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
  29 #include "cpu/pred/2bit_local.hh"
  30
  31 #include "base/intmath.hh"
  32 #include "base/logging.hh"
  33 #include "base/trace.hh"
  34 #include "debug/Fetch.hh"
  35
  36 LocalBP::LocalBP(const LocalBPParams &params)
  37     : BPredUnit(params),
  38       localPredictorSize(params.localPredictorSize),
  39       localCtrBits(params.localCtrBits),
  40       localPredictorSets(localPredictorSize / localCtrBits),
  41       localCtrs(localPredictorSets, SatCounter(localCtrBits)),
  42       indexMask(localPredictorSets - 1)
  43 {
  44     if (!isPowerOf2(localPredictorSize)) {
  45         fatal("Invalid local predictor size!\n");
  46     }
  47
  48     if (!isPowerOf2(localPredictorSets)) {
  49         fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
  50     }
  51
  52     DPRINTF(Fetch, "index mask: %#x\n", indexMask);
  53
  54     DPRINTF(Fetch, "local predictor size: %i\n",
  55             localPredictorSize);
  56
  57     DPRINTF(Fetch, "local counter bits: %i\n", localCtrBits);
  58
  59     DPRINTF(Fetch, "instruction shift amount: %i\n",
  60             instShiftAmt);
  61 }
  62
  63 void
  64 LocalBP::btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history)
  65 {
  66 // Place holder for a function that is called to update predictor history when
  67 // a BTB entry is invalid or not found.
  68 }
  69
  70
  71 bool
  72 LocalBP::lookup(ThreadID tid, Addr branch_addr, void * &bp_history)
  73 {
  74     bool taken;
  75     unsigned local_predictor_idx = getLocalIndex(branch_addr);
  76
  77     DPRINTF(Fetch, "Looking up index %#x\n",
  78             local_predictor_idx);
  79
  80     uint8_t counter_val = localCtrs[local_predictor_idx];
  81
  82     DPRINTF(Fetch, "prediction is %i.\n",
  83             (int)counter_val);
  84
  85     taken = getPrediction(counter_val);
  86
  87     return taken;
  88 }
  89
  90 void
  91 LocalBP::update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
  92                 bool squashed, const StaticInstPtr & inst, Addr corrTarget)
  93 {
  94     assert(bp_history == NULL);
  95     unsigned local_predictor_idx;
  96
  97     // No state to restore, and we do not update on the wrong
  98     // path.
  99     if (squashed) {
 100         return;
 101     }
 102
 103     // Update the local predictor.
 104     local_predictor_idx = getLocalIndex(branch_addr);
 105
 106     DPRINTF(Fetch, "Looking up index %#x\n", local_predictor_idx);
 107
 108     if (taken) {
 109         DPRINTF(Fetch, "Branch updated as taken.\n");
 110         localCtrs[local_predictor_idx]++;
 111     } else {
 112         DPRINTF(Fetch, "Branch updated as not taken.\n");
 113         localCtrs[local_predictor_idx]--;
 114     }
 115 }
 116
 117 inline
 118 bool
 119 LocalBP::getPrediction(uint8_t &count)
 120 {
 121     // Get the MSB of the count
 122     return (count >> (localCtrBits - 1));
 123 }
 124
 125 inline
 126 unsigned
 127 LocalBP::getLocalIndex(Addr &branch_addr)
 128 {
 129     return (branch_addr >> instShiftAmt) & indexMask;
 130 }
 131
 132 void
 133 LocalBP::uncondBranch(ThreadID tid, Addr pc, void *&bp_history)
 134 {
 135 }