| //===-- MachineBlockPlacement.cpp - Basic Block Code Layout optimization --===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements basic block placement transformations using the CFG |
| // structure and branch probability estimates. |
| // |
| // The pass strives to preserve the structure of the CFG (that is, retain |
| // a topological ordering of basic blocks) in the absense of a *strong* signal |
| // to the contrary from probabilities. However, within the CFG structure, it |
| // attempts to choose an ordering which favors placing more likely sequences of |
| // blocks adjacent to each other. |
| // |
| // The algorithm works from the inner-most loop within a function outward, and |
| // at each stage walks through the basic blocks, trying to coalesce them into |
| // sequential chains where allowed by the CFG (or demanded by heavy |
| // probabilities). Finally, it walks the blocks in topological order, and the |
| // first time it reaches a chain of basic blocks, it schedules them in the |
| // function in-order. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "block-placement2" |
| #include "llvm/CodeGen/MachineBasicBlock.h" |
| #include "llvm/CodeGen/MachineBlockFrequencyInfo.h" |
| #include "llvm/CodeGen/MachineBranchProbabilityInfo.h" |
| #include "llvm/CodeGen/MachineFunction.h" |
| #include "llvm/CodeGen/MachineFunctionPass.h" |
| #include "llvm/CodeGen/MachineLoopInfo.h" |
| #include "llvm/CodeGen/MachineModuleInfo.h" |
| #include "llvm/CodeGen/Passes.h" |
| #include "llvm/Support/Allocator.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/ErrorHandling.h" |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/ADT/PostOrderIterator.h" |
| #include "llvm/ADT/SCCIterator.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Target/TargetInstrInfo.h" |
| #include "llvm/Target/TargetLowering.h" |
| #include <algorithm> |
| using namespace llvm; |
| |
| STATISTIC(NumCondBranches, "Number of conditional branches"); |
| STATISTIC(NumUncondBranches, "Number of uncondittional branches"); |
| STATISTIC(CondBranchTakenFreq, |
| "Potential frequency of taking conditional branches"); |
| STATISTIC(UncondBranchTakenFreq, |
| "Potential frequency of taking unconditional branches"); |
| |
| namespace { |
| /// \brief A structure for storing a weighted edge. |
| /// |
| /// This stores an edge and its weight, computed as the product of the |
| /// frequency that the starting block is entered with the probability of |
| /// a particular exit block. |
| struct WeightedEdge { |
| BlockFrequency EdgeFrequency; |
| MachineBasicBlock *From, *To; |
| |
| bool operator<(const WeightedEdge &RHS) const { |
| return EdgeFrequency < RHS.EdgeFrequency; |
| } |
| }; |
| } |
| |
| namespace { |
| class BlockChain; |
| /// \brief Type for our function-wide basic block -> block chain mapping. |
| typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType; |
| } |
| |
| namespace { |
| /// \brief A chain of blocks which will be laid out contiguously. |
| /// |
| /// This is the datastructure representing a chain of consecutive blocks that |
| /// are profitable to layout together in order to maximize fallthrough |
| /// probabilities. We also can use a block chain to represent a sequence of |
| /// basic blocks which have some external (correctness) requirement for |
| /// sequential layout. |
| /// |
| /// Eventually, the block chains will form a directed graph over the function. |
| /// We provide an SCC-supporting-iterator in order to quicky build and walk the |
| /// SCCs of block chains within a function. |
| /// |
| /// The block chains also have support for calculating and caching probability |
| /// information related to the chain itself versus other chains. This is used |
| /// for ranking during the final layout of block chains. |
| class BlockChain { |
| /// \brief The sequence of blocks belonging to this chain. |
| /// |
| /// This is the sequence of blocks for a particular chain. These will be laid |
| /// out in-order within the function. |
| SmallVector<MachineBasicBlock *, 4> Blocks; |
| |
| /// \brief A handle to the function-wide basic block to block chain mapping. |
| /// |
| /// This is retained in each block chain to simplify the computation of child |
| /// block chains for SCC-formation and iteration. We store the edges to child |
| /// basic blocks, and map them back to their associated chains using this |
| /// structure. |
| BlockToChainMapType &BlockToChain; |
| |
| public: |
