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[BOLT] Optimize basic block loops to avoid n^2 loop #156243

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[BOLT] Optimize basic block loops to avoid n^2 loop #156243

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480d48e
[BOLT] Optimize basic block loops to avoid n^2 loop
Mark-Simulacrum Aug 31, 2025
db72cc2
[BOLT] Split getBasicBlocksAfter cache into a distinct function
Mark-Simulacrum Sep 21, 2025
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14 changes: 13 additions & 1 deletion bolt/lib/Core/BinaryFunction.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3591,6 +3591,18 @@ void BinaryFunction::fixBranches() {
auto &MIB = BC.MIB;
MCContext *Ctx = BC.Ctx.get();

// Caches `FunctionLayout::nextBasicBlock(IgnoreSplits = false)`.
// nextBasicBlock uses linear search to find the next block, so the loop
// below becomes O(n^2). This avoids that.
DenseMap<BinaryBasicBlock *, BinaryBasicBlock *> nextBasicBlock(
Layout.block_size());
for (size_t i = 0; i + 1 < Layout.block_size(); i++) {
auto current = Layout.block_begin() + i;
auto next = Layout.block_begin() + i + 1;
if (next != Layout.getFragment((*current)->getFragmentNum()).end())
nextBasicBlock.insert(std::pair(*current, *next));
}

for (BinaryBasicBlock *BB : BasicBlocks) {
const MCSymbol *TBB = nullptr;
const MCSymbol *FBB = nullptr;
Expand All @@ -3605,7 +3617,7 @@ void BinaryFunction::fixBranches() {

// Basic block that follows the current one in the final layout.
const BinaryBasicBlock *const NextBB =
Layout.getBasicBlockAfter(BB, /*IgnoreSplits=*/false);
nextBasicBlock.lookup_or(BB, nullptr);

if (BB->succ_size() == 1) {
// __builtin_unreachable() could create a conditional branch that
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