/*
- * Copyright (c) 2011 ARM Limited
+ * Copyright (c) 2011, 2014 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
* Authors: Kevin Lim
*/
-#include "base/intmath.hh"
#include "cpu/pred/tournament.hh"
-TournamentBP::TournamentBP(unsigned _localPredictorSize,
- unsigned _localCtrBits,
- unsigned _localHistoryTableSize,
- unsigned _localHistoryBits,
- unsigned _globalPredictorSize,
- unsigned _globalHistoryBits,
- unsigned _globalCtrBits,
- unsigned _choicePredictorSize,
- unsigned _choiceCtrBits,
- unsigned _instShiftAmt)
- : localPredictorSize(_localPredictorSize),
- localCtrBits(_localCtrBits),
- localHistoryTableSize(_localHistoryTableSize),
- localHistoryBits(_localHistoryBits),
- globalPredictorSize(_globalPredictorSize),
- globalCtrBits(_globalCtrBits),
- globalHistoryBits(_globalHistoryBits),
- choicePredictorSize(_globalPredictorSize),
- choiceCtrBits(_choiceCtrBits),
- instShiftAmt(_instShiftAmt)
+#include "base/bitfield.hh"
+#include "base/intmath.hh"
+
+TournamentBP::TournamentBP(const TournamentBPParams *params)
+ : BPredUnit(params),
+ localPredictorSize(params->localPredictorSize),
+ localCtrBits(params->localCtrBits),
+ localHistoryTableSize(params->localHistoryTableSize),
+ localHistoryBits(ceilLog2(params->localPredictorSize)),
+ globalPredictorSize(params->globalPredictorSize),
+ globalCtrBits(params->globalCtrBits),
+ globalHistory(params->numThreads, 0),
+ globalHistoryBits(
+ ceilLog2(params->globalPredictorSize) >
+ ceilLog2(params->choicePredictorSize) ?
+ ceilLog2(params->globalPredictorSize) :
+ ceilLog2(params->choicePredictorSize)),
+ choicePredictorSize(params->choicePredictorSize),
+ choiceCtrBits(params->choiceCtrBits)
{
if (!isPowerOf2(localPredictorSize)) {
fatal("Invalid local predictor size!\n");
}
- //Setup the array of counters for the local predictor
+ if (!isPowerOf2(globalPredictorSize)) {
+ fatal("Invalid global predictor size!\n");
+ }
+
+ //Set up the array of counters for the local predictor
localCtrs.resize(localPredictorSize);
for (int i = 0; i < localPredictorSize; ++i)
localCtrs[i].setBits(localCtrBits);
- localPredictorMask = floorPow2(localPredictorSize) - 1;
+ localPredictorMask = mask(localHistoryBits);
if (!isPowerOf2(localHistoryTableSize)) {
fatal("Invalid local history table size!\n");
for (int i = 0; i < localHistoryTableSize; ++i)
localHistoryTable[i] = 0;
- // Setup the local history mask
- localHistoryMask = (1 << localHistoryBits) - 1;
-
- if (!isPowerOf2(globalPredictorSize)) {
- fatal("Invalid global predictor size!\n");
- }
-
//Setup the array of counters for the global predictor
globalCtrs.resize(globalPredictorSize);
for (int i = 0; i < globalPredictorSize; ++i)
globalCtrs[i].setBits(globalCtrBits);
- //Clear the global history
- globalHistory = 0;
- // Setup the global history mask
- globalHistoryMask = (1 << globalHistoryBits) - 1;
+ // 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;
+
//Setup the array of counters for the choice predictor
choiceCtrs.resize(choicePredictorSize);
for (int i = 0; i < choicePredictorSize; ++i)
choiceCtrs[i].setBits(choiceCtrBits);
- // @todo: Allow for different thresholds between the predictors.