| /// \brief Construct a new BlockChain. |
| /// |
| /// This builds a new block chain representing a single basic block in the |
| /// function. It also registers itself as the chain that block participates |
| /// in with the BlockToChain mapping. |
| BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB) |
| : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) { |
| assert(BB && "Cannot create a chain with a null basic block"); |
| BlockToChain[BB] = this; |
| } |
| |
| /// \brief Iterator over blocks within the chain. |
| typedef SmallVectorImpl<MachineBasicBlock *>::const_iterator iterator; |
| |
| /// \brief Beginning of blocks within the chain. |
| iterator begin() const { return Blocks.begin(); } |
| |
| /// \brief End of blocks within the chain. |
| iterator end() const { return Blocks.end(); } |
| |
| /// \brief Merge a block chain into this one. |
| /// |
| /// This routine merges a block chain into this one. It takes care of forming |
| /// a contiguous sequence of basic blocks, updating the edge list, and |
| /// updating the block -> chain mapping. It does not free or tear down the |
| /// old chain, but the old chain's block list is no longer valid. |
| void merge(MachineBasicBlock *BB, BlockChain *Chain) { |
| assert(BB); |
| assert(!Blocks.empty()); |
| |
| // Fast path in case we don't have a chain already. |
| if (!Chain) { |
| assert(!BlockToChain[BB]); |
| Blocks.push_back(BB); |
| BlockToChain[BB] = this; |
| return; |
| } |
| |
| assert(BB == *Chain->begin()); |
| assert(Chain->begin() != Chain->end()); |
| |
| // Update the incoming blocks to point to this chain, and add them to the |
| // chain structure. |
| for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end(); |
| BI != BE; ++BI) { |
| Blocks.push_back(*BI); |
| assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain"); |
| BlockToChain[*BI] = this; |
| } |
| } |
| |
| /// \brief Count of predecessors within the loop currently being processed. |
| /// |
| /// This count is updated at each loop we process to represent the number of |
| /// in-loop predecessors of this chain. |
| unsigned LoopPredecessors; |
| }; |
| } |
| |
| namespace { |
| class MachineBlockPlacement : public MachineFunctionPass { |
| /// \brief A typedef for a block filter set. |
| typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet; |
| |
| /// \brief A handle to the branch probability pass. |
| const MachineBranchProbabilityInfo *MBPI; |
| |
| /// \brief A handle to the function-wide block frequency pass. |
| const MachineBlockFrequencyInfo *MBFI; |
| |
| /// \brief A handle to the loop info. |
| const MachineLoopInfo *MLI; |
| |
| /// \brief A handle to the target's instruction info. |
| const TargetInstrInfo *TII; |
| |
| /// \brief A handle to the target's lowering info. |
| const TargetLowering *TLI; |
| |
| /// \brief Allocator and owner of BlockChain structures. |
| /// |
| /// We build BlockChains lazily by merging together high probability BB |
| /// sequences acording to the "Algo2" in the paper mentioned at the top of |
| /// the file. To reduce malloc traffic, we allocate them using this slab-like |
| /// allocator, and destroy them after the pass completes. |
| SpecificBumpPtrAllocator<BlockChain> ChainAllocator; |
| |
| /// \brief Function wide BasicBlock to BlockChain mapping. |
| /// |
| /// This mapping allows efficiently moving from any given basic block to the |
| /// BlockChain it participates in, if any. We use it to, among other things, |
| /// allow implicitly defining edges between chains as the existing edges |
| /// between basic blocks. |
| DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain; |
| |
| void markChainSuccessors(BlockChain &Chain, |
| MachineBasicBlock *LoopHeaderBB, |
| SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, |
| const BlockFilterSet *BlockFilter = 0); |
| MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB, |
| BlockChain &Chain, |
| const BlockFilterSet *BlockFilter); |
| MachineBasicBlock *selectBestCandidateBlock( |
| BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList, |
| const BlockFilterSet *BlockFilter); |
| MachineBasicBlock *getFirstUnplacedBlock(const BlockChain &PlacedChain, |
| ArrayRef<MachineBasicBlock *> Blocks, |
| unsigned &PrevUnplacedBlockIdx); |
| void buildChain(MachineBasicBlock *BB, BlockChain &Chain, |
| ArrayRef<MachineBasicBlock *> Blocks, |
| SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, |
| const BlockFilterSet *BlockFilter = 0); |
