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bpu: add stats for recomputed vs actual/original prediction diff #685

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Merged
jensen-yan merged 1 commit into from
Jan 12, 2026
Merged

bpu: add stats for recomputed vs actual/original prediction diff #685

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19 changes: 19 additions & 0 deletions src/cpu/pred/btb/btb_tage.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -707,15 +707,25 @@ BTBTAGE::update(const FetchStream &stream) {
}

// Process each BTB entry
bool hasRecomputedVsActualDiff = false;
bool hasRecomputedVsOriginalDiff = false;
for (auto &btb_entry : entries_to_update) {
bool actual_taken = stream.exeTaken && stream.exeBranchInfo == btb_entry;
TagePrediction recomputed;
if (updateOnRead) { // if update on read is enabled, re-read providers using snapshot
// Re-read providers using snapshot (do not rely on prediction-time main/alt)
recomputed = generateSinglePrediction(btb_entry, startAddr, predMeta);
// Track differences for statistics
auto it = predMeta->preds.find(btb_entry.pc);
if (it != predMeta->preds.end() && recomputed.taken != it->second.taken) {
hasRecomputedVsOriginalDiff = true;
}
} else { // otherwise, use the prediction from the prediction-time main/alt
recomputed = predMeta->preds[btb_entry.pc];
}
if (recomputed.taken != actual_taken) {
hasRecomputedVsActualDiff = true;
}

// Update predictor state and check if need to allocate new entry
bool need_allocate = updatePredictorStateAndCheckAllocation(btb_entry, actual_taken, recomputed, stream);
Expand Down Expand Up @@ -760,6 +770,13 @@ BTBTAGE::update(const FetchStream &stream) {
}
#endif
}
// Update recomputed difference statistics (per fetchBlock)
if (hasRecomputedVsActualDiff) {
tageStats.recomputedVsActualDiff++;
}
if (hasRecomputedVsOriginalDiff) {
tageStats.recomputedVsOriginalDiff++;
}
if (getDelay() <2){
checkUtageUpdateMisspred(stream);
}
Expand Down Expand Up @@ -1026,6 +1043,8 @@ BTBTAGE::TageStats::TageStats(statistics::Group* parent, int numPredictors, int
ADD_STAT(updateAllocSuccess, statistics::units::Count::get(), "alloc success when update"),
ADD_STAT(updateMispred, statistics::units::Count::get(), "mispred when update"),
ADD_STAT(updateResetU, statistics::units::Count::get(), "reset u when update"),
ADD_STAT(recomputedVsActualDiff, statistics::units::Count::get(), "fetchBlocks where recomputed.taken != actual_taken"),
ADD_STAT(recomputedVsOriginalDiff, statistics::units::Count::get(), "fetchBlocks where recomputed.taken != original pred.taken"),
ADD_STAT(updateBankConflict, statistics::units::Count::get(), "number of bank conflicts detected"),
ADD_STAT(updateDeferredDueToConflict, statistics::units::Count::get(), "number of updates deferred due to bank conflict (retried later)"),
ADD_STAT(updateBankConflictPerBank, statistics::units::Count::get(), "bank conflicts per bank"),
Expand Down
4 changes: 4 additions & 0 deletions src/cpu/pred/btb/btb_tage.hh
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Expand Up @@ -350,6 +350,10 @@ class BTBTAGE : public TimedBaseBTBPredictor
Scalar updateMispred;
Scalar updateResetU;

// Recomputed prediction difference statistics (per fetchBlock)
Scalar recomputedVsActualDiff; // recomputed.taken != actual_taken
Scalar recomputedVsOriginalDiff; // recomputed.taken != original pred.taken

// Bank conflict statistics
Scalar updateBankConflict; // Number of bank conflicts detected
Scalar updateDeferredDueToConflict; // Number of updates deferred due to bank conflict (retried later)
Expand Down
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