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Mgsc4 align #740

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Mgsc4 align #740

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63a8017
bpu: sc: test sc again
jensen-yan Jan 28, 2026
5871427
bpu: sc: try to use bigger threshold
jensen-yan Jan 27, 2026
34f1ac8
bpu: sc: ensure threadhold is positive
jensen-yan Jan 27, 2026
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2 changes: 1 addition & 1 deletion configs/example/kmhv3.py
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Original file line number Diff line number Diff line change
Expand Up @@ -105,7 +105,7 @@ def setKmhV3Params(args, system):
cpu.branchPred.mbtb.enabled = True
cpu.branchPred.tage.enabled = True
cpu.branchPred.ittage.enabled = True
cpu.branchPred.mgsc.enabled = False
cpu.branchPred.mgsc.enabled = True
cpu.branchPred.ras.enabled = True

# l1 cache per core
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2 changes: 1 addition & 1 deletion src/cpu/pred/BranchPredictor.py
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Expand Up @@ -1126,7 +1126,7 @@ class BTBMGSC(TimedBaseBTBPredictor):
thresholdTablelogSize = Param.Unsigned(6,
"Log size of update threshold counters tables")

updateThresholdWidth = Param.Unsigned(12,
updateThresholdWidth = Param.Unsigned(14,
"Number of bits for the update threshold counter")

pUpdateThresholdWidth = Param.Unsigned(8,
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8 changes: 8 additions & 0 deletions src/cpu/pred/btb/btb_mgsc.cc
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Expand Up @@ -413,6 +413,9 @@ BTBMGSC::generateSinglePrediction(const BTBEntry &btb_entry, const Addr &startPC
int p_update_thres = enablePCThreshold ? findThreshold(pUpdateThreshold, btb_entry.pc) : 0;

int total_thres = (updateThreshold / 8) + p_update_thres;
// Threshold is used as a confidence gate; avoid negative values which
// effectively disable the gate (abs(sum) > negative is almost always true).
total_thres = std::max(total_thres, 0);

bool use_sc_pred = forceUseSC; // Force use SC if configured
if (!use_sc_pred) {
Expand Down Expand Up @@ -656,6 +659,11 @@ void
BTBMGSC::updateGlobalThreshold(Addr pc, bool update_direction)
{
updateCounter(update_direction, updateThresholdWidth, updateThreshold);
// Keep global threshold non-negative; negative thresholds make SC gating
// degenerate and can cause overuse of SC.
if (updateThreshold < 0) {
updateThreshold = 0;
}
}

void
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