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-rw-r--r--src/cpu/pred/ltage.cc93
1 files changed, 69 insertions, 24 deletions
diff --git a/src/cpu/pred/ltage.cc b/src/cpu/pred/ltage.cc
index 73f477745..8f332b3b5 100644
--- a/src/cpu/pred/ltage.cc
+++ b/src/cpu/pred/ltage.cc
@@ -58,8 +58,12 @@ LTAGE::LTAGE(const LTAGEParams *params)
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 @@ LTAGE::lindex(Addr pc_in) const
// 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 @@ LTAGE::getLoop(Addr pc, LTageBranchInfo* bi) const
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 @@ LTAGE::getLoop(Addr pc, LTageBranchInfo* bi) const
}
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 @@ LTAGE::specLoopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
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 @@ LTAGE::loopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
}
}
- 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 @@ LTAGE::loopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
//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 @@ LTAGE::loopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
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 @@ LTAGE::loopUpdate(Addr pc, bool taken, LTageBranchInfo* bi)
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 @@ LTAGE::predict(ThreadID tid, Addr branch_pc, bool cond_branch, void* &b)
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 @@ LTAGE::predict(ThreadID tid, Addr branch_pc, bool cond_branch, void* &b)
"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 @@ LTAGE::condBranchUpdate(Addr branch_pc, bool taken,
{
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 @@ LTAGE::condBranchUpdate(Addr branch_pc, bool taken,
}
}
+ loopUpdate(branch_pc, taken, bi);
+
TAGE::condBranchUpdate(branch_pc, taken, bi, nrand);
}
@@ -269,7 +310,9 @@ LTAGE::squash(ThreadID tid, bool taken, void *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;
}
}
@@ -281,7 +324,9 @@ LTAGE::squash(ThreadID tid, void *bp_history)
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;
}
}
generated by cgit v1.2.3 (git 2.25.1) at 2024-05-18 08:27:55 +0000