- threshold = (1 << (localCtrBits - 1)) - 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 = (ULL(1) << (localCtrBits - 1)) - 1;
+ globalThreshold = (ULL(1) << (globalCtrBits - 1)) - 1;
+ choiceThreshold = (ULL(1) << (choiceCtrBits - 1)) - 1;
}
inline
inline
void
-TournamentBP::updateGlobalHistTaken()
+TournamentBP::updateGlobalHistTaken(ThreadID tid)
{
- globalHistory = (globalHistory << 1) | 1;
- globalHistory = globalHistory & globalHistoryMask;
+ globalHistory[tid] = (globalHistory[tid] << 1) | 1;
+ globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
}
inline
void
-TournamentBP::updateGlobalHistNotTaken()
+TournamentBP::updateGlobalHistNotTaken(ThreadID tid)
{
- globalHistory = (globalHistory << 1);
- globalHistory = globalHistory & globalHistoryMask;
+ globalHistory[tid] = (globalHistory[tid] << 1);
+ globalHistory[tid] = globalHistory[tid] & historyRegisterMask;
}
inline
void
-TournamentBP::BTBUpdate(Addr &branch_addr, void * &bp_history)
+TournamentBP::btbUpdate(ThreadID tid, Addr branch_addr, void * &bp_history)
{
unsigned local_history_idx = calcLocHistIdx(branch_addr);
- //Update Global History to Not Taken
- globalHistory = globalHistory & (globalHistoryMask - 1);
+ //Update Global History to Not Taken (clear LSB)
+ globalHistory[tid] &= (historyRegisterMask & ~ULL(1));
//Update Local History to Not Taken
localHistoryTable[local_history_idx] =
- localHistoryTable[local_history_idx] & (localPredictorMask - 1);
+ localHistoryTable[local_history_idx] & (localPredictorMask & ~ULL(1));
}
bool
-TournamentBP::lookup(Addr &branch_addr, void * &bp_history)
+TournamentBP::lookup(ThreadID tid, Addr branch_addr, void * &bp_history)
{
bool local_prediction;
unsigned local_history_idx;
local_history_idx = calcLocHistIdx(branch_addr);
local_predictor_idx = localHistoryTable[local_history_idx]
& localPredictorMask;
- local_prediction = localCtrs[local_predictor_idx].read() > threshold;
+ local_prediction = localCtrs[local_predictor_idx].read() > localThreshold;
//Lookup in the global predictor to get its branch prediction
- global_prediction = globalCtrs[globalHistory].read() > threshold;
+ global_prediction = globalThreshold <
+ globalCtrs[globalHistory[tid] & globalHistoryMask].read();
//Lookup in the choice predictor to see which one to use
- choice_prediction = choiceCtrs[globalHistory].read() > threshold;
+ choice_prediction = choiceThreshold <
+ choiceCtrs[globalHistory[tid] & choiceHistoryMask].read();
// Create BPHistory and pass it back to be recorded.
BPHistory *history = new BPHistory;
- history->globalHistory = globalHistory;
+ 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(globalHistory < globalPredictorSize &&
- local_history_idx < localHistoryTableSize &&
- local_predictor_idx < localPredictorSize);
+ assert(local_history_idx < localHistoryTableSize);
- // Commented code is for doing speculative update of counters and
- // all histories.
+ // Speculative update of the global history and the
+ // selected local history.
if (choice_prediction) {
if (global_prediction) {
- updateGlobalHistTaken();
+ updateGlobalHistTaken(tid);
updateLocalHistTaken(local_history_idx);
return true;
} else {
- updateGlobalHistNotTaken();
+ updateGlobalHistNotTaken(tid);
updateLocalHistNotTaken(local_history_idx);
return false;
}
} else {
if (local_prediction) {
- updateGlobalHistTaken();
+ updateGlobalHistTaken(tid);
updateLocalHistTaken(local_history_idx);
return true;
} else {
- updateGlobalHistNotTaken();
+ updateGlobalHistNotTaken(tid);
updateLocalHistNotTaken(local_history_idx);
return false;
}
}
void
-TournamentBP::uncondBr(void * &bp_history)
+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;
+ 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();
+ updateGlobalHistTaken(tid);
}
void
-TournamentBP::update(Addr &branch_addr, bool taken, void *bp_history,
- bool squashed)
+TournamentBP::update(ThreadID tid, Addr branch_addr, bool taken,
+ void *bp_history, bool squashed)
{
- unsigned local_history_idx;
- unsigned local_predictor_idx M5_VAR_USED;
- unsigned local_predictor_hist;
+ assert(bp_history);
- // Get the local predictor's current prediction
- local_history_idx = calcLocHistIdx(branch_addr);
- local_predictor_hist = localHistoryTable[local_history_idx];
- local_predictor_idx = local_predictor_hist & localPredictorMask;
+ BPHistory *history = static_cast<BPHistory *>(bp_history);
- // Update the choice predictor to tell it which one was correct if
- // there was a prediction.
