src/cpu/o3/2bit_local_pred.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/o3/2bit_local_pred.hh"
  35
  36 LocalBP::LocalBP(unsigned _localPredictorSize,
  37                  unsigned _localCtrBits,
  38                  unsigned _instShiftAmt)
  39     : localPredictorSize(_localPredictorSize),
  40       localCtrBits(_localCtrBits),
  41       instShiftAmt(_instShiftAmt)
  42 {
  43     if (!isPowerOf2(localPredictorSize)) {
  44         fatal("Invalid local predictor size!\n");
  45     }
  46
  47     localPredictorSets = localPredictorSize / localCtrBits;
  48
  49     if (!isPowerOf2(localPredictorSets)) {
  50         fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
  51     }
  52
  53     // Setup the index mask.
  54     indexMask = localPredictorSets - 1;
  55
  56     DPRINTF(Fetch, "Branch predictor: index mask: %#x\n", indexMask);
  57
  58     // Setup the array of counters for the local predictor.
  59     localCtrs.resize(localPredictorSets);
  60
  61     for (int i = 0; i < localPredictorSets; ++i)
  62         localCtrs[i].setBits(_localCtrBits);
  63
  64     DPRINTF(Fetch, "Branch predictor: local predictor size: %i\n",
  65             localPredictorSize);
  66
  67     DPRINTF(Fetch, "Branch predictor: local counter bits: %i\n", localCtrBits);
  68
  69     DPRINTF(Fetch, "Branch predictor: instruction shift amount: %i\n",
  70             instShiftAmt);
  71 }
  72
  73 void
  74 LocalBP::reset()
  75 {
  76     for (int i = 0; i < localPredictorSets; ++i) {
  77         localCtrs[i].reset();
  78     }
  79 }
  80
  81 bool
  82 LocalBP::lookup(Addr &branch_addr, void * &bp_history)
  83 {
  84     bool taken;
  85     uint8_t counter_val;
  86     unsigned local_predictor_idx = getLocalIndex(branch_addr);
  87
  88     DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
  89             local_predictor_idx);
  90
  91     counter_val = localCtrs[local_predictor_idx].read();
  92
  93     DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
  94             (int)counter_val);
  95
  96     taken = getPrediction(counter_val);
  97
  98 #if 0
  99     // Speculative update.
 100     if (taken) {
 101         DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
 102         localCtrs[local_predictor_idx].increment();
 103     } else {
 104         DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
 105         localCtrs[local_predictor_idx].decrement();
 106     }
 107 #endif
 108
 109     return taken;
 110 }
 111
 112 void
 113 LocalBP::update(Addr &branch_addr, bool taken, void *bp_history)
 114 {
 115     assert(bp_history == NULL);
 116     unsigned local_predictor_idx;
 117
 118     // Update the local predictor.
 119     local_predictor_idx = getLocalIndex(branch_addr);
 120
 121     DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
 122             local_predictor_idx);
 123
 124     if (taken) {
 125         DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
 126         localCtrs[local_predictor_idx].increment();
 127     } else {
 128         DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
 129         localCtrs[local_predictor_idx].decrement();
 130     }
 131 }
 132
 133 inline
 134 bool
 135 LocalBP::getPrediction(uint8_t &count)
 136 {
 137     // Get the MSB of the count
 138     return (count >> (localCtrBits - 1));
 139 }
 140
 141 inline
 142 unsigned
 143 LocalBP::getLocalIndex(Addr &branch_addr)
 144 {
 145     return (branch_addr >> instShiftAmt) & indexMask;
 146 }