btb.cc revision 7720:65d338a8dba4 
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 * Authors: Kevin Lim
29 */
30
31#include "base/intmath.hh"
32#include "base/trace.hh"
33#include "cpu/pred/btb.hh"
34
35DefaultBTB::DefaultBTB(unsigned _numEntries,
36                       unsigned _tagBits,
37                       unsigned _instShiftAmt)
38    : numEntries(_numEntries),
39      tagBits(_tagBits),
40      instShiftAmt(_instShiftAmt)
41{
42    DPRINTF(Fetch, "BTB: Creating BTB object.\n");
43
44    if (!isPowerOf2(numEntries)) {
45        fatal("BTB entries is not a power of 2!");
46    }
47
48    btb.resize(numEntries);
49
50    for (unsigned i = 0; i < numEntries; ++i) {
51        btb[i].valid = false;
52    }
53
54    idxMask = numEntries - 1;
55
56    tagMask = (1 << tagBits) - 1;
57
58    tagShiftAmt = instShiftAmt + floorLog2(numEntries);
59}
60
61void
62DefaultBTB::reset()
63{
64    for (unsigned i = 0; i < numEntries; ++i) {
65        btb[i].valid = false;
66    }
67}
68
69inline
70unsigned
71DefaultBTB::getIndex(Addr instPC)
72{
73    // Need to shift PC over by the word offset.
74    return (instPC >> instShiftAmt) & idxMask;
75}
76
77inline
78Addr
79DefaultBTB::getTag(Addr instPC)
80{
81    return (instPC >> tagShiftAmt) & tagMask;
82}
83
84bool
85DefaultBTB::valid(Addr instPC, ThreadID tid)
86{
87    unsigned btb_idx = getIndex(instPC);
88
89    Addr inst_tag = getTag(instPC);
90
91    assert(btb_idx < numEntries);
92
93    if (btb[btb_idx].valid
94        && inst_tag == btb[btb_idx].tag
95        && btb[btb_idx].tid == tid) {
96        return true;
97    } else {
98        return false;
99    }
100}
101
102// @todo Create some sort of return struct that has both whether or not the
103// address is valid, and also the address.  For now will just use addr = 0 to
104// represent invalid entry.
105TheISA::PCState
106DefaultBTB::lookup(Addr instPC, ThreadID tid)
107{
108    unsigned btb_idx = getIndex(instPC);
109
110    Addr inst_tag = getTag(instPC);
111
112    assert(btb_idx < numEntries);
113
114    if (btb[btb_idx].valid
115        && inst_tag == btb[btb_idx].tag
116        && btb[btb_idx].tid == tid) {
117        return btb[btb_idx].target;
118    } else {
119        return 0;
120    }
121}
122
123void
124DefaultBTB::update(Addr instPC, const TheISA::PCState &target, ThreadID tid)
125{
126    unsigned btb_idx = getIndex(instPC);
127
128    assert(btb_idx < numEntries);
129
130    btb[btb_idx].tid = tid;
131    btb[btb_idx].valid = true;
132    btb[btb_idx].target = target;
133    btb[btb_idx].tag = getTag(instPC);
134}
135