void MachineBlockPlacement::placeChainsTopologically(MachineFunction &F) {
MachineBasicBlock *EntryB = &F.front();
assert(BlockToChain[EntryB] && "Missing chain for entry block");
assert(*BlockToChain[EntryB]->begin() == EntryB &&
"Entry block is not the head of the entry block chain");
// Walk the blocks in RPO, and insert each block for a chain in order the
// first time we see that chain.
MachineFunction::iterator InsertPos = F.begin();
SmallPtrSet<BlockChain *, 16> VisitedChains;
ReversePostOrderTraversal<MachineBasicBlock *> RPOT(EntryB);
typedef ReversePostOrderTraversal<MachineBasicBlock *>::rpo_iterator
rpo_iterator;
for (rpo_iterator I = RPOT.begin(), E = RPOT.end(); I != E; ++I) {
BlockChain *Chain = BlockToChain[*I];
assert(Chain);
if(!VisitedChains.insert(Chain))
continue;
for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end(); BI != BE;
++BI) {
DEBUG(dbgs() << (BI == Chain->begin() ? "Placing chain "
: " ... ")
<< getBlockName(*BI) << "\n");
if (InsertPos != MachineFunction::iterator(*BI))
F.splice(InsertPos, *BI);
else
++InsertPos;
}
}
// Now that every block is in its final position, update all of the
// terminators.
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
// FIXME: It would be awesome of updateTerminator would just return rather
// than assert when the branch cannot be analyzed in order to remove this
// boiler plate.
Cond.clear();
MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
if (!TII->AnalyzeBranch(*FI, TBB, FBB, Cond))
FI->updateTerminator();
}
}
/// \brief Merge a chain with any viable successor.
///
/// This routine walks the predecessors of the current block, looking for
/// viable merge candidates. It has strict rules it uses to determine when
/// a predecessor can be merged with the current block, which center around
/// preserving the CFG structure. It performs the merge if any viable candidate
/// is found.
void MachineBlockPlacement::mergeSuccessor(MachineBasicBlock *BB,
BlockChain *Chain,
BlockFilterSet *Filter) {
assert(BB);
assert(Chain);
// If this block is not at the end of its chain, it cannot merge with any
// other chain.
if (Chain && *llvm::prior(Chain->end()) != BB)
return;
// Walk through the successors looking for the highest probability edge.
MachineBasicBlock *Successor = 0;
BranchProbability BestProb = BranchProbability::getZero();
DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
SE = BB->succ_end();
SI != SE; ++SI) {
if (BB == *SI || (Filter && !Filter->count(*SI)))
continue;
BranchProbability SuccProb = MBPI->getEdgeProbability(BB, *SI);
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb << "\n");
if (!Successor || SuccProb > BestProb || (!(SuccProb < BestProb) &&
BB->isLayoutSuccessor(*SI))) {
Successor = *SI;
BestProb = SuccProb;
}
}
if (!Successor)
return;
// Grab a chain if it exists already for this successor and make sure the
// successor is at the start of the chain as we can't merge mid-chain. Also,
// if the successor chain is the same as our chain, we're already merged.
BlockChain *SuccChain = BlockToChain[Successor];
if (SuccChain && (SuccChain == Chain || Successor != *SuccChain->begin()))
return;
// We only merge chains across a CFG merge when the desired merge path is
// significantly hotter than the incoming edge. We define a hot edge more
// strictly than the BranchProbabilityInfo does, as the two predecessor
// blocks may have dramatically different incoming probabilities we need to
// account for. Therefor we use the "global" edge weight which is the
// branch's probability times the block frequency of the predecessor.
BlockFrequency MergeWeight = MBFI->getBlockFreq(BB);
MergeWeight *= MBPI->getEdgeProbability(BB, Successor);
// We only want to consider breaking the CFG when the merge weight is much
// higher (80% vs. 20%), so multiply it by 1/4. This will require the merged
// edge to be 4x more likely before we disrupt the CFG. This number matches
// the definition of "hot" in BranchProbabilityAnalysis (80% vs. 20%).
MergeWeight *= BranchProbability(1, 4);
for (MachineBasicBlock::pred_iterator PI = Successor->pred_begin(),
PE = Successor->pred_end();
PI != PE; ++PI) {
if (BB == *PI || Successor == *PI) continue;
BlockFrequency PredWeight = MBFI->getBlockFreq(*PI);
PredWeight *= MBPI->getEdgeProbability(*PI, Successor);
// Return on the first predecessor we find which outstrips our merge weight.
if (MergeWeight < PredWeight)
return;
DEBUG(dbgs() << "Breaking CFG edge!\n"
<< " Edge from " << getBlockNum(BB) << " to "
<< getBlockNum(Successor) << ": " << MergeWeight << "\n"
<< " vs. " << getBlockNum(BB) << " to "
<< getBlockNum(*PI) << ": " << PredWeight << "\n");
}
DEBUG(dbgs() << "Merging from " << getBlockNum(BB) << " to "
<< getBlockNum(Successor) << "\n");
Chain->merge(Successor, SuccChain);
}