ltage.hh revision 13455
1/* 2 * Copyright (c) 2014 The University of Wisconsin 3 * 4 * Copyright (c) 2006 INRIA (Institut National de Recherche en 5 * Informatique et en Automatique / French National Research Institute 6 * for Computer Science and Applied Mathematics) 7 * 8 * All rights reserved. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions are 12 * met: redistributions of source code must retain the above copyright 13 * notice, this list of conditions and the following disclaimer; 14 * redistributions in binary form must reproduce the above copyright 15 * notice, this list of conditions and the following disclaimer in the 16 * documentation and/or other materials provided with the distribution; 17 * neither the name of the copyright holders nor the names of its 18 * contributors may be used to endorse or promote products derived from 19 * this software without specific prior written permission. 20 * 21 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 22 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 23 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 24 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 25 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 26 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 27 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 28 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 29 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 30 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 31 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 32 * 33 * Authors: Vignyan Reddy, Dibakar Gope and Arthur Perais, 34 * from André Seznec's code. 35 */ 36 37/* @file 38 * Implementation of a L-TAGE branch predictor. TAGE is a global-history based 39 * branch predictor. It features a PC-indexed bimodal predictor and N 40 * partially tagged tables, indexed with a hash of the PC and the global 41 * branch history. The different lengths of global branch history used to 42 * index the partially tagged tables grow geometrically. A small path history 43 * is also used in the hash. L-TAGE also features a loop predictor that records 44 * iteration count of loops and predicts accordingly. 45 * 46 * All TAGE tables are accessed in parallel, and the one using the longest 47 * history that matches provides the prediction (some exceptions apply). 48 * Entries are allocated in components using a longer history than the 49 * one that predicted when the prediction is incorrect. 50 */ 51 52#ifndef __CPU_PRED_LTAGE 53#define __CPU_PRED_LTAGE 54 55 56#include <vector> 57 58#include "base/types.hh" 59#include "cpu/pred/tage.hh" 60#include "params/LTAGE.hh" 61 62class LTAGE: public TAGE 63{ 64 public: 65 LTAGE(const LTAGEParams *params); 66 67 // Base class methods. 68 void squash(ThreadID tid, void *bp_history) override; 69 70 void regStats() override; 71 72 private: 73 // Prediction Structures 74 // Loop Predictor Entry 75 struct LoopEntry 76 { 77 uint16_t numIter; 78 uint16_t currentIter; 79 uint16_t currentIterSpec; 80 uint8_t confidence; 81 uint16_t tag; 82 uint8_t age; 83 bool dir; 84 85 LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0), 86 confidence(0), tag(0), age(0), dir(0) { } 87 }; 88 89 // more provider types 90 enum { 91 LOOP = LAST_TAGE_PROVIDER_TYPE + 1 92 }; 93 94 // Primary branch history entry 95 struct LTageBranchInfo : public TageBranchInfo 96 { 97 uint16_t loopTag; 98 uint16_t currentIter; 99 100 bool loopPred; 101 bool loopPredValid; 102 int loopIndex; 103 int loopHit; 104 105 LTageBranchInfo(int sz) 106 : TageBranchInfo(sz), 107 loopTag(0), currentIter(0), 108 loopPred(false), 109 loopPredValid(false), loopIndex(0), loopHit(0) 110 {} 111 }; 112 113 /** 114 * Computes the index used to access the 115 * loop predictor. 116 * @param pc_in The unshifted branch PC. 117 */ 118 int lindex(Addr pc_in) const; 119 120 /** 121 * Get a branch prediction from the loop 122 * predictor. 123 * @param pc The unshifted branch PC. 124 * @param bi Pointer to information on the 125 * prediction. 126 */ 127 bool getLoop(Addr pc, LTageBranchInfo* bi) const; 128 129 /** 130 * Updates the loop predictor. 131 * @param pc The unshifted branch PC. 132 * @param taken The actual branch outcome. 133 * @param bi Pointer to information on the 134 * prediction recorded at prediction time. 135 */ 136 void loopUpdate(Addr pc, bool Taken, LTageBranchInfo* bi); 137 138 /** 139 * Speculatively updates the loop predictor 140 * iteration count. 141 * @param pc The unshifted branch PC. 142 * @param taken The predicted branch outcome. 143 * @param bi Pointer to information on the prediction 144 * recorded at prediction time. 145 */ 146 void specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi); 147 148 /** 149 * Update LTAGE for conditional branches. 150 * @param branch_pc The unshifted branch PC. 151 * @param taken Actual branch outcome. 152 * @param bi Pointer to information on the prediction 153 * recorded at prediction time. 154 * @nrand Random int number from 0 to 3 155 */ 156 void condBranchUpdate( 157 Addr branch_pc, bool taken, TageBranchInfo* bi, int nrand) override; 158 159 /** 160 * Get a branch prediction from LTAGE. *NOT* an override of 161 * BpredUnit::predict(). 162 * @param tid The thread ID to select the global 163 * histories to use. 164 * @param branch_pc The unshifted branch PC. 165 * @param cond_branch True if the branch is conditional. 166 * @param b Reference to wrapping pointer to allow storing 167 * derived class prediction information in the base class. 168 */ 169 bool predict( 170 ThreadID tid, Addr branch_pc, bool cond_branch, void* &b) override; 171 172 /** 173 * Restores speculatively updated path and direction histories. 174 * Also recomputes compressed (folded) histories based on the 175 * correct branch outcome. 176 * This version of squash() is called once on a branch misprediction. 177 * @param tid The Thread ID to select the histories to rollback. 178 * @param taken The correct branch outcome. 179 * @param bp_history Wrapping pointer to TageBranchInfo (to allow 180 * storing derived class prediction information in the 181 * base class). 182 * @post bp_history points to valid memory. 183 */ 184 void squash( 185 ThreadID tid, bool taken, void *bp_history) override; 186 187 /** 188 * Update the stats 189 * @param taken Actual branch outcome 190 * @param bi Pointer to information on the prediction 191 * recorded at prediction time. 192 */ 193 void updateStats(bool taken, TageBranchInfo* bi) override; 194 195 const unsigned logSizeLoopPred; 196 const unsigned loopTableAgeBits; 197 const unsigned loopTableConfidenceBits; 198 const unsigned loopTableTagBits; 199 const unsigned loopTableIterBits; 200 const unsigned logLoopTableAssoc; 201 const uint8_t confidenceThreshold; 202 const uint16_t loopTagMask; 203 const uint16_t loopNumIterMask; 204 205 LoopEntry *ltable; 206 207 int8_t loopUseCounter; 208 unsigned withLoopBits; 209 210 // stats 211 Stats::Scalar loopPredictorCorrect; 212 Stats::Scalar loopPredictorWrong; 213}; 214 215#endif // __CPU_PRED_LTAGE 216