MachineBasicBlock *selectBestCandidateBlock(
BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
const BlockFilterSet *BlockFilter);
- MachineBasicBlock *getFirstUnplacedBlock(const BlockChain &PlacedChain,
- ArrayRef<MachineBasicBlock *> Blocks,
- unsigned &PrevUnplacedBlockIdx);
+ MachineBasicBlock *getFirstUnplacedBlock(
+ MachineFunction &F,
+ const BlockChain &PlacedChain,
+ MachineFunction::iterator &PrevUnplacedBlockIt,
+ const BlockFilterSet *BlockFilter);
void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
- ArrayRef<MachineBasicBlock *> Blocks,
SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
const BlockFilterSet *BlockFilter = 0);
void buildLoopChains(MachineFunction &F, MachineLoop &L);
const BranchProbability HotProb(4, 5); // 80%
MachineBasicBlock *BestSucc = 0;
- BranchProbability BestProb = BranchProbability::getZero();
+ // FIXME: Due to the performance of the probability and weight routines in
+ // the MBPI analysis, we manually compute probabilities using the edge
+ // weights. This is suboptimal as it means that the somewhat subtle
+ // definition of edge weight semantics is encoded here as well. We should
+ // improve the MBPI interface to effeciently support query patterns such as
+ // this.
+ uint32_t BestWeight = 0;
+ uint32_t WeightScale = 0;
+ uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
SE = BB->succ_end();
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
continue;
}
+ if (*SI != *SuccChain.begin()) {
+ DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
+ continue;
+ }
- BranchProbability SuccProb = MBPI->getEdgeProbability(BB, *SI);
+ uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
+ BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
// Only consider successors which are either "hot", or wouldn't violate
// any CFG constraints.
- if (SuccChain.LoopPredecessors != 0 && SuccProb < HotProb) {
- DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
- continue;
+ if (SuccChain.LoopPredecessors != 0) {
+ if (SuccProb < HotProb) {
+ DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
+ continue;
+ }
+
+ // Make sure that a hot successor doesn't have a globally more important
+ // predecessor.
+ BlockFrequency CandidateEdgeFreq
+ = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
+ bool BadCFGConflict = false;
+ for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
+ PE = (*SI)->pred_end();
+ PI != PE; ++PI) {
+ if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
+ BlockToChain[*PI] == &Chain)
+ continue;
+ BlockFrequency PredEdgeFreq
+ = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
+ if (PredEdgeFreq >= CandidateEdgeFreq) {
+ BadCFGConflict = true;
+ break;
+ }
+ }
+ if (BadCFGConflict) {
+ DEBUG(dbgs() << " " << getBlockName(*SI)
+ << " -> non-cold CFG conflict\n");
+ continue;
+ }
}
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
<< " (prob)"
<< (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
<< "\n");
- if (BestSucc && BestProb >= SuccProb)
+ if (BestSucc && BestWeight >= SuccWeight)
continue;
BestSucc = *SI;
- BestProb = SuccProb;
+ BestWeight = SuccWeight;
}
return BestSucc;
}
+namespace {
+/// \brief Predicate struct to detect blocks already placed.
+class IsBlockPlaced {
+ const BlockChain &PlacedChain;
+ const BlockToChainMapType &BlockToChain;
+
+public:
+ IsBlockPlaced(const BlockChain &PlacedChain,
+ const BlockToChainMapType &BlockToChain)
+ : PlacedChain(PlacedChain), BlockToChain(BlockToChain) {}
+
+ bool operator()(MachineBasicBlock *BB) const {
+ return BlockToChain.lookup(BB) == &PlacedChain;
+ }
+};
+}
+
/// \brief Select the best block from a worklist.
///
/// This looks through the provided worklist as a list of candidate basic
MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
const BlockFilterSet *BlockFilter) {
+ // Once we need to walk the worklist looking for a candidate, cleanup the
+ // worklist of already placed entries.
+ // FIXME: If this shows up on profiles, it could be folded (at the cost of
+ // some code complexity) into the loop below.
+ WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
+ IsBlockPlaced(Chain, BlockToChain)),
+ WorkList.end());
+
MachineBasicBlock *BestBlock = 0;
BlockFrequency BestFreq;
for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
WBE = WorkList.end();
WBI != WBE; ++WBI) {
- if (BlockFilter && !BlockFilter->count(*WBI))
- continue;
+ assert(!BlockFilter || BlockFilter->count(*WBI));
BlockChain &SuccChain = *BlockToChain[*WBI];
if (&SuccChain == &Chain) {
DEBUG(dbgs() << " " << getBlockName(*WBI)
///
/// This routine is called when we are unable to use the CFG to walk through
/// all of the basic blocks and form a chain due to unnatural loops in the CFG.
-/// We walk through the sequence of blocks, starting from the
-/// LastUnplacedBlockIdx. We update this index to avoid re-scanning the entire
-/// sequence on repeated calls to this routine.
+/// We walk through the function's blocks in order, starting from the
+/// LastUnplacedBlockIt. We update this iterator on each call to avoid
+/// re-scanning the entire sequence on repeated calls to this routine.
MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
- const BlockChain &PlacedChain,
- ArrayRef<MachineBasicBlock *> Blocks,
- unsigned &PrevUnplacedBlockIdx) {
- for (unsigned i = PrevUnplacedBlockIdx, e = Blocks.size(); i != e; ++i) {
- MachineBasicBlock *BB = Blocks[i];
- if (BlockToChain[BB] != &PlacedChain) {
- PrevUnplacedBlockIdx = i;
- return BB;
+ MachineFunction &F, const BlockChain &PlacedChain,
+ MachineFunction::iterator &PrevUnplacedBlockIt,
+ const BlockFilterSet *BlockFilter) {
+ for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
+ ++I) {
+ if (BlockFilter && !BlockFilter->count(I))
+ continue;
+ if (BlockToChain[I] != &PlacedChain) {
+ PrevUnplacedBlockIt = I;
+ // Now select the head of the chain to which the unplaced block belongs
+ // as the block to place. This will force the entire chain to be placed,
+ // and satisfies the requirements of merging chains.
+ return *BlockToChain[I]->begin();
}
}
return 0;
void MachineBlockPlacement::buildChain(
MachineBasicBlock *BB,
BlockChain &Chain,
- ArrayRef<MachineBasicBlock *> Blocks,
SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
const BlockFilterSet *BlockFilter) {
assert(BB);
assert(BlockToChain[BB] == &Chain);
- assert(*Chain.begin() == BB);
- SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
- unsigned PrevUnplacedBlockIdx = 0;
+ MachineFunction &F = *BB->getParent();
+ MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
MachineBasicBlock *LoopHeaderBB = BB;
markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
assert(*llvm::prior(Chain.end()) == BB);
MachineBasicBlock *BestSucc = 0;
- // Check for unreasonable branches, and forcibly merge the existing layout
- // successor for them. We can handle cases that AnalyzeBranch can't: jump
- // tables etc are fine. The case we want to handle specially is when there
- // is potential fallthrough, but the branch cannot be analyzed. This
- // includes blocks without terminators as well as other cases.
- Cond.clear();
- MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
- if (TII->AnalyzeBranch(*BB, TBB, FBB, Cond) && BB->canFallThrough()) {
- MachineFunction::iterator I(BB), NextI(llvm::next(I));
- // Ensure that the layout successor is a viable block, as we know that
- // fallthrough is a possibility.
- assert(NextI != BB->getParent()->end());
- assert(!BlockFilter || BlockFilter->count(NextI));
- BestSucc = NextI;
- }
-
- // Otherwise, look for the best viable successor if there is one to place
- // immediately after this block.
- if (!BestSucc)
- BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
+ // Look for the best viable successor if there is one to place immediately
+ // after this block.
+ BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
// If an immediate successor isn't available, look for the best viable
// block among those we've identified as not violating the loop's CFG at
BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
if (!BestSucc) {
- BestSucc = getFirstUnplacedBlock(Chain, Blocks, PrevUnplacedBlockIdx);
+ BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
+ BlockFilter);
if (!BestSucc)
break;
BlockWorkList.push_back(*BI);
}
- buildChain(*L.block_begin(), LoopChain, L.getBlocks(), BlockWorkList,
- &LoopBlockSet);
+ buildChain(*L.block_begin(), LoopChain, BlockWorkList, &LoopBlockSet);
DEBUG({
// Crash at the end so we get all of the debugging output first.
for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
BCI != BCE; ++BCI)
if (!LoopBlockSet.erase(*BCI)) {
- BadLoop = true;
+ // We don't mark the loop as bad here because there are real situations
+ // where this can occur. For example, with an unanalyzable fallthrough
+ // from a loop block to a non-loop block or vice versa.
dbgs() << "Loop chain contains a block not contained by the loop!\n"
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
// Ensure that every BB in the function has an associated chain to simplify
// the assumptions of the remaining algorithm.
- for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
- BlockToChain[&*FI] =
- new (ChainAllocator.Allocate()) BlockChain(BlockToChain, &*FI);
+ SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
+ for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
+ MachineBasicBlock *BB = FI;
+ BlockChain *&Chain = BlockToChain[BB];
+ Chain = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
+ // Also, merge any blocks which we cannot reason about and must preserve
+ // the exact fallthrough behavior for.
+ for (;;) {
+ Cond.clear();
+ MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
+ if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
+ break;
+
+ MachineFunction::iterator NextFI(llvm::next(FI));
+ MachineBasicBlock *NextBB = NextFI;
+ // Ensure that the layout successor is a viable block, as we know that
+ // fallthrough is a possibility.
+ assert(NextFI != FE && "Can't fallthrough past the last block.");
+ DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
+ << getBlockName(BB) << " -> " << getBlockName(NextBB)
+ << "\n");
+ Chain->merge(NextBB, 0);
+ FI = NextFI;
+ BB = NextBB;
+ }
+ }
// Build any loop-based chains.
for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
++LI)
buildLoopChains(F, **LI);
- // We need a vector of blocks so that buildChain can handle unnatural CFG
- // constructs by searching for unplaced blocks and just concatenating them.
- SmallVector<MachineBasicBlock *, 16> Blocks;
- Blocks.reserve(F.size());
-
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
MachineBasicBlock *BB = &*FI;
- Blocks.push_back(BB);
BlockChain &Chain = *BlockToChain[BB];
if (!UpdatedPreds.insert(&Chain))
continue;
}
BlockChain &FunctionChain = *BlockToChain[&F.front()];
- buildChain(&F.front(), FunctionChain, Blocks, BlockWorkList);
+ buildChain(&F.front(), FunctionChain, BlockWorkList);
typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
DEBUG({
// Splice the blocks into place.
MachineFunction::iterator InsertPos = F.begin();
- SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
for (BlockChain::iterator BI = FunctionChain.begin(),
BE = FunctionChain.end();
BI != BE; ++BI) {
AlignLoops(F);
BlockToChain.clear();
+ ChainAllocator.DestroyAll();
// We always return true as we have no way to track whether the final order
// differs from the original order.
projects / oota-llvm.git / blobdiff