BranchPredictor.py revision 13627
1# Copyright (c) 2012 Mark D. Hill and David A. Wood
2# Copyright (c) 2015 The University of Wisconsin
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
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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,
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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: Nilay Vaish and Dibakar Gope
29
30from m5.SimObject import SimObject
31from m5.params import *
32from m5.proxy import *
33
34class BranchPredictor(SimObject):
35    type = 'BranchPredictor'
36    cxx_class = 'BPredUnit'
37    cxx_header = "cpu/pred/bpred_unit.hh"
38    abstract = True
39
40    numThreads = Param.Unsigned(Parent.numThreads, "Number of threads")
41    BTBEntries = Param.Unsigned(4096, "Number of BTB entries")
42    BTBTagSize = Param.Unsigned(16, "Size of the BTB tags, in bits")
43    RASSize = Param.Unsigned(16, "RAS size")
44    instShiftAmt = Param.Unsigned(2, "Number of bits to shift instructions by")
45
46    useIndirect = Param.Bool(True, "Use indirect branch predictor")
47    indirectHashGHR = Param.Bool(True, "Hash branch predictor GHR")
48    indirectHashTargets = Param.Bool(True, "Hash path history targets")
49    indirectSets = Param.Unsigned(256, "Cache sets for indirect predictor")
50    indirectWays = Param.Unsigned(2, "Ways for indirect predictor")
51    indirectTagSize = Param.Unsigned(16, "Indirect target cache tag bits")
52    indirectPathLength = Param.Unsigned(3,
53        "Previous indirect targets to use for path history")
54
55
56
57class LocalBP(BranchPredictor):
58    type = 'LocalBP'
59    cxx_class = 'LocalBP'
60    cxx_header = "cpu/pred/2bit_local.hh"
61
62    localPredictorSize = Param.Unsigned(2048, "Size of local predictor")
63    localCtrBits = Param.Unsigned(2, "Bits per counter")
64
65
66class TournamentBP(BranchPredictor):
67    type = 'TournamentBP'
68    cxx_class = 'TournamentBP'
69    cxx_header = "cpu/pred/tournament.hh"
70
71    localPredictorSize = Param.Unsigned(2048, "Size of local predictor")
72    localCtrBits = Param.Unsigned(2, "Bits per counter")
73    localHistoryTableSize = Param.Unsigned(2048, "size of local history table")
74    globalPredictorSize = Param.Unsigned(8192, "Size of global predictor")
75    globalCtrBits = Param.Unsigned(2, "Bits per counter")
76    choicePredictorSize = Param.Unsigned(8192, "Size of choice predictor")
77    choiceCtrBits = Param.Unsigned(2, "Bits of choice counters")
78
79
80class BiModeBP(BranchPredictor):
81    type = 'BiModeBP'
82    cxx_class = 'BiModeBP'
83    cxx_header = "cpu/pred/bi_mode.hh"
84
85    globalPredictorSize = Param.Unsigned(8192, "Size of global predictor")
86    globalCtrBits = Param.Unsigned(2, "Bits per counter")
87    choicePredictorSize = Param.Unsigned(8192, "Size of choice predictor")
88    choiceCtrBits = Param.Unsigned(2, "Bits of choice counters")
89
90class TAGEBase(SimObject):
91    type = 'TAGEBase'
92    cxx_class = 'TAGEBase'
93    cxx_header = "cpu/pred/tage_base.hh"
94
95    numThreads = Param.Unsigned(Parent.numThreads, "Number of threads")
96    instShiftAmt = Param.Unsigned(Parent.instShiftAmt,
97        "Number of bits to shift instructions by")
98
99    nHistoryTables = Param.Unsigned(7, "Number of history tables")
100    minHist = Param.Unsigned(5, "Minimum history size of TAGE")
101    maxHist = Param.Unsigned(130, "Maximum history size of TAGE")
102
103    tagTableTagWidths = VectorParam.Unsigned(
104        [0, 9, 9, 10, 10, 11, 11, 12], "Tag size in TAGE tag tables")
105    logTagTableSizes = VectorParam.Int(
106        [13, 9, 9, 9, 9, 9, 9, 9], "Log2 of TAGE table sizes")
107    logRatioBiModalHystEntries = Param.Unsigned(2,
108        "Log num of prediction entries for a shared hysteresis bit " \
109        "for the Bimodal")
110
111    tagTableCounterBits = Param.Unsigned(3, "Number of tag table counter bits")
