diff options
Diffstat (limited to 'llvm/lib/CodeGen/MachineBlockPlacement.cpp')
| -rw-r--r-- | llvm/lib/CodeGen/MachineBlockPlacement.cpp | 3161 |
1 files changed, 3161 insertions, 0 deletions
diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp new file mode 100644 index 000000000000..ac19bc0bd8ea --- /dev/null +++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp @@ -0,0 +1,3161 @@ +//===- MachineBlockPlacement.cpp - Basic Block Code Layout optimization ---===// +// +// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. +// See https://llvm.org/LICENSE.txt for license information. +// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception +// +//===----------------------------------------------------------------------===// +// +// 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 absence 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. +// +//===----------------------------------------------------------------------===// + +#include "BranchFolding.h" +#include "llvm/ADT/ArrayRef.h" +#include "llvm/ADT/DenseMap.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/SetVector.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/BlockFrequencyInfoImpl.h" +#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/MachinePostDominators.h" +#include "llvm/CodeGen/TailDuplicator.h" +#include "llvm/CodeGen/TargetInstrInfo.h" +#include "llvm/CodeGen/TargetLowering.h" +#include "llvm/CodeGen/TargetPassConfig.h" +#include "llvm/CodeGen/TargetSubtargetInfo.h" +#include "llvm/IR/DebugLoc.h" +#include "llvm/IR/Function.h" +#include "llvm/Pass.h" +#include "llvm/Support/Allocator.h" +#include "llvm/Support/BlockFrequency.h" +#include "llvm/Support/BranchProbability.h" +#include "llvm/Support/CodeGen.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Compiler.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Target/TargetMachine.h" +#include <algorithm> +#include <cassert> +#include <cstdint> +#include <iterator> +#include <memory> +#include <string> +#include <tuple> +#include <utility> +#include <vector> + +using namespace llvm; + +#define DEBUG_TYPE "block-placement" + +STATISTIC(NumCondBranches, "Number of conditional branches"); +STATISTIC(NumUncondBranches, "Number of unconditional branches"); +STATISTIC(CondBranchTakenFreq, + "Potential frequency of taking conditional branches"); +STATISTIC(UncondBranchTakenFreq, + "Potential frequency of taking unconditional branches"); + +static cl::opt<unsigned> AlignAllBlock( + "align-all-blocks", + cl::desc("Force the alignment of all blocks in the function in log2 format " + "(e.g 4 means align on 16B boundaries)."), + cl::init(0), cl::Hidden); + +static cl::opt<unsigned> AlignAllNonFallThruBlocks( + "align-all-nofallthru-blocks", + cl::desc("Force the alignment of all blocks that have no fall-through " + "predecessors (i.e. don't add nops that are executed). In log2 " + "format (e.g 4 means align on 16B boundaries)."), + cl::init(0), cl::Hidden); + +// FIXME: Find a good default for this flag and remove the flag. +static cl::opt<unsigned> ExitBlockBias( + "block-placement-exit-block-bias", + cl::desc("Block frequency percentage a loop exit block needs " + "over the original exit to be considered the new exit."), + cl::init(0), cl::Hidden); + +// Definition: +// - Outlining: placement of a basic block outside the chain or hot path. + +static cl::opt<unsigned> LoopToColdBlockRatio( + "loop-to-cold-block-ratio", + cl::desc("Outline loop blocks from loop chain if (frequency of loop) / " + "(frequency of block) is greater than this ratio"), + cl::init(5), cl::Hidden); + +static cl::opt<bool> ForceLoopColdBlock( + "force-loop-cold-block", + cl::desc("Force outlining cold blocks from loops."), + cl::init(false), cl::Hidden); + +static cl::opt<bool> + PreciseRotationCost("precise-rotation-cost", + cl::desc("Model the cost of loop rotation more " + "precisely by using profile data."), + cl::init(false), cl::Hidden); + +static cl::opt<bool> + ForcePreciseRotationCost("force-precise-rotation-cost", + cl::desc("Force the use of precise cost " + "loop rotation strategy."), + cl::init(false), cl::Hidden); + +static cl::opt<unsigned> MisfetchCost( + "misfetch-cost", + cl::desc("Cost that models the probabilistic risk of an instruction " + "misfetch due to a jump comparing to falling through, whose cost " + "is zero."), + cl::init(1), cl::Hidden); + +static cl::opt<unsigned> JumpInstCost("jump-inst-cost", + cl::desc("Cost of jump instructions."), + cl::init(1), cl::Hidden); +static cl::opt<bool> +TailDupPlacement("tail-dup-placement", + cl::desc("Perform tail duplication during placement. " + "Creates more fallthrough opportunites in " + "outline branches."), + cl::init(true), cl::Hidden); + +static cl::opt<bool> +BranchFoldPlacement("branch-fold-placement", + cl::desc("Perform branch folding during placement. " + "Reduces code size."), + cl::init(true), cl::Hidden); + +// Heuristic for tail duplication. +static cl::opt<unsigned> TailDupPlacementThreshold( + "tail-dup-placement-threshold", + cl::desc("Instruction cutoff for tail duplication during layout. " + "Tail merging during layout is forced to have a threshold " + "that won't conflict."), cl::init(2), + cl::Hidden); + +// Heuristic for aggressive tail duplication. +static cl::opt<unsigned> TailDupPlacementAggressiveThreshold( + "tail-dup-placement-aggressive-threshold", + cl::desc("Instruction cutoff for aggressive tail duplication during " + "layout. Used at -O3. Tail merging during layout is forced to " + "have a threshold that won't conflict."), cl::init(4), + cl::Hidden); + +// Heuristic for tail duplication. +static