gem5  v21.2.0.0
2bit_local.cc
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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 namespace gem5
37 {
38 
39 namespace branch_prediction
40 {
41 
42 LocalBP::LocalBP(const LocalBPParams &params)
43  : BPredUnit(params),
44  localPredictorSize(params.localPredictorSize),
45  localCtrBits(params.localCtrBits),
46  localPredictorSets(localPredictorSize / localCtrBits),
47  localCtrs(localPredictorSets, SatCounter8(localCtrBits)),
48  indexMask(localPredictorSets - 1)
49 {
51  fatal("Invalid local predictor size!\n");
52  }
53 
55  fatal("Invalid number of local predictor sets! Check localCtrBits.\n");
56  }
57 
58  DPRINTF(Fetch, "index mask: %#x\n", indexMask);
59 
60  DPRINTF(Fetch, "local predictor size: %i\n",
62 
63  DPRINTF(Fetch, "local counter bits: %i\n", localCtrBits);
64 
65  DPRINTF(Fetch, "instruction shift amount: %i\n",
66  instShiftAmt);
67 }
68 
69 void
70 LocalBP::btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history)
71 {
72 // Place holder for a function that is called to update predictor history when
73 // a BTB entry is invalid or not found.
74 }
75 
76 
77 bool
78 LocalBP::lookup(ThreadID tid, Addr branch_addr, void * &bp_history)
79 {
80  bool taken;
81  unsigned local_predictor_idx = getLocalIndex(branch_addr);
82 
83  DPRINTF(Fetch, "Looking up index %#x\n",
84  local_predictor_idx);
85 
86  uint8_t counter_val = localCtrs[local_predictor_idx];
87 
88  DPRINTF(Fetch, "prediction is %i.\n",
89  (int)counter_val);
90 
91  taken = getPrediction(counter_val);
92 
93  return taken;
94 }
95 
96 void
97 LocalBP::update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history,
98  bool squashed, const StaticInstPtr & inst, Addr corrTarget)
99 {
100  assert(bp_history == NULL);
101  unsigned local_predictor_idx;
102 
103  // No state to restore, and we do not update on the wrong
104  // path.
105  if (squashed) {
106  return;
107  }
108 
109  // Update the local predictor.
110  local_predictor_idx = getLocalIndex(branch_addr);
111 
112  DPRINTF(Fetch, "Looking up index %#x\n", local_predictor_idx);
113 
114  if (taken) {
115  DPRINTF(Fetch, "Branch updated as taken.\n");
116  localCtrs[local_predictor_idx]++;
117  } else {
118  DPRINTF(Fetch, "Branch updated as not taken.\n");
119  localCtrs[local_predictor_idx]--;
120  }
121 }
122 
123 inline
124 bool
126 {
127  // Get the MSB of the count
128  return (count >> (localCtrBits - 1));
129 }
130 
131 inline
132 unsigned
134 {
135  return (branch_addr >> instShiftAmt) & indexMask;
136 }
137 
138 void
139 LocalBP::uncondBranch(ThreadID tid, Addr pc, void *&bp_history)
140 {
141 }
142 
143 } // namespace branch_prediction
144 } // namespace gem5
fatal
#define fatal(...)
This implements a cprintf based fatal() function.
Definition: logging.hh:190
gem5::branch_prediction::LocalBP::localCtrs
std::vector< SatCounter8 > localCtrs
Array of counters that make up the local predictor.
Definition: 2bit_local.hh:125
gem5::branch_prediction::LocalBP::LocalBP
LocalBP(const LocalBPParams &params)
Default branch predictor constructor.
Definition: 2bit_local.cc:42
2bit_local.hh
gem5::isPowerOf2
static constexpr bool isPowerOf2(const T &n)
Definition: intmath.hh:98
gem5::RefCountingPtr< StaticInst >
gem5::GenericSatCounter< uint8_t >
DPRINTF
#define DPRINTF(x,...)
Definition: trace.hh:186
gem5::X86ISA::count
count
Definition: misc.hh:709
gem5::branch_prediction::LocalBP::localPredictorSize
const unsigned localPredictorSize
Size of the local predictor.
Definition: 2bit_local.hh:116
gem5::branch_prediction::LocalBP::indexMask
const unsigned indexMask
Mask to get index bits.
Definition: 2bit_local.hh:128
gem5::branch_prediction::LocalBP::update
void update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history, bool squashed, const StaticInstPtr &inst, Addr corrTarget)
Updates the branch predictor with the actual result of a branch.
Definition: 2bit_local.cc:97
gem5::branch_prediction::LocalBP::localCtrBits
const unsigned localCtrBits
Number of bits of the local predictor's counters.
Definition: 2bit_local.hh:119
gem5::Addr
uint64_t Addr
Address type This will probably be moved somewhere else in the near future.
Definition: types.hh:147
gem5::branch_prediction::LocalBP::getLocalIndex
unsigned getLocalIndex(Addr &PC)
Calculates the local index based on the PC.
Definition: 2bit_local.cc:133
gem5::branch_prediction::BPredUnit::instShiftAmt
const unsigned instShiftAmt
Number of bits to shift instructions by for predictor addresses.
Definition: bpred_unit.hh:338
gem5::branch_prediction::LocalBP::localPredictorSets
const unsigned localPredictorSets
Number of sets.
Definition: 2bit_local.hh:122
gem5::branch_prediction::LocalBP::lookup
bool lookup(ThreadID tid, Addr branch_addr, void *&bp_history)
Looks up the given address in the branch predictor and returns a true/false value as to whether it is...
Definition: 2bit_local.cc:78
gem5::branch_prediction::LocalBP::btbUpdate
void btbUpdate(ThreadID tid, Addr branch_addr, void *&bp_history)
Updates the branch predictor to Not Taken if a BTB entry is invalid or not found.
Definition: 2bit_local.cc:70
gem5::MipsISA::pc
Bitfield< 4 > pc
Definition: pra_constants.hh:243
gem5::branch_prediction::BPredUnit
Basically a wrapper class to hold both the branch predictor and the BTB.
Definition: bpred_unit.hh:68
logging.hh
gem5::branch_prediction::LocalBP::uncondBranch
virtual void uncondBranch(ThreadID tid, Addr pc, void *&bp_history)
Definition: 2bit_local.cc:139
trace.hh
intmath.hh
gem5
Reference material can be found at the JEDEC website: UFS standard http://www.jedec....
Definition: tlb.cc:60
gem5::branch_prediction::LocalBP::getPrediction
bool getPrediction(uint8_t &count)
Returns the taken/not taken prediction given the value of the counter.
Definition: 2bit_local.cc:125
gem5::ThreadID
int16_t ThreadID
Thread index/ID type.
Definition: types.hh:242

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