| void buildLoopChains(MachineFunction &F, MachineLoop &L); |
| void buildCFGChains(MachineFunction &F); |
| void AlignLoops(MachineFunction &F); |
| |
| public: |
| static char ID; // Pass identification, replacement for typeid |
| MachineBlockPlacement() : MachineFunctionPass(ID) { |
| initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnMachineFunction(MachineFunction &F); |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.addRequired<MachineBranchProbabilityInfo>(); |
| AU.addRequired<MachineBlockFrequencyInfo>(); |
| AU.addRequired<MachineLoopInfo>(); |
| MachineFunctionPass::getAnalysisUsage(AU); |
| } |
| |
| const char *getPassName() const { return "Block Placement"; } |
| }; |
| } |
| |
| char MachineBlockPlacement::ID = 0; |
| INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2", |
| "Branch Probability Basic Block Placement", false, false) |
| INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) |
| INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) |
| INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) |
| INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2", |
| "Branch Probability Basic Block Placement", false, false) |
| |
| FunctionPass *llvm::createMachineBlockPlacementPass() { |
| return new MachineBlockPlacement(); |
| } |
| |
| #ifndef NDEBUG |
| /// \brief Helper to print the name of a MBB. |
| /// |
| /// Only used by debug logging. |
| static std::string getBlockName(MachineBasicBlock *BB) { |
| std::string Result; |
| raw_string_ostream OS(Result); |
| OS << "BB#" << BB->getNumber() |
| << " (derived from LLVM BB '" << BB->getName() << "')"; |
| OS.flush(); |
| return Result; |
| } |
| |
| /// \brief Helper to print the number of a MBB. |
| /// |
| /// Only used by debug logging. |
| static std::string getBlockNum(MachineBasicBlock *BB) { |
| std::string Result; |
| raw_string_ostream OS(Result); |
| OS << "BB#" << BB->getNumber(); |
| OS.flush(); |
| return Result; |
| } |
| #endif |
| |
| /// \brief Mark a chain's successors as having one fewer preds. |
| /// |
| /// When a chain is being merged into the "placed" chain, this routine will |
| /// quickly walk the successors of each block in the chain and mark them as |
| /// having one fewer active predecessor. It also adds any successors of this |
| /// chain which reach the zero-predecessor state to the worklist passed in. |
| void MachineBlockPlacement::markChainSuccessors( |
| BlockChain &Chain, |
| MachineBasicBlock *LoopHeaderBB, |
| SmallVectorImpl<MachineBasicBlock *> &BlockWorkList, |
| const BlockFilterSet *BlockFilter) { |
| // Walk all the blocks in this chain, marking their successors as having |
| // a predecessor placed. |
| for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end(); |
| CBI != CBE; ++CBI) { |
| // Add any successors for which this is the only un-placed in-loop |
| // predecessor to the worklist as a viable candidate for CFG-neutral |
| // placement. No subsequent placement of this block will violate the CFG |
| // shape, so we get to use heuristics to choose a favorable placement. |
| for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(), |
| SE = (*CBI)->succ_end(); |
| SI != SE; ++SI) { |
| if (BlockFilter && !BlockFilter->count(*SI)) |
| continue; |
| BlockChain &SuccChain = *BlockToChain[*SI]; |
| // Disregard edges within a fixed chain, or edges to the loop header. |
| if (&Chain == &SuccChain || *SI == LoopHeaderBB) |
| continue; |
| |
| // This is a cross-chain edge that is within the loop, so decrement the |
| // loop predecessor count of the destination chain. |
| if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0) |
| BlockWorkList.push_back(*SI); |
| } |
| } |
| } |
| |
| /// \brief Select the best successor for a block. |
| /// |
| /// This looks across all successors of a particular block and attempts to |
| /// select the "best" one to be the layout successor. It only considers direct |
| /// successors which also pass the block filter. It will attempt to avoid |
| /// breaking CFG structure, but cave and break such structures in the case of |
| /// very hot successor edges. |
| /// |
| /// \returns The best successor block found, or null if none are viable. |
| MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor( |
| MachineBasicBlock *BB, BlockChain &Chain, |
| const BlockFilterSet *BlockFilter) { |
| const BranchProbability HotProb(4, 5); // 80% |
| |
| MachineBasicBlock *BestSucc = 0; |
