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 bool
  83 LocalBP::lookup(Addr &branch_addr, void * &bp_history)
  84 {
  85     bool taken;
  86     uint8_t counter_val;
  87     unsigned local_predictor_idx = getLocalIndex(branch_addr);
  88
  89     DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
  90             local_predictor_idx);
  91
  92     counter_val = localCtrs[local_predictor_idx].read();
  93
  94     DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
  95             (int)counter_val);
  96
  97     taken = getPrediction(counter_val);
  98
  99 #if 0
 100     // Speculative update.
 101     if (taken) {
 102         DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
 103         localCtrs[local_predictor_idx].increment();
 104     } else {
 105         DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
 106         localCtrs[local_predictor_idx].decrement();
 107     }
 108 #endif
 109
 110     return taken;
 111 }
 112
 113 void
 114 LocalBP::update(Addr &branch_addr, bool taken, void *bp_history)
 115 {
 116     assert(bp_history == NULL);
 117     unsigned local_predictor_idx;
 118
 119     // Update the local predictor.
 120     local_predictor_idx = getLocalIndex(branch_addr);
 121
 122     DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
 123             local_predictor_idx);
 124
 125     if (taken) {
 126         DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
 127         localCtrs[local_predictor_idx].increment();
 128     } else {
 129         DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
 130         localCtrs[local_predictor_idx].decrement();
 131     }
 132 }
 133
 134 inline
 135 bool
 136 LocalBP::getPrediction(uint8_t &count)
 137 {
 138     // Get the MSB of the count
 139     return (count >> (localCtrBits - 1));
 140 }
 141
 142 inline
 143 unsigned
 144 LocalBP::getLocalIndex(Addr &branch_addr)
 145 {
 146     return (branch_addr >> instShiftAmt) & indexMask;
 147 }