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Fix path history info target #814

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Merged
jensen-yan merged 1 commit into from
Apr 9, 2026
Merged

Fix path history info target #814

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62 changes: 23 additions & 39 deletions src/cpu/pred/btb/common.hh
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Original file line number Diff line number Diff line change
Expand Up @@ -516,24 +516,29 @@ struct FullBTBPrediction
return (bbStart + predictWidth) & ~mask(floorLog2(predictWidth) - 1);
}

Addr getEntryTarget(const BTBEntry &entry) {
Addr target = entry.target;
// indirect target should come from ipred or ras,
// or btb itself when ipred miss
if (entry.isIndirect) {
if (!entry.isReturn) { // normal indirect, see ittage
auto& pc = entry.pc;
auto it = IndirectTakens_find(indirectTargets, pc);
if (it != indirectTargets.end()) { // found in ittage, use it
target = it->second;
}
} else { // indirect return, use RAS target
target = returnTarget;
}
} // else: normal taken, use btb target
return target;
}

Addr getTarget(Addr predictWidth) {
Addr target;
const auto &entry = getTakenEntry();
if (entry.valid) { // found a taken entry
target = entry.target;
// indirect target should come from ipred or ras,
// or btb itself when ipred miss
if (entry.isIndirect) {
if (!entry.isReturn) { // normal indirect, see ittage
auto& pc = entry.pc;
auto it = IndirectTakens_find(indirectTargets, pc);
if (it != indirectTargets.end()) { // found in ittage, use it
target = it->second;
}
} else { // indirect return, use RAS target
target = returnTarget;
}
} // else: normal taken, use btb target
target = getEntryTarget(entry);
} else {
target = getFallThrough(predictWidth);
}
Expand Down Expand Up @@ -631,32 +636,11 @@ struct FullBTBPrediction

std::tuple<Addr, Addr, bool> getPHistInfo() //path
{
bool taken = false;
Addr pc = 0;
Addr target = 0;
for (auto &entry : btbEntries) {
if (entry.valid) {
if (entry.isCond) {
auto& _pc = entry.pc;
auto it = CondTakens_find(condTakens, _pc);
if (it != condTakens.end()) {
if (it->second) {
taken = true;
pc = entry.pc; // get the pc of the cond branch
target = entry.target;
break;
}
}
} else {
// uncond
taken = true;
pc = entry.pc; // get the pc of the cond branch
target = entry.target;
break;
}
}
const auto &entry = getTakenEntry();
if (entry.valid) {
return std::make_tuple(entry.pc, getEntryTarget(entry), true);
}
return std::make_tuple(pc, target, taken);
return std::make_tuple(0, 0, false);
}

};
Expand Down
49 changes: 49 additions & 0 deletions src/cpu/pred/btb/test/btb_mgsc.test.cc
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Expand Up @@ -846,6 +846,55 @@ TEST(BTBMGSCTest, PTableLearnsOutcomeFromPreviousTakenBranchTarget)
EXPECT_GE(acc, 0.80) << "Accuracy too low for PTable path-target learning: " << acc;
}

TEST(BTBMGSCTest, SpecUpdatePHistUsesIndirectTargetOverride)
{
MgscHarness h;

BTBEntry entry = makeCondBTBEntry(0x1000);
entry.isIndirect = true;
entry.target = 0x2000;
const Addr indirectTarget = 0x3000;

ASSERT_NE(pathHash(entry.pc, entry.target), pathHash(entry.pc, indirectTarget));

FullBTBPrediction pred;
pred.btbEntries = {entry};
pred.condTakens.push_back({entry.pc, true});
pred.indirectTargets.push_back({entry.pc, indirectTarget});

boost::dynamic_bitset<> expected_phr = h.phr;
pHistShiftIn(2, true, expected_phr, entry.pc, indirectTarget);

h.mgsc.specUpdatePHist(h.phr, pred);
h.mgsc.checkFoldedHist(h.ghr, expected_phr, h.lhr,
"indirect target override");
}

TEST(BTBMGSCTest, SpecUpdatePHistUsesReturnTargetOverride)
{
MgscHarness h;

BTBEntry entry = makeCondBTBEntry(0x1000);
entry.isIndirect = true;
entry.isReturn = true;
entry.target = 0x2000;
const Addr returnTarget = 0x3400;

ASSERT_NE(pathHash(entry.pc, entry.target), pathHash(entry.pc, returnTarget));

FullBTBPrediction pred;
pred.btbEntries = {entry};
pred.condTakens.push_back({entry.pc, true});
pred.returnTarget = returnTarget;

boost::dynamic_bitset<> expected_phr = h.phr;
pHistShiftIn(2, true, expected_phr, entry.pc, returnTarget);

h.mgsc.specUpdatePHist(h.phr, pred);
h.mgsc.checkFoldedHist(h.ghr, expected_phr, h.lhr,
"return target override");
}

} // namespace test
} // namespace btb_pred
} // namespace branch_prediction
Expand Down
53 changes: 53 additions & 0 deletions src/cpu/pred/btb/test/btb_tage.test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1102,6 +1102,59 @@ TEST_F(BTBTAGEUpperBoundPathHashTest, PredictionUsesPathHashHistorySnapshot) {
EXPECT_TRUE(predicted);
}

TEST_F(BTBTAGEUpperBoundPathHashTest, PredictionUsesIndirectOverridePathHashSnapshot) {
BTBEntry entry = createBTBEntry(0x1000, true, true, false, -1, 0x2000);
entry.isIndirect = true;
const Addr indirectTarget = 0x3000;

ASSERT_NE(pathHash(entry.pc, entry.target), pathHash(entry.pc, indirectTarget));

boost::dynamic_bitset<> pathHistoryA(128, 0);
boost::dynamic_bitset<> pathHistoryB(128, 0);
applyPathHistoryTaken(pathHistoryB, entry.pc, indirectTarget);

ASSERT_TRUE(tage->insertExactEntry(2, entry.pc, pathHistoryB, 2));

FullBTBPrediction pred;
pred.btbEntries.push_back(entry);
pred.condTakens.push_back({entry.pc, true});
pred.indirectTargets.push_back({entry.pc, indirectTarget});

tage->specUpdatePHist(pathHistoryA, pred);
tage->checkFoldedHist(pathHistoryB, "indirect target override");

bool predicted = predictTAGE(tage, 0x1000, {entry}, outcomeHistory, stagePreds);

EXPECT_TRUE(predicted);
}

TEST_F(BTBTAGEUpperBoundPathHashTest, PredictionUsesReturnOverridePathHashSnapshot) {
BTBEntry entry = createBTBEntry(0x1000, true, true, false, -1, 0x2000);
entry.isIndirect = true;
entry.isReturn = true;
const Addr returnTarget = 0x3400;

ASSERT_NE(pathHash(entry.pc, entry.target), pathHash(entry.pc, returnTarget));

boost::dynamic_bitset<> pathHistoryA(128, 0);
boost::dynamic_bitset<> pathHistoryB(128, 0);
applyPathHistoryTaken(pathHistoryB, entry.pc, returnTarget);

ASSERT_TRUE(tage->insertExactEntry(2, entry.pc, pathHistoryB, 2));

FullBTBPrediction pred;
pred.btbEntries.push_back(entry);
pred.condTakens.push_back({entry.pc, true});
pred.returnTarget = returnTarget;

tage->specUpdatePHist(pathHistoryA, pred);
tage->checkFoldedHist(pathHistoryB, "return target override");

bool predicted = predictTAGE(tage, 0x1000, {entry}, outcomeHistory, stagePreds);

EXPECT_TRUE(predicted);
}


} // namespace test

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