cpu: Added parameters to enable/disable features in LTAGE
They are for the following features in the LTAGE loop predictor:
- Hashing for calculating the loop table entry
- Add direction information
- Add speculative iteration number information
Change-Id: I395f4526163ee0d0229d1e87cde2bb046f1dd43a
Signed-off-by: Pau Cabre <pau.cabre@metempsy.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14597
Reviewed-by: Ilias Vougioukas <ilias.vougioukas@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Louis Delhez <ldelhez@ucla.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
diff --git a/src/cpu/pred/BranchPredictor.py b/src/cpu/pred/BranchPredictor.py
index 2c622cd..0c1e9c2 100644
--- a/src/cpu/pred/BranchPredictor.py
+++ b/src/cpu/pred/BranchPredictor.py
@@ -143,3 +143,19 @@
loopTableIterBits = Param.Unsigned(14, "Nuber of iteration bits per loop")
logLoopTableAssoc = Param.Unsigned(2, "Log loop predictor associativity")
+ # Parameters for enabling modifications to the loop predictor
+ # They have been copied from ISL-TAGE
+ # (https://www.jilp.org/jwac-2/program/03_seznec.tgz)
+ #
+ # All of them should be disabled to match the original LTAGE implementation
+ # (http://hpca23.cse.tamu.edu/taco/camino/cbp2/cbp-src/realistic-seznec.h)
+
+ # Add speculation
+ useSpeculation = Param.Bool(False, "Use speculation")
+
+ # Add hashing for calculating the loop table index
+ useHashing = Param.Bool(False, "Use hashing")
+
+ # Add a direction bit to the loop table entries
+ useDirectionBit = Param.Bool(False, "Use direction info")
+
diff --git a/src/cpu/pred/ltage.cc b/src/cpu/pred/ltage.cc
index 73f4777..8f332b3 100644
--- a/src/cpu/pred/ltage.cc
+++ b/src/cpu/pred/ltage.cc
@@ -58,8 +58,12 @@
confidenceThreshold((1 << loopTableConfidenceBits) - 1),
loopTagMask((1 << loopTableTagBits) - 1),
loopNumIterMask((1 << loopTableIterBits) - 1),
+ loopSetMask((1 << (logSizeLoopPred - logLoopTableAssoc)) - 1),
loopUseCounter(0),
- withLoopBits(params->withLoopBits)
+ withLoopBits(params->withLoopBits),
+ useDirectionBit(params->useDirectionBit),
+ useSpeculation(params->useSpeculation),
+ useHashing(params->useHashing)
{
// we use uint16_t type for these vales, so they cannot be more than
// 16 bits
@@ -82,12 +86,28 @@
// by logLoopTableAssoc in order to return the index of the first of the
// N entries of the set
Addr mask = (ULL(1) << (logSizeLoopPred - logLoopTableAssoc)) - 1;
- return (((pc_in >> instShiftAmt) & mask) << logLoopTableAssoc);
+ Addr pc = pc_in >> instShiftAmt;
+ if (useHashing) {
+ // copied from TAGE-SC-L
+ // (http://www.jilp.org/cbp2016/code/AndreSeznecLimited.tar.gz)
+ pc ^= (pc_in >> (instShiftAmt + logLoopTableAssoc));
+ }
+ return ((pc & mask) << logLoopTableAssoc);
+}
+
+int
+LTAGE::finallindex(int index, int lowPcBits, int way) const
+{
+ // copied from TAGE-SC-L
+ // (http://www.jilp.org/cbp2016/code/AndreSeznecLimited.tar.gz)
+ return (useHashing ? (index ^ ((lowPcBits >> way) << logLoopTableAssoc)) :
+ (index))
+ + way;
}
//loop prediction: only used if high confidence
bool
-LTAGE::getLoop(Addr pc, LTageBranchInfo* bi) const
+LTAGE::getLoop(Addr pc, LTageBranchInfo* bi, bool speculative) const
{
bi->loopHit = -1;
bi->loopPredValid = false;
@@ -95,17 +115,25 @@
unsigned pcShift = instShiftAmt + logSizeLoopPred - logLoopTableAssoc;
bi->loopTag = ((pc) >> pcShift) & loopTagMask;
+ if (useHashing) {
+ bi->loopTag ^= ((pc >> (pcShift + logSizeLoopPred)) & loopTagMask);
