ltage.hh revision 13454
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 private: 71 // Prediction Structures 72 // Loop Predictor Entry 73 struct LoopEntry 74 { 75 uint16_t numIter; 76 uint16_t currentIter; 77 uint16_t currentIterSpec; 78 uint8_t confidence; 79 uint16_t tag; 80 uint8_t age; 81 bool dir; 82 83 LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0), 84 confidence(0), tag(0), age(0), dir(0) { } 85 }; 86 87 // Primary branch history entry 88 struct LTageBranchInfo : public TageBranchInfo 89 { 90 uint16_t loopTag; 91 uint16_t currentIter; 92 93 bool loopPred; 94 bool loopPredValid; 95 int loopIndex; 96 int loopHit; 97 98 LTageBranchInfo(int sz) 99 : TageBranchInfo(sz), 100 loopTag(0), currentIter(0), 101 loopPred(false), 102 loopPredValid(false), loopIndex(0), loopHit(0) 103 {} 104 }; 105 106 /** 107 * Computes the index used to access the 108 * loop predictor. 109 * @param pc_in The unshifted branch PC. 110 */ 111 int lindex(Addr pc_in) const; 112 113 /** 114 * Get a branch prediction from the loop 115 * predictor. 116 * @param pc The unshifted branch PC. 117 * @param bi Pointer to information on the 118 * prediction. 119 */ 120 bool getLoop(Addr pc, LTageBranchInfo* bi) const; 121 122 /** 123 * Updates the loop predictor. 124 * @param pc The unshifted branch PC. 125 * @param taken The actual branch outcome. 126 * @param bi Pointer to information on the 127 * prediction recorded at prediction time. 128 */ 129 void loopUpdate(Addr pc, bool Taken, LTageBranchInfo* bi); 130 131 /** 132 * Speculatively updates the loop predictor 133 * iteration count. 134 * @param pc The unshifted branch PC. 135 * @param taken The predicted branch outcome. 136 * @param bi Pointer to information on the prediction 137 * recorded at prediction time. 138 */ 139 void specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi); 140 141 /** 142 * Update LTAGE for conditional branches. 143 * @param branch_pc The unshifted branch PC. 144 * @param taken Actual branch outcome. 145 * @param bi Pointer to information on the prediction 146 * recorded at prediction time. 147 * @nrand Random int number from 0 to 3 148 */ 149 void condBranchUpdate( 150 Addr branch_pc, bool taken, TageBranchInfo* bi, int nrand) override; 151 152 /** 153 * Get a branch prediction from LTAGE. *NOT* an override of 154 * BpredUnit::predict(). 155 * @param tid The thread ID to select the global 156 * histories to use. 157 * @param branch_pc The unshifted branch PC. 158 * @param cond_branch True if the branch is conditional. 159 * @param b Reference to wrapping pointer to allow storing 160 * derived class prediction information in the base class. 161 */ 162 bool predict( 163 ThreadID tid, Addr branch_pc, bool cond_branch, void* &b) override; 164 165 /** 166 * Restores speculatively updated path and direction histories. 167 * Also recomputes compressed (folded) histories based on the 168 * correct branch outcome. 169 * This version of squash() is called once on a branch misprediction. 170 * @param tid The Thread ID to select the histories to rollback. 171 * @param taken The correct branch outcome. 172 * @param bp_history Wrapping pointer to TageBranchInfo (to allow 173 * storing derived class prediction information in the 174 * base class). 175 * @post bp_history points to valid memory. 176 */ 177 void squash( 178 ThreadID tid, bool taken, void *bp_history) override; 179 180 const unsigned logSizeLoopPred; 181 const unsigned loopTableAgeBits; 182 const unsigned loopTableConfidenceBits; 183 const unsigned loopTableTagBits; 184 const unsigned loopTableIterBits; 185 const unsigned logLoopTableAssoc; 186 const uint8_t confidenceThreshold; 187 const uint16_t loopTagMask; 188 const uint16_t loopNumIterMask; 189 190 LoopEntry *ltable; 191 192 int8_t loopUseCounter; 193 unsigned withLoopBits; 194}; 195 196#endif // __CPU_PRED_LTAGE 197