cl::opt<unsigned> TailDupPlacementPenalty( + "tail-dup-placement-penalty", + cl::desc("Cost penalty for blocks that can avoid breaking CFG by copying. " + "Copying can increase fallthrough, but it also increases icache " + "pressure. This parameter controls the penalty to account for that. " + "Percent as integer."), + cl::init(2), + cl::Hidden); + +// Heuristic for triangle chains. +static cl::opt<unsigned> TriangleChainCount( + "triangle-chain-count", + cl::desc("Number of triangle-shaped-CFG's that need to be in a row for the " + "triangle tail duplication heuristic to kick in. 0 to disable."), + cl::init(2), + cl::Hidden); + +extern cl::opt<unsigned> StaticLikelyProb; +extern cl::opt<unsigned> ProfileLikelyProb; + +// Internal option used to control BFI display only after MBP pass. +// Defined in CodeGen/MachineBlockFrequencyInfo.cpp: +// -view-block-layout-with-bfi= +extern cl::opt<GVDAGType> ViewBlockLayoutWithBFI; + +// Command line option to specify the name of the function for CFG dump +// Defined in Analysis/BlockFrequencyInfo.cpp: -view-bfi-func-name= +extern cl::opt<std::string> ViewBlockFreqFuncName; + +namespace { + +class BlockChain; + +/// Type for our function-wide basic block -> block chain mapping. +using BlockToChainMapType = DenseMap<const MachineBasicBlock *, BlockChain *>; + +/// 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 and code locality. We also can use a block chain to represent +/// a sequence of basic blocks which have some external (correctness) +/// requirement for sequential layout. +/// +/// Chains can be built around a single basic block and can be merged to grow +/// them. They participate in a block-to-chain mapping, which is updated +/// automatically as chains are merged together. +class BlockChain { + /// 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; + + /// 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: + /// 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) { + assert(BB && "Cannot create a chain with a null basic block"); + BlockToChain[BB] = this; + } + + /// Iterator over blocks within the chain. + using iterator = SmallVectorImpl<MachineBasicBlock *>::iterator; + using const_iterator = SmallVectorImpl<MachineBasicBlock *>::const_iterator; + + /// Beginning of blocks within the chain. + iterator begin() { return Blocks.begin(); } + const_iterator begin() const { return Blocks.begin(); } + + /// End of blocks within the chain. + iterator end() { return Blocks.end(); } + const_iterator end() const { return Blocks.end(); } + + bool remove(MachineBasicBlock* BB) { + for(iterator i = begin(); i != end(); ++i) { + if (*i == BB) { + Blocks.erase(i); + return true; + } + } + return false; + } + + /// 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 && "Can't merge a null block."); + assert(!Blocks.empty() && "Can't merge into an empty chain."); + + // Fast path in case we don't have a chain already. + if (!Chain) { + assert(!BlockToChain[BB] && + "Passed chain is null, but BB has entry in BlockToChain."); + Blocks.push_back(BB); + BlockToChain[BB] = this; + return; + } + + assert(BB == *Chain->begin() && "Passed BB is not head of Chain."); + assert(Chain->begin() != Chain->end()); + + // Update the incoming blocks to point to this chain, and add them to the + // chain structure. + for (MachineBasicBlock *ChainBB : *Chain) { + Blocks.push_back(ChainBB); + assert(BlockToChain[ChainBB] == Chain && "Incoming blocks not in chain."); + BlockToChain[ChainBB] = this; + } + } + +#ifndef NDEBUG + /// Dump the blocks in this chain. + LLVM_DUMP_METHOD void dump() { + for (MachineBasicBlock *MBB : *this) + MBB->dump(); + } +#endif // NDEBUG + + /// Count of predecessors of any block within the chain which have not + /// yet been scheduled. In general, we will delay scheduling this chain + /// until those predecessors are scheduled (or we find a sufficiently good + /// reason to override this heuristic.) Note that when forming loop chains, + /// blocks outside the loop are ignored and treated as if they were already + /// scheduled. + /// + /// Note: This field is reinitialized multiple times - once for each loop, + /// and then once for the function as a whole. + unsigned UnscheduledPredecessors = 0; +}; + +class MachineBlockPlacement : public MachineFunctionPass { + /// A type for a block filter set. + using BlockFilterSet = SmallSetVector<const MachineBasicBlock *, 16>; + + /// Pair struct containing basic block and taildup profitability + struct BlockAndTailDupResult { + MachineBasicBlock *BB; + bool ShouldTailDup; + }; + + /// Triple struct containing edge weight and the edge. + struct WeightedEdge { + BlockFrequency Weight; + MachineBasicBlock *Src; + MachineBasicBlock *Dest; + }; + + /// work lists of blocks that are ready to be laid out + SmallVector<MachineBasicBlock *, 16> BlockWorkList; + SmallVector<MachineBasicBlock *, 16> EHPadWorkList; + + /// Edges that have already been computed as optimal. + DenseMap<const MachineBasicBlock *, BlockAndTailDupResult> ComputedEdges; + + /// Machine Function + MachineFunction *F; + + /// A handle to the branch probability pass. + const MachineBranchProbabilityInfo *MBPI; + + /// A handle to the function-wide block frequency pass. + std::unique_ptr<BranchFolder::MBFIWrapper> MBFI; + + /// A handle to the loop info. + MachineLoopInfo *MLI; + + /// Preferred loop exit. + /// Member variable for convenience. It may be removed by duplication deep + /// in the call stack. + MachineBasicBlock *PreferredLoopExit; + + /// A handle to the target's instruction info. + const TargetInstrInfo *TII; + + /// A handle to the target's lowering