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[SSAUpdater] Avoid scanning basic blocks to find instruction order. #123803

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Merged
merged 3 commits into from
Jan 22, 2025
Merged

[SSAUpdater] Avoid scanning basic blocks to find instruction order. #123803

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8fa00f0
[SSAUpdater] Avoid scanning basic blocks to find instruction order.
jcranmer-intel Jan 21, 2025
93e0e47
Remove redundant checks.
jcranmer-intel Jan 21, 2025
99ac764
Address review comments.
jcranmer-intel Jan 21, 2025
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26 changes: 11 additions & 15 deletions llvm/lib/Transforms/Utils/SSAUpdater.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -432,27 +432,27 @@ void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
}
}

// If so, we can queue them all as live in loads. We don't have an
// efficient way to tell which on is first in the block and don't want to
// scan large blocks, so just add all loads as live ins.
// If so, we can queue them all as live in loads.
if (!HasStore) {
for (Instruction *I : BlockUses)
LiveInLoads.push_back(cast<LoadInst>(I));
BlockUses.clear();
continue;
}

// Sort all of the interesting instructions in the block so that we don't
// have to scan a large block just to find a few instructions.
std::sort(BlockUses.begin(), BlockUses.end(),
[](Instruction *A, Instruction *B) { return A->comesBefore(B); });
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@nikic nikic Jan 21, 2025

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Suggested change
std::sort(BlockUses.begin(), BlockUses.end(),
[](Instruction *A, Instruction *B) { return A->comesBefore(B); });
llvm::sort(BlockUses,
[](Instruction *A, Instruction *B) { return A->comesBefore(B); });


// Otherwise, we have mixed loads and stores (or just a bunch of stores).
// Since SSAUpdater is purely for cross-block values, we need to determine
// the order of these instructions in the block. If the first use in the
// block is a load, then it uses the live in value. The last store defines
// the live out value. We handle this by doing a linear scan of the block.
// the live out value.
Value *StoredValue = nullptr;
for (Instruction &I : *BB) {
if (LoadInst *L = dyn_cast<LoadInst>(&I)) {
// If this is a load from an unrelated pointer, ignore it.
if (!isInstInList(L, Insts)) continue;

for (Instruction *I : BlockUses) {
if (LoadInst *L = dyn_cast<LoadInst>(I)) {
// If we haven't seen a store yet, this is a live in use, otherwise
// use the stored value.
if (StoredValue) {
Expand All @@ -465,18 +465,14 @@ void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
continue;
}

if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
// If this is a store to an unrelated pointer, ignore it.
if (!isInstInList(SI, Insts)) continue;
if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
updateDebugInfo(SI);

// Remember that this is the active value in the block.
StoredValue = SI->getOperand(0);
} else if (auto *AI = dyn_cast<AllocaInst>(&I)) {
} else if (auto *AI = dyn_cast<AllocaInst>(I)) {
// Check if this an alloca, in which case we treat it as a store of
// getValueToUseForAlloca.
if (!isInstInList(AI, Insts))
continue;
StoredValue = getValueToUseForAlloca(AI);
}
}
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