taken, BranchInfo* bi);
+
+ /**
+ * Get a branch prediction from the loop
+ * predictor.
+ * @param pc The unshifted branch PC.
+ * @param bi Pointer to information on the
+ * prediction.
+ */
+ bool getLoop(Addr pc, BranchInfo* bi) const;
+
+ /**
+ * Updates the loop predictor.
+ * @param pc The unshifted branch PC.
+ * @param taken The actual branch outcome.
+ * @param bi Pointer to information on the
+ * prediction recorded at prediction time.
+ */
+ void loopUpdate(Addr pc, bool Taken, BranchInfo* bi);
+
+ /**
+ * (Speculatively) updates the global branch history.
+ * @param h Reference to pointer to global branch history.
+ * @param dir (Predicted) outcome to update the histories
+ * with.
+ * @param tab
+ * @param PT Reference to path history.
+ */
+ void updateGHist(uint8_t * &h, bool dir, uint8_t * tab, int &PT);
+
+ /**
+ * Get a branch prediction from L-TAGE. *NOT* an override of
+ * BpredUnit::predict().
+ * @param tid The thread ID to select the global
+ * histories to use.
+ * @param branch_pc The unshifted branch PC.
+ * @param cond_branch True if the branch is conditional.
+ * @param b Reference to wrapping pointer to allow storing
+ * derived class prediction information in the base class.
+ */
+ bool predict(ThreadID tid, Addr branch_pc, bool cond_branch, void* &b);
+
+ /**
+ * Update L-TAGE. Called at execute to repair histories on a misprediction
+ * and at commit to update the tables.
+ * @param tid The thread ID to select the global
+ * histories to use.
+ * @param branch_pc The unshifted branch PC.
+ * @param taken Actual branch outcome.
+ * @param bi Pointer to information on the prediction
+ * recorded at prediction time.
+ */
+ void update(ThreadID tid, Addr branch_pc, bool taken, BranchInfo* bi);
+
+ /**
+ * (Speculatively) updates global histories (path and direction).
+ * Also recomputes compressed (folded) histories based on the
+ * branch direction.
+ * @param tid The thread ID to select the histories
+ * to update.
+ * @param branch_pc The unshifted branch PC.
+ * @param taken (Predicted) branch direction.
+ * @param b Wrapping pointer to BranchInfo (to allow
+ * storing derived class prediction information in the
+ * base class).
+ */
+ void updateHistories(ThreadID tid, Addr branch_pc, bool taken, void* b);
+
+ /**
+ * Restores speculatively updated path and direction histories.
+ * Also recomputes compressed (folded) histories based on the
+ * correct branch outcome.
+ * This version of squash() is called once on a branch misprediction.
+ * @param tid The Thread ID to select the histories to rollback.
+ * @param taken The correct branch outcome.
+ * @param bp_history Wrapping pointer to BranchInfo (to allow
+ * storing derived class prediction information in the
+ * base class).
+ * @post bp_history points to valid memory.
+ */
+ void squash(ThreadID tid, bool taken, void *bp_history);
+
+ /**
+ * Speculatively updates the loop predictor
+ * iteration count.
+ * @param pc The unshifted branch PC.
+ * @param taken The predicted branch outcome.
+ * @param bi Pointer to information on the prediction
+ * recorded at prediction time.
+ */
+ void specLoopUpdate(Addr pc, bool taken, BranchInfo* bi);
+
+ const unsigned logSizeBiMP;
+ const unsigned logSizeTagTables;
+ const unsigned logSizeLoopPred;
+ const unsigned nHistoryTables;
+ const unsigned tagTableCounterBits;
+ const unsigned histBufferSize;
+ const unsigned minHist;
+ const unsigned maxHist;
+ const unsigned minTagWidth;
+
+ BimodalEntry *btable;
+ TageEntry **gtable;
+ LoopEntry *ltable;
+
+ // Keep per-thread histories to
+ // support SMT.
+ struct ThreadHistory {
+ // Speculative path history
+ // (LSB of branch address)
+ int pathHist;
+
+ // Speculative branch direction
+ // history (circular buffer)
+ // @TODO Convert to std::vector<bool>
+ uint8_t *globalHistory;
+
+ // Pointer to most recent branch outcome
+ uint8_t* gHist;
+
+ // Index to most recent branch outcome
+ int ptGhist;
+
+ // Speculative folded histories.
+ FoldedHistory *computeIndices;
+ FoldedHistory *computeTags[2];
+ };
+
+ std::vector<ThreadHistory> threadHistory;
+
+ int tagWidths[15];
+ int tagTableSizes[15];
+ int *histLengths;
+ int *tableIndices;
+ int *tableTags;
+
+ int8_t loopUseCounter;
+ int8_t useAltPredForNewlyAllocated;
+ int tCounter;
+ int logTick;
+};
+
+#endif // __CPU_PRED_LTAGE
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