info. + const TargetLoweringBase *TLI; + + /// A handle to the post dominator tree. + MachinePostDominatorTree *MPDT; + + /// Duplicator used to duplicate tails during placement. + /// + /// Placement decisions can open up new tail duplication opportunities, but + /// since tail duplication affects placement decisions of later blocks, it + /// must be done inline. + TailDuplicator TailDup; + + /// Allocator and owner of BlockChain structures. + /// + /// We build BlockChains lazily while processing the loop structure of + /// a function. To reduce malloc traffic, we allocate them using this + /// slab-like allocator, and destroy them after the pass completes. An + /// important guarantee is that this allocator produces stable pointers to + /// the chains. + SpecificBumpPtrAllocator<BlockChain> ChainAllocator; + + /// 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<const MachineBasicBlock *, BlockChain *> BlockToChain; + +#ifndef NDEBUG + /// The set of basic blocks that have terminators that cannot be fully + /// analyzed. These basic blocks cannot be re-ordered safely by + /// MachineBlockPlacement, and we must preserve physical layout of these + /// blocks and their successors through the pass. + SmallPtrSet<MachineBasicBlock *, 4> BlocksWithUnanalyzableExits; +#endif + + /// Decrease the UnscheduledPredecessors count for all blocks in chain, and + /// if the count goes to 0, add them to the appropriate work list. + void markChainSuccessors( + const BlockChain &Chain, const MachineBasicBlock *LoopHeaderBB, + const BlockFilterSet *BlockFilter = nullptr); + + /// Decrease the UnscheduledPredecessors count for a single block, and + /// if the count goes to 0, add them to the appropriate work list. + void markBlockSuccessors( + const BlockChain &Chain, const MachineBasicBlock *BB, + const MachineBasicBlock *LoopHeaderBB, + const BlockFilterSet *BlockFilter = nullptr); + + BranchProbability + collectViableSuccessors( + const MachineBasicBlock *BB, const BlockChain &Chain, + const BlockFilterSet *BlockFilter, + SmallVector<MachineBasicBlock *, 4> &Successors); + bool shouldPredBlockBeOutlined( + const MachineBasicBlock *BB, const MachineBasicBlock *Succ, + const BlockChain &Chain, const BlockFilterSet *BlockFilter, + BranchProbability SuccProb, BranchProbability HotProb); + bool repeatedlyTailDuplicateBlock( + MachineBasicBlock *BB, MachineBasicBlock *&LPred, + const MachineBasicBlock *LoopHeaderBB, + BlockChain &Chain, BlockFilterSet *BlockFilter, + MachineFunction::iterator &PrevUnplacedBlockIt); + bool maybeTailDuplicateBlock( + MachineBasicBlock *BB, MachineBasicBlock *LPred, + BlockChain &Chain, BlockFilterSet *BlockFilter, + MachineFunction::iterator &PrevUnplacedBlockIt, + bool &DuplicatedToLPred); + bool hasBetterLayoutPredecessor( + const MachineBasicBlock *BB, const MachineBasicBlock *Succ, + const BlockChain &SuccChain, BranchProbability SuccProb, + BranchProbability RealSuccProb, const BlockChain &Chain, + const BlockFilterSet *BlockFilter); + BlockAndTailDupResult selectBestSuccessor( + const MachineBasicBlock *BB, const BlockChain &Chain, + const BlockFilterSet *BlockFilter); + MachineBasicBlock *selectBestCandidateBlock( + const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList); + MachineBasicBlock *getFirstUnplacedBlock( + const BlockChain &PlacedChain, + MachineFunction::iterator &PrevUnplacedBlockIt, + const BlockFilterSet *BlockFilter); + + /// Add a basic block to the work list if it is appropriate. + /// + /// If the optional parameter BlockFilter is provided, only MBB + /// present in the set will be added to the worklist. If nullptr + /// is provided, no filtering occurs. + void fillWorkLists(const MachineBasicBlock *MBB, + SmallPtrSetImpl<BlockChain *> &UpdatedPreds, + const BlockFilterSet *BlockFilter); + + void buildChain(const MachineBasicBlock *BB, BlockChain &Chain, + BlockFilterSet *BlockFilter = nullptr); + bool canMoveBottomBlockToTop(const MachineBasicBlock *BottomBlock, + const MachineBasicBlock *OldTop); + bool hasViableTopFallthrough(const MachineBasicBlock *Top, + const BlockFilterSet &LoopBlockSet); + BlockFrequency TopFallThroughFreq(const MachineBasicBlock *Top, + const BlockFilterSet &LoopBlockSet); + BlockFrequency FallThroughGains(const MachineBasicBlock *NewTop, + const MachineBasicBlock *OldTop, + const MachineBasicBlock *ExitBB, + const BlockFilterSet &LoopBlockSet); + MachineBasicBlock *findBestLoopTopHelper(MachineBasicBlock *OldTop, + const MachineLoop &L, const BlockFilterSet &LoopBlockSet); + MachineBasicBlock *findBestLoopTop( + const MachineLoop &L, const BlockFilterSet &LoopBlockSet); + MachineBasicBlock *findBestLoopExit( + const MachineLoop &L, const BlockFilterSet &LoopBlockSet, + BlockFrequency &ExitFreq); + BlockFilterSet collectLoopBlockSet(const MachineLoop &L); + void buildLoopChains(const MachineLoop &L); + void rotateLoop( + BlockChain &LoopChain, const MachineBasicBlock *ExitingBB, + BlockFrequency ExitFreq, const BlockFilterSet &LoopBlockSet); + void rotateLoopWithProfile( + BlockChain &LoopChain, const MachineLoop &L, + const BlockFilterSet &LoopBlockSet); + void buildCFGChains(); + void optimizeBranches(); + void alignBlocks(); + /// Returns true if a block should be tail-duplicated to increase fallthrough + /// opportunities. + bool shouldTailDuplicate(MachineBasicBlock *BB); + /// Check the edge frequencies to see if tail duplication will increase + /// fallthroughs. + bool isProfitableToTailDup( + const MachineBasicBlock *BB, const MachineBasicBlock *Succ, + BranchProbability QProb, + const BlockChain &Chain, const BlockFilterSet *BlockFilter); + + /// Check for a trellis layout. + bool isTrellis(const MachineBasicBlock *BB, + const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs, + const BlockChain &Chain, const BlockFilterSet *BlockFilter); + + /// Get the best successor given a trellis layout. + BlockAndTailDupResult getBestTrellisSuccessor( + const MachineBasicBlock *BB, + const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs, + BranchProbability AdjustedSumProb, const BlockChain &Chain, + const BlockFilterSet *BlockFilter); + + /// Get the best pair of non-conflicting edges. + static std::pair<WeightedEdge, WeightedEdge> getBestNonConflictingEdges( + const MachineBasicBlock *BB, + MutableArrayRef<SmallVector<WeightedEdge, 8>> Edges); + + /// Returns true if a block can tail duplicate into all unplaced + /// predecessors. Filters based on loop. + bool canTailDuplicateUnplacedPreds( + const MachineBasicBlock *BB, MachineBasicBlock *Succ, + const BlockChain &Chain, const BlockFilterSet *BlockFilter); + + /// Find chains of triangles to tail-duplicate where a global analysis works, + /// but a local analysis would not find them. + void precomputeTriangleChains(); + +public: + static char ID; // Pass identification, replacement for typeid + + MachineBlockPlacement() : MachineFunctionPass(ID) { + initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry()); + } + + bool runOnMachineFunction(MachineFunction &F) override; + + bool allowTailDupPlacement() const { + assert(F); + return TailDupPlacement && !F->getTarget().requiresStructuredCFG(); + } + + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.addRequired<MachineBranchProbabilityInfo>(); + AU.addRequired<MachineBlockFrequencyInfo>(); + if (TailDupPlacement) + AU.addRequired<MachinePostDominatorTree>(); + AU.addRequired<MachineLoopInfo>(); + AU.addRequired<TargetPassConfig>(); + MachineFunctionPass::getAnalysisUsage(AU); + } +}; + +} // end anonymous namespace + +char MachineBlockPlacement::ID = 0; + +char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID; + +INITIALIZE_PASS_BEGIN(MachineBlockPlacement, DEBUG_TYPE, + "Branch Probability Basic Block Placement", false, false) +INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo) +INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo) +INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTree) +INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) +INITIALIZE_PASS_END(MachineBlockPlacement, DEBUG_TYPE, + "Branch Probability Basic Block Placement", false, false) + +#ifndef NDEBUG +/// Helper to print the name of a MBB. +/// +/// Only used by debug logging. +static std::string getBlockName(const MachineBasicBlock *BB) { + std::string Result; + raw_string_ostream OS(Result); + OS << printMBBReference(*BB); + OS << " ('" << BB->getName() << "')"; + OS.flush(); + return Result; +} +#endif + +/// 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 appropriate worklist. +void MachineBlockPlacement::markChainSuccessors( + const BlockChain &Chain, const MachineBasicBlock *LoopHeaderBB, + const BlockFilterSet *BlockFilter) { + // Walk all the blocks in this chain, marking their successors as having + // a predecessor placed. + for (MachineBasicBlock *MBB : Chain) { + markBlockSuccessors(Chain, MBB, LoopHeaderBB, BlockFilter); + } +} + +/// Mark a single block's successors as having one fewer preds. +/// +/// Under normal circumstances, this is only called by markChainSuccessors, +/// but if a block that was to be placed is completely tail-duplicated away, +/// and was duplicated into the chain end, we need to redo markBlockSuccessors +/// for just that block. +void MachineBlockPlacement::markBlockSuccessors( + const BlockChain &Chain, const MachineBasicBlock *MBB, + const MachineBasicBlock *LoopHeaderBB, const BlockFilterSet *BlockFilter) { + // 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 : MBB->successors()) { + if (BlockFilter && !BlockFilter->count(Succ)) + continue; + BlockChain &SuccChain = *BlockToChain[Succ]; + // Disregard edges within a fixed chain, or edges to the loop header. + if (&Chain == &SuccChain || Succ == 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.UnscheduledPredecessors == 0 || + --SuccChain.UnscheduledPredecessors > 0) + continue; + + auto *NewBB = *SuccChain.begin(); + if (NewBB->isEHPad()) + EHPadWorkList.push_back(NewBB); + else + BlockWorkList.push_back(NewBB); + } +} + +/// This helper function collects the set of successors of block +/// \p BB that are allowed to be its layout successors, and return +/// the total branch probability of edges from \p BB to those +/// blocks. +BranchProbability MachineBlockPlacement::collectViableSuccessors( + const MachineBasicBlock *BB, const BlockChain &Chain, + const BlockFilterSet *BlockFilter, + SmallVector<MachineBasicBlock *, 4> &Successors) { + // Adjust edge probabilities by excluding edges pointing to blocks that is + // either not in BlockFilter or is already in the current chain. Consider the + // following CFG: + // + // --->A + // | / \ + // | B C + // | \ / \ + // ----D E + // + // Assume A->C is very hot (>90%), and C->D has a 50% probability, then after + // A->C is chosen as a fall-through, D won't be selected as a successor of C + // due to CFG constraint (the probability of C->D is not greater than + // HotProb to break topo-order). If we exclude E that is not in BlockFilter + // when calculating the probability of C->D, D will be selected and we + // will get A C D B as the layout of this loop. + auto AdjustedSumProb = BranchProbability::getOne(); + for (MachineBasicBlock *Succ : BB->successors()) { + bool SkipSucc = false; + if (Succ->isEHPad() || (BlockFilter && !BlockFilter->count(Succ))) { + SkipSucc = true; + } else { + BlockChain *SuccChain = BlockToChain[Succ]; + if (SuccChain == &Chain) { + SkipSucc = true; + } else if (Succ != *SuccChain->begin()) { + LLVM_DEBUG(dbgs() << " " << getBlockName(Succ) + << " -> Mid chain!