| // 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(); |
| SI != SE; ++SI) { |
| if (BlockFilter && !BlockFilter->count(*SI)) |
| continue; |
| BlockChain &SuccChain = *BlockToChain[*SI]; |
| if (&SuccChain == &Chain) { |
| DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n"); |
| continue; |
| } |
| |
| 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; |
| } |
| |
| DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb |
| << " (prob)" |
| << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "") |
| << "\n"); |
| if (BestSucc && BestWeight >= SuccWeight) |
| continue; |
| BestSucc = *SI; |
| 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 |
| /// blocks and select the most profitable one to place. The definition of |
| /// profitable only really makes sense in the context of a loop. This returns |
| /// the most frequently visited block in the worklist, which in the case of |
| /// a loop, is the one most desirable to be physically close to the rest of the |
| /// loop body in order to improve icache behavior. |
| /// |
| /// \returns The best block found, or null if none are viable. |
| 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) { |
| assert(!BlockFilter || BlockFilter->count(*WBI)); |
| BlockChain &SuccChain = *BlockToChain[*WBI]; |
| if (&SuccChain == &Chain) { |
| DEBUG(dbgs() << " " << getBlockName(*WBI) |
| << " -> Already merged!\n"); |
| continue; |
| } |
| assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block"); |
| |
| BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI); |
| DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> " << CandidateFreq |
| << " (freq)\n"); |
| if (BestBlock && BestFreq >= CandidateFreq) |
| continue; |
| BestBlock = *WBI; |
| BestFreq = CandidateFreq; |
| } |
| return BestBlock; |
| } |
| |
| /// \brief Retrieve the first unplaced basic block. |
| /// |
| /// 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. |
| 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; |
| } |
| } |
| 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; |
| |
| MachineBasicBlock *LoopHeaderBB = BB; |
| markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter); |
| BB = *llvm::prior(Chain.end()); |
| for (;;) { |
| assert(BB); |
| assert(BlockToChain[BB] == &Chain); |
| 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. Note that this may not be a valid block |
| // in the loop, but we allow that to cope with degenerate situations. |
| assert(NextI != BB->getParent()->end()); |
| 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); |
| |
| // 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 |
| // this point. This won't be a fallthrough, but it will increase locality. |
| if (!BestSucc) |
| BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter); |
| |
| if (!BestSucc) { |
| BestSucc = getFirstUnplacedBlock(Chain, Blocks, PrevUnplacedBlockIdx); |
| if (!BestSucc) |
| break; |
| |
| DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the " |
| "layout successor until the CFG reduces\n"); |
| } |
| |
| // Place this block, updating the datastructures to reflect its placement. |
| BlockChain &SuccChain = *BlockToChain[BestSucc]; |
| // Zero out LoopPredecessors for the successor we're about to merge in case |
| // we selected a successor that didn't fit naturally into the CFG. |
| SuccChain.LoopPredecessors = 0; |
| DEBUG(dbgs() << "Merging from " << getBlockNum(BB) |
| << " to " << getBlockNum(BestSucc) << "\n"); |
| markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter); |
| Chain.merge(BestSucc, &SuccChain); |
| BB = *llvm::prior(Chain.end()); |
| }; |
| |
| DEBUG(dbgs() << "Finished forming chain for header block " |
| << getBlockNum(*Chain.begin()) << "\n"); |
| } |
| |
| /// \brief Forms basic block chains from the natural loop structures. |
| /// |
| /// These chains are designed to preserve the existing *structure* of the code |
| /// as much as possible. We can then stitch the chains together in a way which |
| /// both preserves the topological structure and minimizes taken conditional |
| /// branches. |
| void MachineBlockPlacement::buildLoopChains(MachineFunction &F, |
| MachineLoop &L) { |
| // First recurse through any nested loops, building chains for those inner |
| // loops. |
| for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI) |
| buildLoopChains(F, **LI); |
| |
| SmallVector<MachineBasicBlock *, 16> BlockWorkList; |
| BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end()); |
