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 Params *params)
  38     : BPredUnit(params),
  39       localPredictorSize(params->localPredictorSize),
  40       localCtrBits(params->localCtrBits),
  41       instShiftAmt(params->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, "index mask: %#x\n", indexMask);
  57
  58     // Setup the array of counters for the local predictor.
  59     localCtrs.resize(localPredictorSets);
  60
  61     for (unsigned i = 0; i < localPredictorSets; ++i)
  62         localCtrs[i].setBits(localCtrBits);
  63
  64     DPRINTF(Fetch, "local predictor size: %i\n",
  65             localPredictorSize);
  66
  67     DPRINTF(Fetch, "local counter bits: %i\n", localCtrBits);
  68
  69     DPRINTF(Fetch, "instruction shift amount: %i\n",
  70             instShiftAmt);
  71 }
  72
  73 void
  74 LocalBP::reset()
  75 {
  76     for (unsigned i = 0; i < localPredictorSets; ++i) {
  77         localCtrs[i].reset();
  78     }
  79 }
  80
  81 void
  82 LocalBP::btbUpdate(Addr branch_addr, void * &bp_history)
  83 {
  84 // Place holder for a function that is called to update predictor history when
  85 // a BTB entry is invalid or not found.
  86 }
  87
  88
  89 bool
  90 LocalBP::lookup(Addr branch_addr, void * &bp_history)
  91 {
  92     bool taken;
  93     uint8_t counter_val;
  94     unsigned local_predictor_idx = getLocalIndex(branch_addr);
  95
  96     DPRINTF(Fetch, "Looking up index %#x\n",
  97             local_predictor_idx);
  98
  99     counter_val = localCtrs[local_predictor_idx].read();
 100
 101     DPRINTF(Fetch, "prediction is %i.\n",
 102             (int)counter_val);
 103
 104     taken = getPrediction(counter_val);
 105
 106 #if 0
 107     // Speculative update.
 108     if (taken) {
 109         DPRINTF(Fetch, "Branch updated as taken.\n");
 110         localCtrs[local_predictor_idx].increment();
 111     } else {
 112         DPRINTF(Fetch, "Branch updated as not taken.\n");
 113         localCtrs[local_predictor_idx].decrement();
 114     }
 115 #endif
 116
 117     return taken;
 118 }
 119
 120 void
 121 LocalBP::update(Addr branch_addr, bool taken, void *bp_history, bool squashed)
 122 {
 123     assert(bp_history == NULL);
 124     unsigned local_predictor_idx;
 125
 126     // Update the local predictor.
 127     local_predictor_idx = getLocalIndex(branch_addr);
 128
 129     DPRINTF(Fetch, "Looking up index %#x\n", local_predictor_idx);
 130
 131     if (taken) {
 132         DPRINTF(Fetch, "Branch updated as taken.\n");
 133         localCtrs[local_predictor_idx].increment();
 134     } else {
 135         DPRINTF(Fetch, "Branch updated as not taken.\n");
 136         localCtrs[local_predictor_idx].decrement();
 137     }
 138 }
 139
 140 inline
 141 bool
 142 LocalBP::getPrediction(uint8_t &count)
 143 {
 144     // Get the MSB of the count
 145     return (count >> (localCtrBits - 1));
 146 }
 147
 148 inline
 149 unsigned
 150 LocalBP::getLocalIndex(Addr &branch_addr)
 151 {
 152     return (branch_addr >> instShiftAmt) & indexMask;
 153 }
 154
 155 void
 156 LocalBP::uncondBranch(void *&bp_history)
 157 {
 158 }