\n"); + continue; + } + } + if (SkipSucc) + AdjustedSumProb -= MBPI->getEdgeProbability(BB, Succ); + else + Successors.push_back(Succ); + } + + return AdjustedSumProb; +} + +/// The helper function returns the branch probability that is adjusted +/// or normalized over the new total \p AdjustedSumProb. +static BranchProbability +getAdjustedProbability(BranchProbability OrigProb, + BranchProbability AdjustedSumProb) { + BranchProbability SuccProb; + uint32_t SuccProbN = OrigProb.getNumerator(); + uint32_t SuccProbD = AdjustedSumProb.getNumerator(); + if (SuccProbN >= SuccProbD) + SuccProb = BranchProbability::getOne(); + else + SuccProb = BranchProbability(SuccProbN, SuccProbD); + + return SuccProb; +} + +/// Check if \p BB has exactly the successors in \p Successors. +static bool +hasSameSuccessors(MachineBasicBlock &BB, + SmallPtrSetImpl<const MachineBasicBlock *> &Successors) { + if (BB.succ_size() != Successors.size()) + return false; + // We don't want to count self-loops + if (Successors.count(&BB)) + return false; + for (MachineBasicBlock *Succ : BB.successors()) + if (!Successors.count(Succ)) + return false; + return true; +} + +/// Check if a block should be tail duplicated to increase fallthrough +/// opportunities. +/// \p BB Block to check. +bool MachineBlockPlacement::shouldTailDuplicate(MachineBasicBlock *BB) { + // Blocks with single successors don't create additional fallthrough + // opportunities. Don't duplicate them. TODO: When conditional exits are + // analyzable, allow them to be duplicated. + bool IsSimple = TailDup.isSimpleBB(BB); + + if (BB->succ_size() == 1) + return false; + return TailDup.shouldTailDuplicate(IsSimple, *BB); +} + +/// Compare 2 BlockFrequency's with a small penalty for \p A. +/// In order to be conservative, we apply a X% penalty to account for +/// increased icache pressure and static heuristics. For small frequencies +/// we use only the numerators to improve accuracy. For simplicity, we assume the +/// penalty is less than 100% +/// TODO(iteratee): Use 64-bit fixed point edge frequencies everywhere. +static bool greaterWithBias(BlockFrequency A, BlockFrequency B, + uint64_t EntryFreq) { + BranchProbability ThresholdProb(TailDupPlacementPenalty, 100); + BlockFrequency Gain = A - B; + return (Gain / ThresholdProb).getFrequency() >= EntryFreq; +} + +/// Check the edge frequencies to see if tail duplication will increase +/// fallthroughs. It only makes sense to call this function when +/// \p Succ would not be chosen otherwise. Tail duplication of \p Succ is +/// always locally profitable if we would have picked \p Succ without +/// considering duplication. +bool MachineBlockPlacement::isProfitableToTailDup( + const MachineBasicBlock *BB, const MachineBasicBlock *Succ, + BranchProbability QProb, + const BlockChain &Chain, const BlockFilterSet *BlockFilter) { + // We need to do a probability calculation to make sure this is profitable. + // First: does succ have a successor that post-dominates? This affects the + // calculation. The 2 relevant cases are: + // BB BB + // | \Qout | \Qout + // P| C |P C + // = C' = C' + // | /Qin | /Qin + // | / | / + // Succ Succ + // / \ | \ V + // U/ =V |U \ + // / \ = D + // D E | / + // | / + // |/ + // PDom + // '=' : Branch taken for that CFG edge + // In the second case, Placing Succ while duplicating it into C prevents the + // fallthrough of Succ into either D or PDom, because they now have C as an + // unplaced predecessor + + // Start by figuring out which case we fall into + MachineBasicBlock *PDom = nullptr; + SmallVector<MachineBasicBlock *, 4> SuccSuccs; + // Only scan the relevant successors + auto AdjustedSuccSumProb = + collectViableSuccessors(Succ, Chain, BlockFilter, SuccSuccs); + BranchProbability PProb = MBPI->getEdgeProbability(BB, Succ); + auto BBFreq = MBFI->getBlockFreq(BB); + auto SuccFreq = MBFI->getBlockFreq(Succ); + BlockFrequency P = BBFreq * PProb; + BlockFrequency Qout = BBFreq * QProb; + uint64_t EntryFreq = MBFI->getEntryFreq(); + // If there are no more successors, it is profitable to copy, as it strictly + // increases fallthrough. + if (SuccSuccs.size() == 0) + return greaterWithBias(P, Qout, EntryFreq); + + auto BestSuccSucc = BranchProbability::getZero(); + // Find the PDom or the best Succ if no PDom exists. + for (MachineBasicBlock *SuccSucc : SuccSuccs) { + auto Prob = MBPI->getEdgeProbability(Succ, SuccSucc); + if (Prob > BestSuccSucc) + BestSuccSucc = Prob; + if (PDom == nullptr) + if (MPDT->dominates(SuccSucc, Succ)) { + PDom = SuccSucc; + break; + } + } + // For the comparisons, we need to know Succ's best incoming edge that isn't + // from BB. + auto SuccBestPred = BlockFrequency(0); + for (MachineBasicBlock *SuccPred : Succ->predecessors()) { + if (SuccPred == Succ || SuccPred == BB + || BlockToChain[SuccPred] == &Chain + || (BlockFilter && !BlockFilter->count(SuccPred))) + continue; + auto Freq = MBFI->getBlockFreq(SuccPred) + * MBPI->getEdgeProbability(SuccPred, Succ); + if (Freq > SuccBestPred) + SuccBestPred = Freq; + } + // Qin is Succ's best unplaced incoming edge that isn't BB + BlockFrequency Qin = SuccBestPred; + // If it doesn't have a post-dominating successor, here is the calculation: + // BB BB + // | \Qout | \ + // P| C | = + // = C' | C + // | /Qin | | + // | / | C' (+Succ) + // Succ Succ /| + // / \ | \/ | + // U/ =V | == | + // / \ | / \| + // D E D E + // '=' : Branch taken for that CFG edge + // Cost in the first case is: P + V + // For this calculation, we always assume P > Qout. If Qout > P + // The result of this function will be ignored at the caller. + // Let F = SuccFreq - Qin + // Cost in the second case is: Qout + min(Qin, F) * U + max(Qin, F) * V + + if (PDom == nullptr || !Succ->isSuccessor(PDom)) { + BranchProbability UProb = BestSuccSucc; + BranchProbability VProb = AdjustedSuccSumProb - UProb; + BlockFrequency F = SuccFreq - Qin; + BlockFrequency V = SuccFreq * VProb; + BlockFrequency QinU = std::min(Qin, F) * UProb; + BlockFrequency BaseCost = P + V; + BlockFrequency DupCost = Qout + QinU + std::max(Qin, F) * VProb; + return greaterWithBias(BaseCost, DupCost, EntryFreq); + } + BranchProbability UProb = MBPI->getEdgeProbability(Succ, PDom); + BranchProbability VProb = AdjustedSuccSumProb - UProb; + BlockFrequency U = SuccFreq * UProb; + BlockFrequency V = SuccFreq * VProb; + BlockFrequency F = SuccFreq - Qin; + // If there is a post-dominating successor, here is the calculation: + // BB BB BB BB + // | \Qout | \ | \Qout | \ + // |P C | = |P C | = + // = C' |P C = C' |P C + // | /Qin | | | /Qin | | + // | / | C' (+Succ) | / | C' (+Succ) + // Succ Succ /| Succ Succ /| + // | \ V | \/ | | \ V | \/ | + // |U \ |U /\ =? |U = |U /\ | + // = D = = =?| | D | = =| + // | / |/ D | / |/ D + // | / | / | = | / + // |/ | / |/ | = + // Dom Dom Dom Dom + // '=' : Branch taken for that CFG edge + // The cost for taken branches in the first case is P + U + // Let F = SuccFreq - Qin + // The cost in the second case (assuming independence), given the layout: + // BB, Succ, (C+Succ), D, Dom or the layout: + // BB, Succ, D, Dom, (C+Succ) + // is Qout + max(F, Qin) * U + min(F, Qin) + // compare P + U vs Qout + P * U + Qin. + // + // The 3rd and 4th cases cover when Dom would be chosen to follow Succ. + // + // For the 3rd case, the cost is P + 2 * V + // For the 4th case, the cost is Qout + min(Qin, F) * U + max(Qin, F) * V + V + // We choose 4 over 3 when (P + V) > Qout + min(Qin, F) * U + max(Qin, F) * V + if (UProb > AdjustedSuccSumProb / 2 && + !hasBetterLayoutPredecessor(Succ, PDom, *BlockToChain[PDom], UProb, UProb, + Chain, BlockFilter)) + // Cases 3 & 4 + return greaterWithBias( + (P + V), (Qout + std::max(Qin, F) * VProb + std::min(Qin, F) * UProb), + EntryFreq); + // Cases 1 & 2 + return greaterWithBias((P + U), + (Qout + std::min(Qin, F) * AdjustedSuccSumProb + + std::max(Qin, F) * UProb), + EntryFreq); +} + +/// Check for a trellis layout. \p BB is the upper part of a trellis if its +/// successors form the lower part of a trellis. A successor set S forms the +/// lower part of a trellis if all of the predecessors of S are either in S or +/// have all of S as successors. We ignore trellises where BB doesn't have 2 +/// successors because for fewer than 2, it's trivial, and for 3 or greater they +/// are very uncommon and complex to compute optimally. Allowing edges within S +/// is not strictly a trellis, but the same algorithm works, so we allow it. +bool MachineBlockPlacement::isTrellis( + const MachineBasicBlock *BB, + const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs, + const BlockChain &Chain, const BlockFilterSet *BlockFilter) { + // Technically BB could form a trellis with branching factor higher than 2. + // But that's extremely uncommon. + if (BB->succ_size() != 2 || ViableSuccs.size() != 2) + return false; + + SmallPtrSet<const MachineBasicBlock *, 2> Successors(BB->succ_begin(), + BB->succ_end()); + // To avoid reviewing the same predecessors twice. + SmallPtrSet<const MachineBasicBlock *, 8> SeenPreds; + + for (MachineBasicBlock *Succ : ViableSuccs) { + int PredCount = 0; + for (auto SuccPred : Succ->predecessors()) { + // Allow triangle successors, but don't count them. + if (Successors.count(SuccPred)) { + // Make sure that it is actually a triangle. + for (MachineBasicBlock *CheckSucc : SuccPred->successors()) + if (!Successors.count(CheckSucc)) + return false; + continue; + } + const BlockChain *PredChain = BlockToChain[SuccPred]; + if (SuccPred == BB || (BlockFilter && !BlockFilter->count(SuccPred)) || + PredChain == &Chain || PredChain == BlockToChain[Succ]) + continue; + ++PredCount; + // Perform the successor check only once. + if (!SeenPreds.insert(SuccPred).second) + continue; + if (!hasSameSuccessors(*SuccPred, Successors)) + return false; + } + // If one of the successors has only BB as a predecessor, it is not a + // trellis. + if (PredCount < 1) + return false; + } + return true; +} + +/// Pick the highest total weight pair of edges that can both be laid out. +/// The edges in \p Edges[0] are assumed to have a different destination than +/// the edges in \p Edges[1]. Simple counting shows that the best pair is either +/// the individual highest weight edges to the 2 different destinations, or in +/// case of a conflict, one of them should be replaced with a 2nd best edge. +std::pair<MachineBlockPlacement::WeightedEdge, + MachineBlockPlacement::WeightedEdge> +MachineBlockPlacement::getBestNonConflictingEdges( + const MachineBasicBlock *BB, + MutableArrayRef<SmallVector<MachineBlockPlacement::WeightedEdge, 8>> + Edges) { + // Sort the edges, and then for each successor, find the best incoming + // predecessor. If the best incoming predecessors aren't the same, + // then that is clearly the best layout. If there is a conflict, one of the + // successors will have to fallthrough from the second best predecessor. We + // compare which combination is better overall. + + // Sort for