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[DFAJumpThreading] Precompute value => successor mapping #162824

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merged 2 commits into from
Oct 10, 2025
Merged

[DFAJumpThreading] Precompute value => successor mapping #162824

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[DFAJumpThreading] Precompute value - successor mapping
XChy Oct 10, 2025
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XChy Oct 10, 2025
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62 changes: 39 additions & 23 deletions llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -386,22 +386,6 @@ inline raw_ostream &operator<<(raw_ostream &OS, const PathType &Path) {
return OS;
}

/// Helper to get the successor corresponding to a particular case value for
/// a switch statement.
static BasicBlock *getNextCaseSuccessor(SwitchInst *Switch,
const APInt &NextState) {
BasicBlock *NextCase = nullptr;
for (auto Case : Switch->cases()) {
if (Case.getCaseValue()->getValue() == NextState) {
NextCase = Case.getCaseSuccessor();
break;
}
}
if (!NextCase)
NextCase = Switch->getDefaultDest();
return NextCase;
}

namespace {
/// ThreadingPath is a path in the control flow of a loop that can be threaded
/// by cloning necessary basic blocks and replacing conditional branches with
Expand Down Expand Up @@ -835,19 +819,32 @@ struct AllSwitchPaths {
TPaths = std::move(TempList);
}

/// Fast helper to get the successor corresponding to a particular case value
/// for a switch statement.
BasicBlock *getNextCaseSuccessor(const APInt &NextState) {
// Precompute the value => successor mapping
if (CaseValToDest.empty()) {
for (auto Case : Switch->cases()) {
APInt CaseVal = Case.getCaseValue()->getValue();
CaseValToDest[CaseVal] = Case.getCaseSuccessor();
}
}

auto SuccIt = CaseValToDest.find(NextState);
return SuccIt == CaseValToDest.end() ? Switch->getDefaultDest()
: SuccIt->second;
}

// Two states are equivalent if they have the same switch destination.
// Unify the states in different threading path if the states are equivalent.
void unifyTPaths() {
llvm::SmallDenseMap<BasicBlock *, APInt> DestToState;
SmallDenseMap<BasicBlock *, APInt> DestToState;
for (ThreadingPath &Path : TPaths) {
APInt NextState = Path.getExitValue();
BasicBlock *Dest = getNextCaseSuccessor(Switch, NextState);
auto StateIt = DestToState.find(Dest);
if (StateIt == DestToState.end()) {
DestToState.insert({Dest, NextState});
BasicBlock *Dest = getNextCaseSuccessor(NextState);
auto [StateIt, Inserted] = DestToState.try_emplace(Dest, NextState);
if (Inserted)
continue;
}

if (NextState != StateIt->second) {
LLVM_DEBUG(dbgs() << "Next state in " << Path << " is equivalent to "
<< StateIt->second << "\n");
Expand All @@ -861,6 +858,7 @@ struct AllSwitchPaths {
BasicBlock *SwitchBlock;
OptimizationRemarkEmitter *ORE;
std::vector<ThreadingPath> TPaths;
DenseMap<APInt, BasicBlock *> CaseValToDest;
LoopInfo *LI;
Loop *SwitchOuterLoop;
};
Expand Down Expand Up @@ -1162,6 +1160,24 @@ struct TransformDFA {
SSAUpdate.RewriteAllUses(&DTU->getDomTree());
}

/// Helper to get the successor corresponding to a particular case value for
/// a switch statement.
/// TODO: Unify it with SwitchPaths->getNextCaseSuccessor(SwitchInst *Switch)
/// by updating cached value => successor mapping during threading.
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Hm, the need for a second copy is unfortunate...

static BasicBlock *getNextCaseSuccessor(SwitchInst *Switch,
const APInt &NextState) {
BasicBlock *NextCase = nullptr;
for (auto Case : Switch->cases()) {
if (Case.getCaseValue()->getValue() == NextState) {
NextCase = Case.getCaseSuccessor();
break;
}
}
if (!NextCase)
NextCase = Switch->getDefaultDest();
return NextCase;
}

/// Clones a basic block, and adds it to the CFG.
///
/// This function also includes updating phi nodes in the successors of the
Expand Down