cpu: implement an L-TAGE branch predictor
This patch implements an L-TAGE predictor, based on André Seznec's code available from CBP-2 (http://hpca23.cse.tamu.edu/taco/camino/cbp2/cbp-src/realistic-seznec.h). Signed-off-by Jason Lowe-Power <jason@lowepower.com>
This commit is contained in:
Arthur Perais
parent
497cc2d373
commit
c9d933efb0
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@ -86,3 +86,19 @@ class BiModeBP(BranchPredictor):
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choicePredictorSize = Param.Unsigned(8192, "Size of choice predictor")
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choiceCtrBits = Param.Unsigned(2, "Bits of choice counters")
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class LTAGE(BranchPredictor):
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type = 'LTAGE'
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cxx_class = 'LTAGE'
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cxx_header = "cpu/pred/ltage.hh"
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logSizeBiMP = Param.Unsigned(14, "Log size of Bimodal predictor in bits")
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logSizeTagTables = Param.Unsigned(11, "Log size of tag table in LTAGE")
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logSizeLoopPred = Param.Unsigned(8, "Log size of the loop predictor")
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nHistoryTables = Param.Unsigned(12, "Number of history tables")
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tagTableCounterBits = Param.Unsigned(3, "Number of tag table counter bits")
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histBufferSize = Param.Unsigned(2097152,
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"A large number to track all branch histories(2MEntries default)")
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minHist = Param.Unsigned(4, "Minimum history size of LTAGE")
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maxHist = Param.Unsigned(640, "Maximum history size of LTAGE")
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minTagWidth = Param.Unsigned(7, "Minimum tag size in tag tables")
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@ -43,6 +43,7 @@ Source('indirect.cc')
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Source('ras.cc')
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Source('tournament.cc')
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Source ('bi_mode.cc')
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Source('ltage.cc')
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DebugFlag('FreeList')
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DebugFlag('Branch')
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DebugFlag('LTage')
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@ -0,0 +1,746 @@
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/*
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* Copyright (c) 2014 The University of Wisconsin
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*
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* Copyright (c) 2006 INRIA (Institut National de Recherche en
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* Informatique et en Automatique / French National Research Institute
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* for Computer Science and Applied Mathematics)
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*
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions are
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* met: redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer;
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* redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution;
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* neither the name of the copyright holders nor the names of its
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* contributors may be used to endorse or promote products derived from
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* this software without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*
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* Authors: Vignyan Reddy, Dibakar Gope and Arthur Perais,
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* from André Seznec's code.
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*/
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/* @file
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* Implementation of a L-TAGE branch predictor
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*/
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#include "cpu/pred/ltage.hh"
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#include "base/intmath.hh"
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#include "base/misc.hh"
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#include "base/random.hh"
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#include "base/trace.hh"
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#include "debug/Fetch.hh"
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#include "debug/LTage.hh"
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LTAGE::LTAGE(const LTAGEParams *params)
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: BPredUnit(params),
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logSizeBiMP(params->logSizeBiMP),
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logSizeTagTables(params->logSizeTagTables),
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logSizeLoopPred(params->logSizeLoopPred),
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nHistoryTables(params->nHistoryTables),
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tagTableCounterBits(params->tagTableCounterBits),
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histBufferSize(params->histBufferSize),
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minHist(params->minHist),
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maxHist(params->maxHist),
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minTagWidth(params->minTagWidth),
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threadHistory(params->numThreads)
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{
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assert(params->histBufferSize > params->maxHist * 2);
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useAltPredForNewlyAllocated = 0;
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logTick = 19;
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tCounter = ULL(1) << (logTick - 1);
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for (auto& history : threadHistory) {
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history.pathHist = 0;
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history.globalHistory = new uint8_t[histBufferSize];
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history.gHist = history.globalHistory;
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memset(history.gHist, 0, histBufferSize);
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history.ptGhist = 0;
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}
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histLengths = new int [nHistoryTables+1];
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histLengths[1] = minHist;
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histLengths[nHistoryTables] = maxHist;
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for (int i = 2; i <= nHistoryTables; i++) {
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histLengths[i] = (int) (((double) minHist *
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pow ((double) (maxHist) / (double) minHist,
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(double) (i - 1) / (double) ((nHistoryTables- 1))))
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+ 0.5);
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}
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tagWidths[1] = minTagWidth;
