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cpu: Fixed useful counter handling in LTAGE

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Increased to 2 bits of useful counter per TAGE entry as described in the
LTAGE paper (and made the size configurable)

Changed how the useful counters are incremented/decremented as described
in the LTAGE paper

Change-Id: I8c692cc7c180d29897cb77781681ff498a1d16c8
Signed-off-by: Pau Cabre <pau.cabre@metempsy.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14215
Reviewed-by: Ilias Vougioukas <ilias.vougioukas@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
This commit is contained in:
Pau Cabre
2018-11-17 23:21:15 +01:00
parent 7e48e92888
commit 290a35401b
3 changed files with 13 additions and 9 deletions
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1
src/cpu/pred/BranchPredictor.py
View File

@@ -100,6 +100,7 @@ class LTAGE(BranchPredictor):
logSizeLoopPred = Param.Unsigned(8, "Log size of the loop predictor")
nHistoryTables = Param.Unsigned(12, "Number of history tables")
tagTableCounterBits = Param.Unsigned(3, "Number of tag table counter bits")
tagTableUBits = Param.Unsigned(2, "Number of tag table u bits")
histBufferSize = Param.Unsigned(2097152,
"A large number to track all branch histories(2MEntries default)")
minHist = Param.Unsigned(4, "Minimum history size of LTAGE")

18
src/cpu/pred/ltage.cc
View File

@@ -55,6 +55,7 @@ LTAGE::LTAGE(const LTAGEParams *params)
logSizeLoopPred(params->logSizeLoopPred),
nHistoryTables(params->nHistoryTables),
tagTableCounterBits(params->tagTableCounterBits),
tagTableUBits(params->tagTableUBits),
histBufferSize(params->histBufferSize),
minHist(params->minHist),
maxHist(params->maxHist),
@@ -68,6 +69,11 @@ LTAGE::LTAGE(const LTAGEParams *params)
loopNumIterMask((1 << loopTableIterBits) - 1),
threadHistory(params->numThreads)
{
// Current method for periodically resetting the u counter bits only
// works for 1 or 2 bits
// Also make sure that it is not 0
assert(tagTableUBits <= 2 && (tagTableUBits > 0));
// we use uint16_t type for these vales, so they cannot be more than
// 16 bits
assert(loopTableTagBits <= 16);
@@ -576,7 +582,7 @@ LTAGE::update(ThreadID tid, Addr branch_pc, bool taken, void* bp_history,
if (alloc) {
// is there some "unuseful" entry to allocate
int8_t min = 1;
uint8_t min = 1;
for (int i = nHistoryTables; i > bi->hitBank; i--) {
if (gtable[i][bi->tableIndices[i]].u < min) {
min = gtable[i][bi->tableIndices[i]].u;
@@ -605,7 +611,6 @@ LTAGE::update(ThreadID tid, Addr branch_pc, bool taken, void* bp_history,
if ((gtable[i][bi->tableIndices[i]].u == 0)) {
gtable[i][bi->tableIndices[i]].tag = bi->tableTags[i];
gtable[i][bi->tableIndices[i]].ctr = (taken) ? 0 : -1;
gtable[i][bi->tableIndices[i]].u = 0; //?
break;
}
}
@@ -643,12 +648,9 @@ LTAGE::update(ThreadID tid, Addr branch_pc, bool taken, void* bp_history,
}
// update the u counter
if (longest_match_pred != bi->altTaken) {
if (longest_match_pred == taken) {
if (gtable[bi->hitBank][bi->hitBankIndex].u < 1) {
gtable[bi->hitBank][bi->hitBankIndex].u++;
}
}
if (bi->tagePred != bi->altTaken) {
unsignedCtrUpdate(gtable[bi->hitBank][bi->hitBankIndex].u,
bi->tagePred == taken, tagTableUBits);
}
} else {
baseUpdate(pc, taken, bi);

3
src/cpu/pred/ltage.hh
View File

@@ -94,7 +94,7 @@ class LTAGE: public BPredUnit
{
int8_t ctr;
uint16_t tag;
int8_t u;
uint8_t u;
TageEntry() : ctr(0), tag(0), u(0) { }
};
@@ -360,6 +360,7 @@ class LTAGE: public BPredUnit
const unsigned logSizeLoopPred;
const unsigned nHistoryTables;
const unsigned tagTableCounterBits;
const unsigned tagTableUBits;
const unsigned histBufferSize;
const unsigned minHist;
const unsigned maxHist;