src/cpu/o3/2bit_local_pred.cc

   1 /*
   2  * Copyright (c) 2004-2005 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
  29 #include "base/trace.hh"
  30 #include "cpu/o3/2bit_local_pred.hh"
  31
  32 DefaultBP::DefaultBP(unsigned _localPredictorSize,
  33                      unsigned _localCtrBits,
  34                      unsigned _instShiftAmt)
  35     : localPredictorSize(_localPredictorSize),
  36       localCtrBits(_localCtrBits),
  37       instShiftAmt(_instShiftAmt)
  38 {
  39     // Should do checks here to make sure sizes are correct (powers of 2).
  40
  41     // Setup the index mask.
  42     indexMask = localPredictorSize - 1;
  43
  44     DPRINTF(Fetch, "Branch predictor: index mask: %#x\n", indexMask);
  45
  46     // Setup the array of counters for the local predictor.
  47     localCtrs = new SatCounter[localPredictorSize];
  48
  49     for (int i = 0; i < localPredictorSize; ++i)
  50         localCtrs[i].setBits(_localCtrBits);
  51
  52     DPRINTF(Fetch, "Branch predictor: local predictor size: %i\n",
  53             localPredictorSize);
  54
  55     DPRINTF(Fetch, "Branch predictor: local counter bits: %i\n", localCtrBits);
  56
  57     DPRINTF(Fetch, "Branch predictor: instruction shift amount: %i\n",
  58             instShiftAmt);
  59 }
  60
  61 bool
  62 DefaultBP::lookup(Addr &branch_addr)
  63 {
  64     bool taken;
  65     uint8_t local_prediction;
  66     unsigned local_predictor_idx = getLocalIndex(branch_addr);
  67
  68     DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
  69             local_predictor_idx);
  70
  71     assert(local_predictor_idx < localPredictorSize);
  72
  73     local_prediction = localCtrs[local_predictor_idx].read();
  74
  75     DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
  76             (int)local_prediction);
  77
  78     taken = getPrediction(local_prediction);
  79
  80 #if 0
  81     // Speculative update.
  82     if (taken) {
  83         DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
  84         localCtrs[local_predictor_idx].increment();
  85     } else {
  86         DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
  87         localCtrs[local_predictor_idx].decrement();
  88     }
  89 #endif
  90
  91     return taken;
  92 }
  93
  94 void
  95 DefaultBP::update(Addr &branch_addr, bool taken)
  96 {
  97     unsigned local_predictor_idx;
  98
  99     // Update the local predictor.
 100     local_predictor_idx = getLocalIndex(branch_addr);
 101
 102     DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
 103             local_predictor_idx);
 104
 105     assert(local_predictor_idx < localPredictorSize);
 106
 107     if (taken) {
 108         DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
 109         localCtrs[local_predictor_idx].increment();
 110     } else {
 111         DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
 112         localCtrs[local_predictor_idx].decrement();
 113     }
 114 }
 115
 116 inline
 117 bool
 118 DefaultBP::getPrediction(uint8_t &count)
 119 {
 120     // Get the MSB of the count
 121     return (count >> (localCtrBits - 1));
 122 }
 123
 124 inline
 125 unsigned
 126 DefaultBP::getLocalIndex(Addr &branch_addr)
 127 {
 128     return (branch_addr >> instShiftAmt) & indexMask;
 129 }