+ bi->loopLowPcBits = (pc >> pcShift) & loopSetMask;
+ }
+
for (int i = 0; i < (1 << logLoopTableAssoc); i++) {
- if (ltable[bi->loopIndex + i].tag == bi->loopTag) {
+ int idx = finallindex(bi->loopIndex, bi->loopLowPcBits, i);
+ if (ltable[idx].tag == bi->loopTag) {
bi->loopHit = i;
bi->loopPredValid =
- ltable[bi->loopIndex + i].confidence == confidenceThreshold;
- bi->currentIter = ltable[bi->loopIndex + i].currentIterSpec;
- if (ltable[bi->loopIndex + i].currentIterSpec + 1 ==
- ltable[bi->loopIndex + i].numIter) {
- return !(ltable[bi->loopIndex + i].dir);
- }else {
- return (ltable[bi->loopIndex + i].dir);
+ ltable[idx].confidence == confidenceThreshold;
+
+ uint16_t iter = speculative ? ltable[idx].currentIterSpec
+ : ltable[idx].currentIter;
+
+ if ((iter + 1) == ltable[idx].numIter) {
+ return useDirectionBit ? !(ltable[idx].dir) : false;
+ } else {
+ return useDirectionBit ? (ltable[idx].dir) : true;
}
}
}
@@ -113,10 +141,10 @@
}
void
-LTAGE::specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
+LTAGE::specLoopUpdate(bool taken, LTageBranchInfo* bi)
{
if (bi->loopHit>=0) {
- int index = lindex(pc);
+ int index = finallindex(bi->loopIndex, bi->loopLowPcBits, bi->loopHit);
if (taken != ltable[index].dir) {
ltable[index].currentIterSpec = 0;
} else {
@@ -129,7 +157,7 @@
void
LTAGE::loopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
{
- int idx = bi->loopIndex + bi->loopHit;
+ int idx = finallindex(bi->loopIndex, bi->loopLowPcBits, bi->loopHit);
if (bi->loopHit >= 0) {
//already a hit
if (bi->loopPredValid) {
@@ -158,7 +186,7 @@
}
}
- if (taken != ltable[idx].dir) {
+ if (taken != (useDirectionBit ? ltable[idx].dir : true)) {
if (ltable[idx].currentIter == ltable[idx].numIter) {
DPRINTF(LTage, "Loop End predicted successfully:%lx\n", pc);
@@ -167,7 +195,7 @@
//just do not predict when the loop count is 1 or 2
if (ltable[idx].numIter < 3) {
// free the entry
- ltable[idx].dir = taken;
+ ltable[idx].dir = taken; // ignored if no useDirectionBit
ltable[idx].numIter = 0;
ltable[idx].age = 0;
ltable[idx].confidence = 0;
@@ -189,7 +217,9 @@
ltable[idx].currentIter = 0;
}
- } else if (taken) {
+ } else if (useDirectionBit ?
+ ((bi->loopPredValid ? bi->loopPred : bi->tagePred) != taken) :
+ taken) {
//try to allocate an entry on taken branch
int nrand = random_mt.random<int>();
for (int i = 0; i < (1 << logLoopTableAssoc); i++) {
@@ -198,7 +228,7 @@
if (ltable[idx].age == 0) {
DPRINTF(LTage, "Allocating loop pred entry for branch %lx\n",
pc);
- ltable[idx].dir = !taken;
+ ltable[idx].dir = !taken; // ignored if no useDirectionBit
ltable[idx].tag = bi->loopTag;
ltable[idx].numIter = 0;
ltable[idx].age = (1 << loopTableAgeBits) - 1;
@@ -224,7 +254,8 @@
bool pred_taken = tagePredict(tid, branch_pc, cond_branch, bi);
if (cond_branch) {
- bi->loopPred = getLoop(branch_pc, bi); // loop prediction
+ // loop prediction
+ bi->loopPred = getLoop(branch_pc, bi, useSpeculation);
if ((loopUseCounter >= 0) && bi->loopPredValid) {
pred_taken = bi->loopPred;
@@ -234,9 +265,12 @@
"loopValid?:%d, loopUseCounter:%d, tagePred:%d, altPred:%d\n",
branch_pc, pred_taken, bi->loopPred, bi->loopPredValid,
loopUseCounter, bi->tagePred, bi->altTaken);
+
+ if (useSpeculation) {
+ specLoopUpdate(pred_taken, bi);
+ }
}
- specLoopUpdate(branch_pc, pred_taken, bi);
return pred_taken;
}
@@ -246,8 +280,13 @@
{
LTageBranchInfo* bi = static_cast<LTageBranchInfo*>(tage_bi);
