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/*
* Copyright (c) 2011, 2014 ARM Limited
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*
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*
* Copyright (c) 2004-2006 The Regents of The University of Michigan
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#include "cpu/pred/tournament.hh"
#include "base/bitfield.hh"
#include "base/intmath.hh"
namespace gem5
{
namespace branch_prediction
{
TournamentBP::TournamentBP(const TournamentBPParams &params)
: BPredUnit(params),
localPredictorSize(params.localPredictorSize),
localCtrBits(params.localCtrBits),
localCtrs(localPredictorSize, SatCounter8(localCtrBits)),
localHistoryTableSize(params.localHistoryTableSize),
localHistoryBits(ceilLog2(params.localPredictorSize)),
globalPredictorSize(params.globalPredictorSize),
globalCtrBits(params.globalCtrBits),
globalCtrs(globalPredictorSize, SatCounter8(globalCtrBits)),
globalHistory(params.numThreads, 0),
globalHistoryBits(
ceilLog2(params.globalPredictorSize) >
ceilLog2(params.choicePredictorSize) ?
ceilLog2(params.globalPredictorSize) :
ceilLog2(params.choicePredictorSize)),
choicePredictorSize(params.choicePredictorSize),
choiceCtrBits(params.choiceCtrBits),
choiceCtrs(choicePredictorSize, SatCounter8(choiceCtrBits))
{
if (!isPowerOf2(localPredictorSize)) {
fatal("Invalid local predictor size!\n");
}
if (!isPowerOf2(globalPredictorSize)) {
fatal("Invalid global predictor size!\n");
}
localPredictorMask = mask(localHistoryBits);
if (!isPowerOf2(localHistoryTableSize)) {
fatal("Invalid local history table size!\n");
}
//Setup the history table for the local table
localHistoryTable.resize(localHistoryTableSize);
for (int i = 0; i < localHistoryTableSize; ++i)
localHistoryTable[i] = 0;
// Set up the global history mask
// this is equivalent to mask(log2(globalPredictorSize)
globalHistoryMask = globalPredictorSize - 1;
if (!isPowerOf2(choicePredictorSize)) {
fatal("Invalid choice predictor size!\n");
}
// Set up choiceHistoryMask
// this is equivalent to mask(log2(choicePredictorSize)
choiceHistoryMask = choicePredictorSize - 1;
//Set up historyRegisterMask
historyRegisterMask = mask(globalHistoryBits);
//Check that predictors don't use more bits than they have available
if (globalHistoryMask > historyRegisterMask) {
fatal("Global predictor too large for global history bits!\n");
}
if (choiceHistoryMask > historyRegisterMask) {
fatal("Choice predictor too large for global history bits!\n");
}
if (globalHistoryMask < historyRegisterMask &&
choiceHistoryMask < historyRegisterMask) {
inform("More global history bits than required by predictors\n");
}
// Set thresholds for the three predictors' counters
// This is equivalent to (2^(Ctr))/2 - 1
localThreshold = (1ULL << (localCtrBits - 1)) - 1;
globalThreshold = (1ULL << (globalCtrBits - 1)) - 1;
choiceThreshold = (1ULL << (choiceCtrBits - 1)) - 1;
}
inline
unsigned
TournamentBP::calcLocHistIdx(Addr &branch_addr)
{
// Get low order bits after removing instruction offset.
return (branch_addr >> instShiftAmt) & (localHistoryTableSize - 1);
}
inline
void
TournamentBP::updateGlobalHistTaken(ThreadID tid)
{
globalHistory[tid] = (globalHistory[tid] << 1) | 1;
globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
}
inline
void
TournamentBP::updateGlobalHistNotTaken(ThreadID tid)
{
globalHistory[tid] = (globalHistory[tid] << 1);
globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
}
inline
void
TournamentBP::updateLocalHistTaken(unsigned local_history_idx)
{
localHistoryTable[local_history_idx] =
(localHistoryTable[local_history_idx] << 1) | 1;
}
inline
void
TournamentBP::updateLocalHistNotTaken(unsigned local_history_idx)
{
localHistoryTable[local_history_idx] =
(localHistoryTable[local_history_idx] << 1);
}
void
TournamentBP::btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history)
{
unsigned local_history_idx = calcLocHistIdx(branch_addr);
//Update Global History to Not Taken (clear LSB)
globalHistory[tid] &= (historyRegisterMask & ~1ULL);
//Update Local History to Not Taken
localHistoryTable[local_history_idx] =
localHistoryTable[local_history_idx] & (localPredictorMask & ~1ULL);
}
bool
TournamentBP::lookup(ThreadID tid, Addr branch_addr, void * &bp_history)
{
bool local_prediction;
unsigned local_history_idx;
unsigned local_predictor_idx;
bool global_prediction;
bool choice_prediction;
//Lookup in the local predictor to get its branch prediction
local_history_idx = calcLocHistIdx(branch_addr);
local_predictor_idx = localHistoryTable[local_history_idx]
& localPredictorMask;
local_prediction = localCtrs[local_predictor_idx] > localThreshold;
//Lookup in the global predictor to get its branch prediction
global_prediction = globalThreshold <
globalCtrs[globalHistory[tid] & globalHistoryMask];
//Lookup in the choice predictor to see which one to use
choice_prediction = choiceThreshold <
choiceCtrs[globalHistory[tid] & choiceHistoryMask];
// Create BPHistory and pass it back to be recorded.