112    tagTableUBits = Param.Unsigned(2, "Number of tag table u bits")
113
114    histBufferSize = Param.Unsigned(2097152,
115            "A large number to track all branch histories(2MEntries default)")
116
117    pathHistBits = Param.Unsigned(16, "Path history size")
118    logUResetPeriod = Param.Unsigned(18,
119        "Log period in number of branches to reset TAGE useful counters")
120    numUseAltOnNa = Param.Unsigned(1, "Number of USE_ALT_ON_NA counters")
121    useAltOnNaBits = Param.Unsigned(4, "Size of the USE_ALT_ON_NA counter")
122
123    maxNumAlloc = Param.Unsigned(1,
124        "Max number of TAGE entries allocted on mispredict")
125
126    # List of enabled TAGE tables. If empty, all are enabled
127    noSkip = VectorParam.Bool([], "Vector of enabled TAGE tables")
128
129    speculativeHistUpdate = Param.Bool(True,
130        "Use speculative update for histories")
131
132# TAGE branch predictor as described in https://www.jilp.org/vol8/v8paper1.pdf
133# The default sizes below are for the 8C-TAGE configuration (63.5 Kbits)
134class TAGE(BranchPredictor):
135    type = 'TAGE'
136    cxx_class = 'TAGE'
137    cxx_header = "cpu/pred/tage.hh"
138    tage = Param.TAGEBase(TAGEBase(), "Tage object")
139
140class LTAGE_TAGE(TAGEBase):
141    nHistoryTables = 12
142    minHist = 4
143    maxHist = 640
144    tagTableTagWidths = [0, 7, 7, 8, 8, 9, 10, 11, 12, 12, 13, 14, 15]
145    logTagTableSizes = [14, 10, 10, 11, 11, 11, 11, 10, 10, 10, 10, 9, 9]
146    logUResetPeriod = 19
147
148class LoopPredictor(SimObject):
149    type = 'LoopPredictor'
150    cxx_class = 'LoopPredictor'
151    cxx_header = 'cpu/pred/loop_predictor.hh'
152
153    logSizeLoopPred = Param.Unsigned(8, "Log size of the loop predictor")
154    withLoopBits = Param.Unsigned(7, "Size of the WITHLOOP counter")
155    loopTableAgeBits = Param.Unsigned(8, "Number of age bits per loop entry")
156    loopTableConfidenceBits = Param.Unsigned(2,
157            "Number of confidence bits per loop entry")
158    loopTableTagBits = Param.Unsigned(14, "Number of tag bits per loop entry")
159    loopTableIterBits = Param.Unsigned(14, "Nuber of iteration bits per loop")
160    logLoopTableAssoc = Param.Unsigned(2, "Log loop predictor associativity")
161
162    # Parameters for enabling modifications to the loop predictor
163    # They have been copied from TAGE-GSC-IMLI
164    # (http://www.irisa.fr/alf/downloads/seznec/TAGE-GSC-IMLI.tar)
165    #
166    # All of them should be disabled to match the original LTAGE implementation
167    # (http://hpca23.cse.tamu.edu/taco/camino/cbp2/cbp-src/realistic-seznec.h)
168
169    # Add speculation
170    useSpeculation = Param.Bool(False, "Use speculation")
171
172    # Add hashing for calculating the loop table index
173    useHashing = Param.Bool(False, "Use hashing")
174
175    # Add a direction bit to the loop table entries
176    useDirectionBit = Param.Bool(False, "Use direction info")
177
178    # If true, use random to decide whether to allocate or not, and only try
179    # with one entry
180    restrictAllocation = Param.Bool(False,
181        "Restrict the allocation conditions")
182
183    initialLoopIter = Param.Unsigned(1, "Initial iteration number")
184    initialLoopAge = Param.Unsigned(255, "Initial age value")
185    optionalAgeReset = Param.Bool(True,
186        "Reset age bits optionally in some cases")
187
188
189# LTAGE branch predictor as described in
190# https://www.irisa.fr/caps/people/seznec/L-TAGE.pdf
191# It is basically a TAGE predictor plus a loop predictor
192# The differnt TAGE sizes are updated according to the paper values (256 Kbits)
193class LTAGE(TAGE):
194    type = 'LTAGE'
195    cxx_class = 'LTAGE'
196    cxx_header = "cpu/pred/ltage.hh"
197
198    tage = LTAGE_TAGE()
199    loop_predictor = Param.LoopPredictor(LoopPredictor(), "Loop predictor")
200