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tagWidths[2] = minTagWidth;
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tagWidths[3] = minTagWidth + 1;
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tagWidths[4] = minTagWidth + 1;
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tagWidths[5] = minTagWidth + 2;
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tagWidths[6] = minTagWidth + 3;
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tagWidths[7] = minTagWidth + 4;
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tagWidths[8] = minTagWidth + 5;
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tagWidths[9] = minTagWidth + 5;
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tagWidths[10] = minTagWidth + 6;
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tagWidths[11] = minTagWidth + 7;
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tagWidths[12] = minTagWidth + 8;
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for (int i = 1; i <= 2; i++)
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tagTableSizes[i] = logSizeTagTables - 1;
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for (int i = 3; i <= 6; i++)
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tagTableSizes[i] = logSizeTagTables;
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for (int i = 7; i <= 10; i++)
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tagTableSizes[i] = logSizeTagTables - 1;
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for (int i = 11; i <= 12; i++)
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tagTableSizes[i] = logSizeTagTables - 2;
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for (auto& history : threadHistory) {
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history.computeIndices = new FoldedHistory[nHistoryTables+1];
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history.computeTags[0] = new FoldedHistory[nHistoryTables+1];
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history.computeTags[1] = new FoldedHistory[nHistoryTables+1];
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for (int i = 1; i <= nHistoryTables; i++) {
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history.computeIndices[i].init(histLengths[i], (tagTableSizes[i]));
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history.computeTags[0][i].init(
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history.computeIndices[i].origLength, tagWidths[i]);
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history.computeTags[1][i].init(
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history.computeIndices[i].origLength, tagWidths[i] - 1);
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DPRINTF(LTage, "HistLength:%d, TTSize:%d, TTTWidth:%d\n",
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histLengths[i], tagTableSizes[i], tagWidths[i]);
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}
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}
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btable = new BimodalEntry[ULL(1) << logSizeBiMP];
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ltable = new LoopEntry[ULL(1) << logSizeLoopPred];
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gtable = new TageEntry*[nHistoryTables + 1];
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for (int i = 1; i <= nHistoryTables; i++) {
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gtable[i] = new TageEntry[1<<(tagTableSizes[i])];
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}
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tableIndices = new int [nHistoryTables+1];
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tableTags = new int [nHistoryTables+1];
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loopUseCounter = 0;
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}
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int
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LTAGE::bindex(Addr pc_in) const
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{
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return ((pc_in) & ((ULL(1) << (logSizeBiMP)) - 1));
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}
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int
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LTAGE::lindex(Addr pc_in) const
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{
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return (((pc_in) & ((ULL(1) << (logSizeLoopPred - 2)) - 1)) << 2);
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}
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int
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LTAGE::F(int A, int size, int bank) const
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{
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int A1, A2;
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A = A & ((ULL(1) << size) - 1);
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A1 = (A & ((ULL(1) << tagTableSizes[bank]) - 1));
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A2 = (A >> tagTableSizes[bank]);
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A2 = ((A2 << bank) & ((ULL(1) << tagTableSizes[bank]) - 1))
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+ (A2 >> (tagTableSizes[bank] - bank));
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A = A1 ^ A2;
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A = ((A << bank) & ((ULL(1) << tagTableSizes[bank]) - 1))
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+ (A >> (tagTableSizes[bank] - bank));
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return (A);
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}
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// gindex computes a full hash of pc, ghist and pathHist
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int
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LTAGE::gindex(ThreadID tid, Addr pc, int bank) const
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{
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int index;
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int hlen = (histLengths[bank] > 16) ? 16 : histLengths[bank];
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index =
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(pc) ^ ((pc) >> ((int) abs(tagTableSizes[bank] - bank) + 1)) ^
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threadHistory[tid].computeIndices[bank].comp ^
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F(threadHistory[tid].pathHist, hlen, bank);
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return (index & ((ULL(1) << (tagTableSizes[bank])) - 1));
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}
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// Tag computation
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uint16_t
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LTAGE::gtag(ThreadID tid, Addr pc, int bank) const
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{
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int tag = (pc) ^ threadHistory[tid].computeTags[0][bank].comp
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^ (threadHistory[tid].computeTags[1][bank].comp << 1);
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return (tag & ((ULL(1) << tagWidths[bank]) - 1));
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}
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// Up-down saturating counter
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void
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LTAGE::ctrUpdate(int8_t & ctr, bool taken, int nbits)
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{
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assert(nbits <= sizeof(int8_t) << 3);
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if (taken) {
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if (ctr < ((1 << (nbits - 1)) - 1))
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ctr++;
