cpu/o3/btb.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/intmath.hh"
  30 #include "base/trace.hh"
  31 #include "cpu/o3/btb.hh"
  32
  33 DefaultBTB::DefaultBTB(unsigned _numEntries,
  34                        unsigned _tagBits,
  35                        unsigned _instShiftAmt)
  36     : numEntries(_numEntries),
  37       tagBits(_tagBits),
  38       instShiftAmt(_instShiftAmt)
  39 {
  40     // @todo Check to make sure num_entries is valid (a power of 2)
  41
  42     DPRINTF(Fetch, "BTB: Creating BTB object.\n");
  43
  44     btb = new BTBEntry[numEntries];
  45
  46     for (int i = 0; i < numEntries; ++i)
  47     {
  48         btb[i].valid = false;
  49     }
  50
  51     idxMask = numEntries - 1;
  52
  53     tagMask = (1 << tagBits) - 1;
  54
  55     tagShiftAmt = instShiftAmt + FloorLog2(numEntries);
  56 }
  57
  58 inline
  59 unsigned
  60 DefaultBTB::getIndex(const Addr &inst_PC)
  61 {
  62     // Need to shift PC over by the word offset.
  63     return (inst_PC >> instShiftAmt) & idxMask;
  64 }
  65
  66 inline
  67 Addr
  68 DefaultBTB::getTag(const Addr &inst_PC)
  69 {
  70     return (inst_PC >> tagShiftAmt) & tagMask;
  71 }
  72
  73 bool
  74 DefaultBTB::valid(const Addr &inst_PC)
  75 {
  76     unsigned btb_idx = getIndex(inst_PC);
  77
  78     Addr inst_tag = getTag(inst_PC);
  79
  80     assert(btb_idx < numEntries);
  81
  82     if (btb[btb_idx].valid && inst_tag == btb[btb_idx].tag) {
  83         return true;
  84     } else {
  85         return false;
  86     }
  87 }
  88
  89 // @todo Create some sort of return struct that has both whether or not the
  90 // address is valid, and also the address.  For now will just use addr = 0 to
  91 // represent invalid entry.
  92 Addr
  93 DefaultBTB::lookup(const Addr &inst_PC)
  94 {
  95     unsigned btb_idx = getIndex(inst_PC);
  96
  97     Addr inst_tag = getTag(inst_PC);
  98
  99     assert(btb_idx < numEntries);
 100
 101     if (btb[btb_idx].valid && inst_tag == btb[btb_idx].tag) {
 102         return btb[btb_idx].target;
 103     } else {
 104         return 0;
 105     }
 106 }
 107
 108 void
 109 DefaultBTB::update(const Addr &inst_PC, const Addr &target)
 110 {
 111     unsigned btb_idx = getIndex(inst_PC);
 112
 113     assert(btb_idx < numEntries);
 114
 115     btb[btb_idx].valid = true;
 116     btb[btb_idx].target = target;
 117     btb[btb_idx].tag = getTag(inst_PC);
 118 }