- // first update the loop predictor
- loopUpdate(branch_pc, taken, bi);
+ if (useSpeculation) {
+ // recalculate loop prediction without speculation
+ // It is ok to overwrite the loop prediction fields in bi
+ // as the stats have already been updated with the previous
+ // values
+ bi->loopPred = getLoop(branch_pc, bi, false);
+ }
if (bi->loopPredValid) {
if (bi->tagePred != bi->loopPred) {
@@ -257,6 +296,8 @@
}
}
+ loopUpdate(branch_pc, taken, bi);
+
TAGE::condBranchUpdate(branch_pc, taken, bi, nrand);
}
@@ -269,7 +310,9 @@
if (bi->condBranch) {
if (bi->loopHit >= 0) {
- int idx = bi->loopIndex + bi->loopHit;
+ int idx = finallindex(bi->loopIndex,
+ bi->loopLowPcBits,
+ bi->loopHit);
ltable[idx].currentIterSpec = bi->currentIter;
}
}
@@ -281,7 +324,9 @@
LTageBranchInfo* bi = (LTageBranchInfo*)(bp_history);
if (bi->condBranch) {
if (bi->loopHit >= 0) {
- int idx = bi->loopIndex + bi->loopHit;
+ int idx = finallindex(bi->loopIndex,
+ bi->loopLowPcBits,
+ bi->loopHit);
ltable[idx].currentIterSpec = bi->currentIter;
}
}
diff --git a/src/cpu/pred/ltage.hh b/src/cpu/pred/ltage.hh
index e9e34b7..94ff968 100644
--- a/src/cpu/pred/ltage.hh
+++ b/src/cpu/pred/ltage.hh
@@ -76,11 +76,11 @@
{
uint16_t numIter;
uint16_t currentIter;
- uint16_t currentIterSpec;
+ uint16_t currentIterSpec; // only for useSpeculation
uint8_t confidence;
uint16_t tag;
uint8_t age;
- bool dir;
+ bool dir; // only for useDirectionBit
LoopEntry() : numIter(0), currentIter(0), currentIterSpec(0),
confidence(0), tag(0), age(0), dir(0) { }
@@ -100,13 +100,14 @@
bool loopPred;
bool loopPredValid;
int loopIndex;
+ int loopLowPcBits; // only for useHashing
int loopHit;
LTageBranchInfo(int sz)
: TageBranchInfo(sz),
loopTag(0), currentIter(0),
loopPred(false),
- loopPredValid(false), loopIndex(0), loopHit(0)
+ loopPredValid(false), loopIndex(0), loopLowPcBits(0), loopHit(0)
{}
};
@@ -118,13 +119,24 @@
int lindex(Addr pc_in) const;
/**
+ * Computes the index used to access the
+ * ltable structures.
+ * It may take hashing into account
+ * @param index Result of lindex function
+ * @param lowPcBits PC bits masked with set size
+ * @param way Way to be used
+ */
+ int finallindex(int lindex, int lowPcBits, int way) const;
+
+ /**
* Get a branch prediction from the loop
* predictor.
* @param pc The unshifted branch PC.
* @param bi Pointer to information on the
* prediction.
+ * @param speculative Use speculative number of iterations
*/
- bool getLoop(Addr pc, LTageBranchInfo* bi) const;
+ bool getLoop(Addr pc, LTageBranchInfo* bi, bool speculative) const;
/**
* Updates the loop predictor.
@@ -137,13 +149,12 @@
/**
* Speculatively updates the loop predictor
- * iteration count.
- * @param pc The unshifted branch PC.
+ * iteration count (only for useSpeculation).
* @param taken The predicted branch outcome.
* @param bi Pointer to information on the prediction
* recorded at prediction time.
*/
- void specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi);
+ void specLoopUpdate(bool taken, LTageBranchInfo* bi);
/**
* Update LTAGE for conditional branches.
@@ -201,12 +212,17 @@
const uint8_t confidenceThreshold;
const uint16_t loopTagMask;
const uint16_t loopNumIterMask;
+ const int loopSetMask;
LoopEntry *ltable;
int8_t loopUseCounter;
unsigned withLoopBits;
+ const bool useDirectionBit;
+ const bool useSpeculation;
+ const bool useHashing;
+
// stats
Stats::Scalar loopPredictorCorrect;
Stats::Scalar loopPredictorWrong;