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} else {
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if (ctr > -(1 << (nbits - 1)))
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ctr--;
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}
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}
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// Bimodal prediction
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bool
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LTAGE::getBimodePred(Addr pc, BranchInfo* bi) const
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{
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return (btable[bi->bimodalIndex].pred > 0);
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}
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// Update the bimodal predictor: a hysteresis bit is shared among 4 prediction
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// bits
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void
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LTAGE::baseUpdate(Addr pc, bool taken, BranchInfo* bi)
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{
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int inter = (btable[bi->bimodalIndex].pred << 1)
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+ btable[bi->bimodalIndex ].hyst;
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if (taken) {
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if (inter < 3)
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inter++;
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} else if (inter > 0) {
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inter--;
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}
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btable[bi->bimodalIndex].pred = inter >> 1;
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btable[bi->bimodalIndex].hyst = (inter & 1);
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DPRINTF(LTage, "Updating branch %lx, pred:%d, hyst:%d\n",
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pc, btable[bi->bimodalIndex].pred,btable[bi->bimodalIndex].hyst);
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}
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//loop prediction: only used if high confidence
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bool
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LTAGE::getLoop(Addr pc, BranchInfo* bi) const
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{
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bi->loopHit = -1;
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bi->loopPredValid = false;
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bi->loopIndex = lindex(pc);
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bi->loopTag = ((pc) >> (logSizeLoopPred - 2));
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for (int i = 0; i < 4; i++) {
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if (ltable[bi->loopIndex + i].tag == bi->loopTag) {
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bi->loopHit = i;
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bi->loopPredValid = (ltable[bi->loopIndex + i].confidence >= 3);
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bi->currentIter = ltable[bi->loopIndex + i].currentIterSpec;
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if (ltable[bi->loopIndex + i].currentIterSpec + 1 ==
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ltable[bi->loopIndex + i].numIter) {
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return !(ltable[bi->loopIndex + i].dir);
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}else {
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return (ltable[bi->loopIndex + i].dir);
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}
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}
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}
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return false;
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}
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void
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LTAGE::specLoopUpdate(Addr pc, bool taken, BranchInfo* bi)
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{
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if (bi->loopHit>=0) {
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int index = lindex(pc);
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if (taken != ltable[index].dir) {
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ltable[index].currentIterSpec = 0;
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} else {
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ltable[index].currentIterSpec++;
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}
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}
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}
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void
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LTAGE::loopUpdate(Addr pc, bool taken, BranchInfo* bi)
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{
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int idx = bi->loopIndex + bi->loopHit;
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if (bi->loopHit >= 0) {
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//already a hit
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if (bi->loopPredValid) {
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if (taken != bi->loopPred) {
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// free the entry
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ltable[idx].numIter = 0;
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ltable[idx].age = 0;
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ltable[idx].confidence = 0;
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ltable[idx].currentIter = 0;
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return;
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} else if (bi->loopPred != bi->tagePred) {
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DPRINTF(LTage, "Loop Prediction success:%lx\n",pc);
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if (ltable[idx].age < 7)
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ltable[idx].age++;
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}
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}
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ltable[idx].currentIter++;
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if (ltable[idx].currentIter > ltable[idx].numIter) {
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ltable[idx].confidence = 0;
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if (ltable[idx].numIter != 0) {
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// free the entry
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ltable[idx].numIter = 0;
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ltable[idx].age = 0;
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ltable[idx].confidence = 0;
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}
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}
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if (taken != ltable[idx].dir) {
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if (ltable[idx].currentIter == ltable[idx].numIter) {
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DPRINTF(LTage, "Loop End predicted successfully:%lx\n", pc);
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if (ltable[idx].confidence < 7) {
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ltable[idx].confidence++;
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}
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//just do not predict when the loop count is 1 or 2
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if (ltable[idx].numIter < 3) {
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// free the entry