| BlockChain &LoopChain = *BlockToChain[L.getHeader()]; |
| |
| // FIXME: This is a really lame way of walking the chains in the loop: we |
| // walk the blocks, and use a set to prevent visiting a particular chain |
| // twice. |
| SmallPtrSet<BlockChain *, 4> UpdatedPreds; |
| for (MachineLoop::block_iterator BI = L.block_begin(), |
| BE = L.block_end(); |
| BI != BE; ++BI) { |
| BlockChain &Chain = *BlockToChain[*BI]; |
| if (!UpdatedPreds.insert(&Chain) || BI == L.block_begin()) |
| continue; |
| |
| assert(Chain.LoopPredecessors == 0); |
| for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end(); |
| BCI != BCE; ++BCI) { |
| assert(BlockToChain[*BCI] == &Chain); |
| for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(), |
| PE = (*BCI)->pred_end(); |
| PI != PE; ++PI) { |
| if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI)) |
| continue; |
| ++Chain.LoopPredecessors; |
| } |
| } |
| |
| if (Chain.LoopPredecessors == 0) |
| BlockWorkList.push_back(*BI); |
| } |
| |
| buildChain(*L.block_begin(), LoopChain, L.getBlocks(), BlockWorkList, |
| &LoopBlockSet); |
| |
| DEBUG({ |
| // Crash at the end so we get all of the debugging output first. |
| bool BadLoop = false; |
| if (LoopChain.LoopPredecessors) { |
| BadLoop = true; |
| dbgs() << "Loop chain contains a block without its preds placed!\n" |
| << " Loop header: " << getBlockName(*L.block_begin()) << "\n" |
| << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"; |
| } |
| for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end(); |
| BCI != BCE; ++BCI) |
| if (!LoopBlockSet.erase(*BCI)) { |
| // 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. |
| // FIXME: Such constructs shouldn't exist. Track them down and fix them. |
| 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" |
| << " Bad block: " << getBlockName(*BCI) << "\n"; |
| } |
| |
| if (!LoopBlockSet.empty()) { |
| BadLoop = true; |
| for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(), |
| LBE = LoopBlockSet.end(); |
| LBI != LBE; ++LBI) |
| dbgs() << "Loop contains blocks never placed into a chain!\n" |
| << " Loop header: " << getBlockName(*L.block_begin()) << "\n" |
| << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n" |
| << " Bad block: " << getBlockName(*LBI) << "\n"; |
| } |
| assert(!BadLoop && "Detected problems with the placement of this loop."); |
| }); |
| } |
| |
| 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); |
| |
| // 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; |
| |
| assert(Chain.LoopPredecessors == 0); |
| for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end(); |
| BCI != BCE; ++BCI) { |
| assert(BlockToChain[*BCI] == &Chain); |
| for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(), |
| PE = (*BCI)->pred_end(); |
| PI != PE; ++PI) { |
| if (BlockToChain[*PI] == &Chain) |
| continue; |
| ++Chain.LoopPredecessors; |
| } |
| } |
| |
| if (Chain.LoopPredecessors == 0) |
| BlockWorkList.push_back(BB); |
| } |
| |
| BlockChain &FunctionChain = *BlockToChain[&F.front()]; |
| buildChain(&F.front(), FunctionChain, Blocks, BlockWorkList); |
| |
| typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType; |
| DEBUG({ |
| // Crash at the end so we get all of the debugging output first. |
| bool BadFunc = false; |
| FunctionBlockSetType FunctionBlockSet; |
| for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) |
| FunctionBlockSet.insert(FI); |
| |
| for (BlockChain::iterator BCI = FunctionChain.begin(), |
| BCE = FunctionChain.end(); |
| BCI != BCE; ++BCI) |
| if (!FunctionBlockSet.erase(*BCI)) { |
| BadFunc = true; |
| dbgs() << "Function chain contains a block not in the function!\n" |
| << " Bad block: " << getBlockName(*BCI) << "\n"; |
| } |
| |
| if (!FunctionBlockSet.empty()) { |
| BadFunc = true; |
| for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(), |
| FBE = FunctionBlockSet.end(); |
| FBI != FBE; ++FBI) |
| dbgs() << "Function contains blocks never placed into a chain!\n" |
| << " Bad block: " << getBlockName(*FBI) << "\n"; |
| } |
| assert(!BadFunc && "Detected problems with the block placement."); |
| }); |
| |
| // 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) { |
| DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain " |
| : " ... ") |
| << getBlockName(*BI) << "\n"); |
| if (InsertPos != MachineFunction::iterator(*BI)) |
| F.splice(InsertPos, *BI); |
| else |
| ++InsertPos; |
| |
| // Update the terminator of the previous block. |
| if (BI == FunctionChain.begin()) |