BPHistory *history = new BPHistory;
history->globalHistory = globalHistory[tid];
history->localPredTaken = local_prediction;
history->globalPredTaken = global_prediction;
history->globalUsed = choice_prediction;
history->localHistoryIdx = local_history_idx;
history->localHistory = local_predictor_idx;
bp_history = (void *)history;
assert(local_history_idx < localHistoryTableSize);
// Speculative update of the global history and the
// selected local history.
if (choice_prediction) {
if (global_prediction) {
updateGlobalHistTaken(tid);
updateLocalHistTaken(local_history_idx);
return true;
} else {
updateGlobalHistNotTaken(tid);
updateLocalHistNotTaken(local_history_idx);
return false;
}
} else {
if (local_prediction) {
updateGlobalHistTaken(tid);
updateLocalHistTaken(local_history_idx);
return true;
} else {
updateGlobalHistNotTaken(tid);
updateLocalHistNotTaken(local_history_idx);
return false;
}
}
}
void
TournamentBP::uncondBranch(ThreadID tid, Addr pc, void * &bp_history)
{
// Create BPHistory and pass it back to be recorded.
BPHistory *history = new BPHistory;
history->globalHistory = globalHistory[tid];
history->localPredTaken = true;
history->globalPredTaken = true;
history->globalUsed = true;
history->localHistoryIdx = invalidPredictorIndex;
history->localHistory = invalidPredictorIndex;
bp_history = static_cast<void *>(history);
updateGlobalHistTaken(tid);
}
void
TournamentBP::update(ThreadID tid, Addr branch_addr, bool taken,
void *bp_history, bool squashed,
const StaticInstPtr & inst, Addr corrTarget)
{
assert(bp_history);
BPHistory *history = static_cast<BPHistory *>(bp_history);
unsigned local_history_idx = calcLocHistIdx(branch_addr);
assert(local_history_idx < localHistoryTableSize);
// Unconditional branches do not use local history.
bool old_local_pred_valid = history->localHistory !=
invalidPredictorIndex;
// If this is a misprediction, restore the speculatively
// updated state (global history register and local history)
// and update again.
if (squashed) {
// Global history restore and update
globalHistory[tid] = (history->globalHistory << 1) | taken;
globalHistory[tid] &= historyRegisterMask;
// Local history restore and update.
if (old_local_pred_valid) {
localHistoryTable[local_history_idx] =
(history->localHistory << 1) | taken;
}
return;
}
unsigned old_local_pred_index = history->localHistory &
localPredictorMask;
assert(old_local_pred_index < localPredictorSize);
// Update the choice predictor to tell it which one was correct if
// there was a prediction.
if (history->localPredTaken != history->globalPredTaken &&
old_local_pred_valid)
{
// If the local prediction matches the actual outcome,
// decrement the counter. Otherwise increment the
// counter.
unsigned choice_predictor_idx =
history->globalHistory & choiceHistoryMask;
if (history->localPredTaken == taken) {
choiceCtrs[choice_predictor_idx]--;
} else if (history->globalPredTaken == taken) {
choiceCtrs[choice_predictor_idx]++;
}
}
// Update the counters with the proper
// resolution of the branch. Histories are updated
// speculatively, restored upon squash() calls, and
// recomputed upon update(squash = true) calls,
// so they do not need to be updated.
unsigned global_predictor_idx =
history->globalHistory & globalHistoryMask;
if (taken) {
globalCtrs[global_predictor_idx]++;
if (old_local_pred_valid) {
localCtrs[old_local_pred_index]++;
}
} else {
globalCtrs[global_predictor_idx]--;
if (old_local_pred_valid) {
localCtrs[old_local_pred_index]--;
}
}
// We're done with this history, now delete it.
delete history;
}
void
TournamentBP::squash(ThreadID tid, void *bp_history)
{
BPHistory *history = static_cast<BPHistory *>(bp_history);
// Restore global history to state prior to this branch.
globalHistory[tid] = history->globalHistory;
// Restore local history
if (history->localHistoryIdx != invalidPredictorIndex) {
localHistoryTable[history->localHistoryIdx] = history->localHistory;
}
// Delete this BPHistory now that we're done with it.
delete history;
}
#ifdef GEM5_DEBUG
int
TournamentBP::BPHistory::newCount = 0;
#endif
} // namespace branch_prediction
} // namespace gem5