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ltable[idx].dir = taken;
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ltable[idx].numIter = 0;
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ltable[idx].age = 0;
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ltable[idx].confidence = 0;
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}
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} else {
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DPRINTF(LTage, "Loop End predicted incorrectly:%lx\n", pc);
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if (ltable[idx].numIter == 0) {
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// first complete nest;
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ltable[idx].confidence = 0;
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ltable[idx].numIter = ltable[idx].currentIter;
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} else {
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//not the same number of iterations as last time: free the
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//entry
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ltable[idx].numIter = 0;
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ltable[idx].age = 0;
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ltable[idx].confidence = 0;
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}
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}
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ltable[idx].currentIter = 0;
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}
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} else if (taken) {
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//try to allocate an entry on taken branch
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int nrand = random_mt.random<int>();
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for (int i = 0; i < 4; i++) {
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int loop_hit = (nrand + i) & 3;
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idx = bi->loopIndex + loop_hit;
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if (ltable[idx].age == 0) {
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DPRINTF(LTage, "Allocating loop pred entry for branch %lx\n",
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pc);
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ltable[idx].dir = !taken;
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ltable[idx].tag = bi->loopTag;
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ltable[idx].numIter = 0;
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ltable[idx].age = 7;
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ltable[idx].confidence = 0;
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ltable[idx].currentIter = 1;
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break;
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}
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else
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ltable[idx].age--;
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}
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}
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}
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// shifting the global history: we manage the history in a big table in order
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// to reduce simulation time
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void
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LTAGE::updateGHist(uint8_t * &h, bool dir, uint8_t * tab, int &pt)
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{
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if (pt == 0) {
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DPRINTF(LTage, "Rolling over the histories\n");
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// Copy beginning of globalHistoryBuffer to end, such that
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// the last maxHist outcomes are still reachable
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// through pt[0 .. maxHist - 1].
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for (int i = 0; i < maxHist; i++)
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tab[histBufferSize - maxHist + i] = tab[i];
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pt = histBufferSize - maxHist;
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h = &tab[pt];
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}
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pt--;
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h--;
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h[0] = (dir) ? 1 : 0;
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}
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// Get GHR for hashing indirect predictor
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// Build history backwards from pointer in
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// bp_history.
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unsigned
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LTAGE::getGHR(ThreadID tid, void *bp_history) const
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{
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BranchInfo* bi = static_cast<BranchInfo*>(bp_history);
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unsigned val = 0;
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for (unsigned i = 0; i < 32; i++) {
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// Make sure we don't go out of bounds
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int gh_offset = bi->ptGhist + i;
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assert(&(threadHistory[tid].globalHistory[gh_offset]) <
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threadHistory[tid].globalHistory + histBufferSize);
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val |= ((threadHistory[tid].globalHistory[gh_offset] & 0x1) << i);
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}
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return val;
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}
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//prediction
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bool
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LTAGE::predict(ThreadID tid, Addr branch_pc, bool cond_branch, void* &b)
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{
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BranchInfo *bi = new BranchInfo(nHistoryTables+1);
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b = (void*)(bi);
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Addr pc = branch_pc;
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bool pred_taken = true;
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bi->loopHit = -1;
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if (cond_branch) {
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// TAGE prediction
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// computes the table addresses and the partial tags
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for (int i = 1; i <= nHistoryTables; i++) {
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tableIndices[i] = gindex(tid, pc, i);
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bi->tableIndices[i] = tableIndices[i];
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tableTags[i] = gtag(tid, pc, i);
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bi->tableTags[i] = tableTags[i];
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}
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bi->bimodalIndex = bindex(pc);
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||||
|
||||
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);
|
||||
}
|
||||
|
|
@ -0,0 +1,408 @@
|
|||
/*
|
||||
* 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
|
||||
Loading…
Reference in New Issue