| continue; |
| MachineBasicBlock *PrevBB = llvm::prior(MachineFunction::iterator(*BI)); |
| |
| // 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(*PrevBB, TBB, FBB, Cond)) |
| PrevBB->updateTerminator(); |
| } |
| |
| // Fixup the last block. |
| Cond.clear(); |
| MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch. |
| if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond)) |
| F.back().updateTerminator(); |
| } |
| |
| /// \brief Recursive helper to align a loop and any nested loops. |
| static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) { |
| // Recurse through nested loops. |
| for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I) |
| AlignLoop(F, *I, Align); |
| |
| L->getTopBlock()->setAlignment(Align); |
| } |
| |
| /// \brief Align loop headers to target preferred alignments. |
| void MachineBlockPlacement::AlignLoops(MachineFunction &F) { |
| if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize)) |
| return; |
| |
| unsigned Align = TLI->getPrefLoopAlignment(); |
| if (!Align) |
| return; // Don't care about loop alignment. |
| |
| for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I) |
| AlignLoop(F, *I, Align); |
| } |
| |
| bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) { |
| // Check for single-block functions and skip them. |
| if (llvm::next(F.begin()) == F.end()) |
| return false; |
| |
| MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); |
| MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); |
| MLI = &getAnalysis<MachineLoopInfo>(); |
| TII = F.getTarget().getInstrInfo(); |
| TLI = F.getTarget().getTargetLowering(); |
| assert(BlockToChain.empty()); |
| |
| buildCFGChains(F); |
| 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. |
| return true; |
| } |
| |
| namespace { |
| /// \brief A pass to compute block placement statistics. |
| /// |
| /// A separate pass to compute interesting statistics for evaluating block |
| /// placement. This is separate from the actual placement pass so that they can |
| /// be computed in the absense of any placement transformations or when using |
| /// alternative placement strategies. |
| class MachineBlockPlacementStats : public MachineFunctionPass { |
| /// \brief A handle to the branch probability pass. |
| const MachineBranchProbabilityInfo *MBPI; |
| |
| /// \brief A handle to the function-wide block frequency pass. |
| const MachineBlockFrequencyInfo *MBFI; |
| |
| public: |
| static char ID; // Pass identification, replacement for typeid |
| MachineBlockPlacementStats() : MachineFunctionPass(ID) { |
| initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnMachineFunction(MachineFunction &F); |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.addRequired<MachineBranchProbabilityInfo>(); |
| AU.addRequired<MachineBlockFrequencyInfo>(); |
| AU.setPreservesAll(); |
| MachineFunctionPass::getAnalysisUsage(AU); |
| } |
| |
| const char *getPassName() const { return "Block Placement Stats"; } |
| }; |
| } |
| |
| char MachineBlockPlacementStats::ID = 0; |
| INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats", |
| "Basic Block Placement Stats", false, false) |
| INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) |
| INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) |
| INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats", |
| "Basic Block Placement Stats", false, false) |
| |
| FunctionPass *llvm::createMachineBlockPlacementStatsPass() { |
| return new MachineBlockPlacementStats(); |
| } |
| |
| bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) { |
| // Check for single-block functions and skip them. |
| if (llvm::next(F.begin()) == F.end()) |
| return false; |
| |
| MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); |
| MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); |
| |
| for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { |
| BlockFrequency BlockFreq = MBFI->getBlockFreq(I); |
| Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches |
| : NumUncondBranches; |
| Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq |
| : UncondBranchTakenFreq; |
| for (MachineBasicBlock::succ_iterator SI = I->succ_begin(), |
| SE = I->succ_end(); |
| SI != SE; ++SI) { |
| // Skip if this successor is a fallthrough. |
| if (I->isLayoutSuccessor(*SI)) |
| continue; |
| |
| BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI); |
| ++NumBranches; |
| BranchTakenFreq += EdgeFreq.getFrequency(); |
| } |
| } |
| |
| return false; |
| } |
| |