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(unsigned _localPredictorSize,
  38                  unsigned _localCtrBits,
  39                  unsigned _instShiftAmt)
  40     : localPredictorSize(_localPredictorSize),
  41       localCtrBits(_localCtrBits),
  42       instShiftAmt(_instShiftAmt)
  43 {
  44     if (!isPowerOf2(localPredictorSize)) {
  45         fatal("Invalid local predictor size!\n");
  46     }
  47
  48     localPredictorSets = localPredictorSize / localCtrBits;
  49
  50     if (!isPowerOf2(localPredictorSets)) {
  51         fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
  52     }
  53
  54     // Setup the index mask.
  55     indexMask = localPredictorSets - 1;
  56
  57     DPRINTF(Fetch, "Branch predictor: index mask: %#x\n", indexMask);
  58
  59     // Setup the array of counters for the local predictor.
  60     localCtrs.resize(localPredictorSets);
  61
  62     for (unsigned i = 0; i < localPredictorSets; ++i)
  63         localCtrs[i].setBits(_localCtrBits);
  64
  65     DPRINTF(Fetch, "Branch predictor: local predictor size: %i\n",
  66             localPredictorSize);
  67
  68     DPRINTF(Fetch, "Branch predictor: local counter bits: %i\n", localCtrBits);
  69
  70     DPRINTF(Fetch, "Branch predictor: instruction shift amount: %i\n",
  71             instShiftAmt);
  72 }
  73
  74 void
  75 LocalBP::reset()
  76 {
  77     for (unsigned i = 0; i < localPredictorSets; ++i) {
  78         localCtrs[i].reset();
  79     }
  80 }
  81
  82 void
  83 LocalBP::BTBUpdate(Addr &branch_addr, void * &bp_history)
  84 {
  85 // Place holder for a function that is called to update predictor history when
  86 // a BTB entry is invalid or not found.
  87 }
  88
  89
  90 bool
  91 LocalBP::lookup(Addr &branch_addr, void * &bp_history)
  92 {
  93     bool taken;
  94     uint8_t counter_val;
  95     unsigned local_predictor_idx = getLocalIndex(branch_addr);
  96
  97     DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
  98             local_predictor_idx);
  99
 100     counter_val = localCtrs[local_predictor_idx].read();
 101
 102     DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
 103             (int)counter_val);
 104
 105     taken = getPrediction(counter_val);
 106
 107 #if 0
 108     // Speculative update.
 109     if (taken) {
 110         DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
 111         localCtrs[local_predictor_idx].increment();
 112     } else {
 113         DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
 114         localCtrs[local_predictor_idx].decrement();
 115     }
 116 #endif
 117
 118     return taken;
 119 }
 120
 121 void
 122 LocalBP::update(Addr &branch_addr, bool taken, void *bp_history)
 123 {
 124     assert(bp_history == NULL);
 125     unsigned local_predictor_idx;
 126
 127     // Update the local predictor.
 128     local_predictor_idx = getLocalIndex(branch_addr);
 129
 130     DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
 131             local_predictor_idx);
 132
 133     if (taken) {
 134         DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
 135         localCtrs[local_predictor_idx].increment();
 136     } else {
 137         DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
 138         localCtrs[local_predictor_idx].decrement();
 139     }
 140 }
 141
 142 inline
 143 bool
 144 LocalBP::getPrediction(uint8_t &count)
 145 {
 146     // Get the MSB of the count
 147     return (count >> (localCtrBits - 1));
 148 }
 149
 150 inline
 151 unsigned
 152 LocalBP::getLocalIndex(Addr &branch_addr)
 153 {
 154     return (branch_addr >> instShiftAmt) & indexMask;
 155 }