- if (bp_history) {
- BPHistory *history = static_cast<BPHistory *>(bp_history);
- if (history->localPredTaken != history->globalPredTaken) {
- // If the local prediction matches the actual outcome,
- // decerement the counter. Otherwise increment the
- // counter.
- if (history->localPredTaken == taken) {
- choiceCtrs[history->globalHistory].decrement();
- } else if (history->globalPredTaken == taken){
- choiceCtrs[history->globalHistory].increment();
- }
+ unsigned local_history_idx = calcLocHistIdx(branch_addr);
- }
+ assert(local_history_idx < localHistoryTableSize);
- // Update the counters and local history with the proper
- // resolution of the branch. Global history is updated
- // speculatively and restored upon squash() calls, so it does not
- // need to be updated.
- unsigned old_local_pred_index = history->localHistory
- & localPredictorMask;
- if (taken) {
- globalCtrs[history->globalHistory].increment();
- if (old_local_pred_index != invalidPredictorIndex) {
- localCtrs[old_local_pred_index].increment();
- }
- } else {
- globalCtrs[history->globalHistory].decrement();
- if (old_local_pred_index != invalidPredictorIndex) {
- localCtrs[old_local_pred_index].decrement();
- }
- }
-
- if (squashed) {
- if (taken) {
- globalHistory = (history->globalHistory << 1) | 1;
- globalHistory = globalHistory & globalHistoryMask;
- if (history->localHistory != invalidPredictorIndex)
- localHistoryTable[local_history_idx] =
- (history->localHistory << 1) | 1;
- } else {
- globalHistory = (history->globalHistory << 1);
- globalHistory = globalHistory & globalHistoryMask;
- if (history->localHistory != invalidPredictorIndex) {
- localHistoryTable[local_history_idx] =
- history->localHistory << 1;
- }
- }
+ // 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;
}
- // We're done with this history, now delete it.
- delete history;
+
+ return;
}
- assert(globalHistory < globalPredictorSize &&
- local_history_idx < localHistoryTableSize &&
- local_predictor_idx < localPredictorSize);
+ 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].decrement();
+ } else if (history->globalPredTaken == taken) {
+ choiceCtrs[choice_predictor_idx].increment();
+ }
+ }
+
+ // 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].increment();
+ if (old_local_pred_valid) {
+ localCtrs[old_local_pred_index].increment();
+ }
+ } else {
+ globalCtrs[global_predictor_idx].decrement();
+ if (old_local_pred_valid) {
+ localCtrs[old_local_pred_index].decrement();
+ }
+ }
+
+ // We're done with this history, now delete it.
+ delete history;
}
void
-TournamentBP::squash(void *bp_history)
+TournamentBP::squash(ThreadID tid, void *bp_history)
{
BPHistory *history = static_cast<BPHistory *>(bp_history);
// Restore global history to state prior to this branch.
- globalHistory = history->globalHistory;
+ 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;
}
+TournamentBP*
+TournamentBPParams::create()
+{
+ return new TournamentBP(this);
+}
+
+unsigned
+TournamentBP::getGHR(ThreadID tid, void *bp_history) const
+{
+ return static_cast<BPHistory *>(bp_history)->globalHistory;
+}
+
#ifdef DEBUG
int
TournamentBP::BPHistory::newCount = 0;
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