src/cpu/pred/2bit_local.cc

   1 /*
   2  * Copyright (c) 2004-2006 The Regents of The University of Michigan
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions are
   7  * met: redistributions of source code must retain the above copyright
   8  * notice, this list of conditions and the following disclaimer;
   9  * redistributions in binary form must reproduce the above copyright
  10  * notice, this list of conditions and the following disclaimer in the
  11  * documentation and/or other materials provided with the distribution;
  12  * neither the name of the copyright holders nor the names of its
  13  * contributors may be used to endorse or promote products derived from
  14  * this software without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  17  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  18  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  19  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  20  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  21  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  22  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  23  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  24  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  25  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  26  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  27  *
  28  * Authors: Kevin Lim
  29  */
  30
  31 #include "base/intmath.hh"
  32 #include "base/misc.hh"
  33 #include "base/trace.hh"
  34 #include "cpu/pred/2bit_local.hh"
  35 #include "debug/Fetch.hh"
  36
  37 LocalBP::LocalBP(const LocalBPParams *params)
  38     : BPredUnit(params),
  39       localPredictorSize(params->localPredictorSize),
  40       localCtrBits(params->localCtrBits)
  41 {
  42     if (!isPowerOf2(localPredictorSize)) {
  43         fatal("Invalid local predictor size!\n");
  44     }
  45
  46     localPredictorSets = localPredictorSize / localCtrBits;
  47
  48     if (!isPowerOf2(localPredictorSets)) {
  49         fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
  50     }
  51
  52     // Setup the index mask.
  53     indexMask = localPredictorSets - 1;
  54
  55     DPRINTF(Fetch, "index mask: %#x\n", indexMask);
  56
  57     // Setup the array of counters for the local predictor.
  58     localCtrs.resize(localPredictorSets);
  59
  60     for (unsigned i = 0; i < localPredictorSets; ++i)
  61         localCtrs[i].setBits(localCtrBits);
  62
  63     DPRINTF(Fetch, "local predictor size: %i\n",
  64             localPredictorSize);
  65
  66     DPRINTF(Fetch, "local counter bits: %i\n", localCtrBits);
  67
  68     DPRINTF(Fetch, "instruction shift amount: %i\n",
  69             instShiftAmt);
  70 }
  71
  72 void
  73 LocalBP::reset()
  74 {
  75     for (unsigned i = 0; i < localPredictorSets; ++i) {
  76         localCtrs[i].reset();
  77     }
  78 }
  79
  80 void
  81 LocalBP::btbUpdate(Addr branch_addr, void * &bp_history)
  82 {
  83 // Place holder for a function that is called to update predictor history when
  84 // a BTB entry is invalid or not found.
  85 }
  86
  87
  88 bool
  89 LocalBP::lookup(Addr branch_addr, void * &bp_history)
  90 {
  91     bool taken;
  92     uint8_t counter_val;
  93     unsigned local_predictor_idx = getLocalIndex(branch_addr);
  94
  95     DPRINTF(Fetch, "Looking up index %#x\n",
  96             local_predictor_idx);
  97
  98     counter_val = localCtrs[local_predictor_idx].read();
  99
 100     DPRINTF(Fetch, "prediction is %i.\n",
 101             (int)counter_val);
 102
 103     taken = getPrediction(counter_val);
 104
 105 #if 0
 106     // Speculative update.
 107     if (taken) {
 108         DPRINTF(Fetch, "Branch updated as taken.\n");
 109         localCtrs[local_predictor_idx].increment();
 110     } else {
 111         DPRINTF(Fetch, "Branch updated as not taken.\n");
 112         localCtrs[local_predictor_idx].decrement();
 113     }
 114 #endif
 115
 116     return taken;
 117 }
 118
 119 void
 120 LocalBP::update(Addr branch_addr, bool taken, void *bp_history, bool squashed)
 121 {
 122     assert(bp_history == NULL);
 123     unsigned local_predictor_idx;
 124
 125     // Update the local predictor.
 126     local_predictor_idx = getLocalIndex(branch_addr);
 127
 128     DPRINTF(Fetch, "Looking up index %#x\n", local_predictor_idx);
 129
 130     if (taken) {
 131         DPRINTF(Fetch, "Branch updated as taken.\n");
 132         localCtrs[local_predictor_idx].increment();
 133     } else {
 134         DPRINTF(Fetch, "Branch updated as not taken.\n");
 135         localCtrs[local_predictor_idx].decrement();
 136     }
 137 }
 138
 139 inline
 140 bool
 141 LocalBP::getPrediction(uint8_t &count)
 142 {
 143     // Get the MSB of the count
 144     return (count >> (localCtrBits - 1));
 145 }
 146
 147 inline
 148 unsigned
 149 LocalBP::getLocalIndex(Addr &branch_addr)
 150 {
 151     return (branch_addr >> instShiftAmt) & indexMask;
 152 }
 153
 154 void
 155 LocalBP::uncondBranch(Addr pc, void *&bp_history)
 156 {
 157 }
 158
 159 LocalBP*
 160 LocalBPParams::create()
 161 {
 162     return new LocalBP(this);
 163 }