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[DomTree] Avoid duplicate hash lookups in runDFS() (NFCI) #96460

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Jun 25, 2024
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[DomTree] Avoid duplicate hash lookups in runDFS() (NFCI) #96460

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21 changes: 5 additions & 16 deletions llvm/include/llvm/Support/GenericDomTreeConstruction.h
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Original file line number Diff line number Diff line change
Expand Up @@ -180,15 +180,17 @@ struct SemiNCAInfo {
unsigned AttachToNum,
const NodeOrderMap *SuccOrder = nullptr) {
assert(V);
SmallVector<NodePtr, 64> WorkList = {V};
SmallVector<std::pair<NodePtr, unsigned>, 64> WorkList = {{V, AttachToNum}};
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This looks like a pretty large stack allocation (16B * 64 => 1kB?). Is this what we want?

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It's on the bigger side, but as runDFS is essentially a leaf function, I don't think it's a problem.

NodeToInfo[V].Parent = AttachToNum;

while (!WorkList.empty()) {
const NodePtr BB = WorkList.pop_back_val();
const auto [BB, ParentNum] = WorkList.pop_back_val();
auto &BBInfo = NodeToInfo[BB];
BBInfo.ReverseChildren.push_back(ParentNum);

// Visited nodes always have positive DFS numbers.
if (BBInfo.DFSNum != 0) continue;
BBInfo.Parent = ParentNum;
BBInfo.DFSNum = BBInfo.Semi = BBInfo.Label = ++LastNum;
NumToNode.push_back(BB);

Expand All @@ -201,22 +203,9 @@ struct SemiNCAInfo {
});

for (const NodePtr Succ : Successors) {
const auto SIT = NodeToInfo.find(Succ);
// Don't visit nodes more than once but remember to collect
// ReverseChildren.
if (SIT != NodeToInfo.end() && SIT->second.DFSNum != 0) {
if (Succ != BB) SIT->second.ReverseChildren.push_back(LastNum);
continue;
}

if (!Condition(BB, Succ)) continue;

// It's fine to add Succ to the map, because we know that it will be
// visited later.
auto &SuccInfo = NodeToInfo[Succ];
WorkList.push_back(Succ);
SuccInfo.Parent = LastNum;
SuccInfo.ReverseChildren.push_back(LastNum);
WorkList.push_back({Succ, LastNum});
}
}

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