highest frequency. + auto Cmp = [](WeightedEdge A, WeightedEdge B) { return A.Weight > B.Weight; }; + + llvm::stable_sort(Edges[0], Cmp); + llvm::stable_sort(Edges[1], Cmp); + auto BestA = Edges[0].begin(); + auto BestB = Edges[1].begin(); + // Arrange for the correct answer to be in BestA and BestB + // If the 2 best edges don't conflict, the answer is already there. + if (BestA->Src == BestB->Src) { + // Compare the total fallthrough of (Best + Second Best) for both pairs + auto SecondBestA = std::next(BestA); + auto SecondBestB = std::next(BestB); + BlockFrequency BestAScore = BestA->Weight + SecondBestB->Weight; + BlockFrequency BestBScore = BestB->Weight + SecondBestA->Weight; + if (BestAScore < BestBScore) + BestA = SecondBestA; + else + BestB = SecondBestB; + } + // Arrange for the BB edge to be in BestA if it exists. + if (BestB->Src == BB) + std::swap(BestA, BestB); + return std::make_pair(*BestA, *BestB); +} + +/// Get the best successor from \p BB based on \p BB being part of a trellis. +/// We only handle trellises with 2 successors, so the algorithm is +/// straightforward: Find the best pair of edges that don't conflict. We find +/// the best incoming edge for each successor in the trellis. If those conflict, +/// we consider which of them should be replaced with the second best. +/// Upon return the two best edges will be in \p BestEdges. If one of the edges +/// comes from \p BB, it will be in \p BestEdges[0] +MachineBlockPlacement::BlockAndTailDupResult +MachineBlockPlacement::getBestTrellisSuccessor( + const MachineBasicBlock *BB, + const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs, + BranchProbability AdjustedSumProb, const BlockChain &Chain, + const BlockFilterSet *BlockFilter) { + + BlockAndTailDupResult Result = {nullptr, false}; + SmallPtrSet<const MachineBasicBlock *, 4> Successors(BB->succ_begin(), + BB->succ_end()); + + // We assume size 2 because it's common. For general n, we would have to do + // the Hungarian algorithm, but it's not worth the complexity because more + // than 2 successors is fairly uncommon, and a trellis even more so. + if (Successors.size() != 2 || ViableSuccs.size() != 2) + return Result; + + // Collect the edge frequencies of all edges that form the trellis. + SmallVector<WeightedEdge, 8> Edges[2]; + int SuccIndex = 0; + for (auto Succ : ViableSuccs) { + for (MachineBasicBlock *SuccPred : Succ->predecessors()) { + // Skip any placed predecessors that are not BB + if (SuccPred != BB) + if ((BlockFilter && !BlockFilter->count(SuccPred)) || + BlockToChain[SuccPred] == &Chain || + BlockToChain[SuccPred] == BlockToChain[Succ]) + continue; + BlockFrequency EdgeFreq = MBFI->getBlockFreq(SuccPred) * + MBPI->getEdgeProbability(SuccPred, Succ); + Edges[SuccIndex].push_back({EdgeFreq, SuccPred, Succ}); + } + ++SuccIndex; + } + + // Pick the best combination of 2 edges from all the edges in the trellis. + WeightedEdge BestA, BestB; + std::tie(BestA, BestB) = getBestNonConflictingEdges(BB, Edges); + + if (BestA.Src != BB) { + // If we have a trellis, and BB doesn't have the best fallthrough edges, + // we shouldn't choose any successor. We've already looked and there's a + // better fallthrough edge for all the successors. + LLVM_DEBUG(dbgs() << "Trellis, but not one of the chosen edges.\n"); + return Result; + } + + // Did we pick the triangle edge? If tail-duplication is profitable, do + // that instead. Otherwise merge the triangle edge now while we know it is + // optimal. + if (BestA.Dest == BestB.Src) { + // The edges are BB->Succ1->Succ2, and we're looking to see if BB->Succ2 + // would be better. + MachineBasicBlock *Succ1 = BestA.Dest; + MachineBasicBlock *Succ2 = BestB.Dest; + // Check to see if tail-duplication would be profitable. + if (allowTailDupPlacement() && shouldTailDuplicate(Succ2) && + canTailDuplicateUnplacedPreds(BB, Succ2, Chain, BlockFilter) && + isProfitableToTailDup(BB, Succ2, MBPI->getEdgeProbability(BB, Succ1), + Chain, BlockFilter)) { + LLVM_DEBUG(BranchProbability Succ2Prob = getAdjustedProbability( + MBPI->getEdgeProbability(BB, Succ2), AdjustedSumProb); + dbgs() << " Selected: " << getBlockName(Succ2) + << ", probability: " << Succ2Prob + << " (Tail Duplicate)\n"); + Result.BB = Succ2; + Result.ShouldTailDup = true; + return Result; + } + } + // We have already computed the optimal edge for the other side of the + // trellis. + ComputedEdges[BestB.Src] = { BestB.Dest, false }; + + auto TrellisSucc = BestA.Dest; + LLVM_DEBUG(BranchProbability SuccProb = getAdjustedProbability( + MBPI->getEdgeProbability(BB, TrellisSucc), AdjustedSumProb); + dbgs() << " Selected: " << getBlockName(TrellisSucc) + << ", probability: " << SuccProb << " (Trellis)\n"); + Result.BB = TrellisSucc; + return Result; +} + +/// When the option allowTailDupPlacement() is on, this method checks if the +/// fallthrough candidate block \p Succ (of block \p BB) can be tail-duplicated +/// into all of its unplaced, unfiltered predecessors, that are not BB. +bool MachineBlockPlacement::canTailDuplicateUnplacedPreds( + const MachineBasicBlock *BB, MachineBasicBlock *Succ, + const BlockChain &Chain, const BlockFilterSet *BlockFilter) { + if (!shouldTailDuplicate(Succ)) + return false; + + // For CFG checking. + SmallPtrSet<const MachineBasicBlock *, 4> Successors(BB->succ_begin(), + BB->succ_end()); + for (MachineBasicBlock *Pred : Succ->predecessors()) { + // Make sure all unplaced and unfiltered predecessors can be + // tail-duplicated into. + // Skip any blocks that are already placed or not in this loop. + if (Pred == BB || (BlockFilter && !BlockFilter->count(Pred)) + || BlockToChain[Pred] == &Chain) + continue; + if (!TailDup.canTailDuplicate(Succ, Pred)) { + if (Successors.size() > 1 && hasSameSuccessors(*Pred, Successors)) + // This will result in a trellis after tail duplication, so we don't + // need to copy Succ into this predecessor. In the presence + // of a trellis tail duplication can continue to be profitable. + // For example: + // A A + // |\ |\ + // | \ | \ + // | C | C+BB + // | / | | + // |/ | | + // BB => BB | + // |\ |\/| + // | \ |/\| + // | D | D + // | / | / + // |/ |/ + // Succ Succ + // + // After BB was duplicated into C, the layout looks like the one on the + // right. BB and C now have the same successors. When considering + // whether Succ can be duplicated into all its unplaced predecessors, we + // ignore C. + // We can do this because C already has a profitable fallthrough, namely + // D. TODO(iteratee): ignore sufficiently cold predecessors for + // duplication and for this test. + // + // This allows trellises to be laid out in 2 separate chains + // (A,B,Succ,...) and later (C,D,...) This is a reasonable heuristic + // because it allows the creation of 2 fallthrough paths with links + // between them, and we correctly identify the best layout for these + // CFGs. We want to extend trellises that the user created in addition + // to trellises created by tail-duplication, so we just look for the + // CFG. + continue; + return false; + } + } + return true; +} + +/// Find chains of triangles where we believe it would be profitable to +/// tail-duplicate them all, but a local analysis would not find them. +/// There are 3 ways this can be profitable: +/// 1) The post-dominators marked 50% are actually taken 55% (This shrinks with +/// longer chains) +/// 2) The chains are statically correlated. Branch probabilities have a very +/// U-shaped distribution. +/// [http://nrs.harvard.edu/urn-3:HUL.InstRepos:24015805] +/// If the branches in a chain are likely to be from the same side of the +/// distribution as their predecessor, but are independent at runtime, this +/// transformation is profitable. (Because the cost of being wrong is a small +/// fixed cost, unlike the standard triangle layout where the cost of being +/// wrong scales with the # of triangles.) +/// 3) The chains are dynamically correlated. If the probability that a previous +/// branch was taken positively influences whether the next branch will be +/// taken +/// We believe that 2 and 3 are common enough to justify the small margin in 1. +void MachineBlockPlacement::precomputeTriangleChains() { + struct TriangleChain { + std::vector<MachineBasicBlock *> Edges; + + TriangleChain(MachineBasicBlock *src, MachineBasicBlock *dst) + : Edges({src, dst}) {} + + void append(MachineBasicBlock *dst) { + assert(getKey()->isSuccessor(dst) && + "Attempting to append a block that is not a successor."); + Edges.push_back(dst); + } + + unsigned count() const { return Edges.size() - 1; } + + MachineBasicBlock *getKey() const { + return Edges.back(); + } + }; + + if (TriangleChainCount == 0) + return; + + LLVM_DEBUG(dbgs() << "Pre-computing triangle chains.\n"); + // Map from last block to the chain that contains it. This allows us to extend + // chains as we find new triangles. + DenseMap<const MachineBasicBlock *, TriangleChain> TriangleChainMap; + for (MachineBasicBlock &BB : *F) { + // If BB doesn't have 2 successors, it doesn't start a triangle. + if (BB.succ_size() != 2) + continue; + MachineBasicBlock *PDom = nullptr; + for (MachineBasicBlock *Succ : BB.successors()) { + if (!MPDT->dominates(Succ, &BB)) + continue; + PDom = Succ; + break; + } + // If BB doesn't have a post-dominating successor, it doesn't form a + // triangle. + if (PDom == nullptr) + continue; + // If PDom has a hint that it is low probability, skip this triangle. + if (MBPI->getEdgeProbability(&BB, PDom) < BranchProbability(50, 100)) + continue; + // If PDom isn't eligible for duplication, this isn't the kind of triangle + // we're looking for. + if (!shouldTailDuplicate(PDom)) + continue; + bool CanTailDuplicate = true; + // If PDom can't tail-duplicate into it's non-BB predecessors, then this + // isn't the kind of triangle we're looking for. + for (MachineBasicBlock* Pred : PDom->predecessors()) { + if (Pred == &BB) + continue; + if (!TailDup.canTailDuplicate(PDom, Pred)) { + CanTailDuplicate = false; + break; + } + } + // If we can't tail-duplicate PDom to its predecessors, then skip this + // triangle. + if (!CanTailDuplicate) + continue; + + // Now we have an interesting triangle. Insert it if it's not part of an + // existing chain. + // Note: This cannot be replaced with a call insert() or emplace() because + // the find key is BB, but the insert/emplace key is PDom. + auto Found = TriangleChainMap.find(&BB); + // If it is, remove the chain from the map, grow it, and put it back in the + // map with the end as the new key. + if (Found != TriangleChainMap.end()) { + TriangleChain Chain = std::move(Found->second); + TriangleChainMap.erase(Found); + Chain.append(PDom); + TriangleChainMap.insert(std::make_pair(Chain.getKey(), std::move(Chain))); + } else { + auto InsertResult = TriangleChainMap.try_emplace(PDom, &BB, PDom); + assert(InsertResult.second && "Block seen twice."); + (void)InsertResult; + } + } + + // Iterating over a DenseMap is safe here, because the only thing in the body + // of the loop is inserting into another DenseMap (ComputedEdges). + // ComputedEdges is never iterated, so this doesn't lead to non-determinism. + for (auto &ChainPair : TriangleChainMap) { + TriangleChain &Chain = ChainPair.second; + // Benchmarking has shown that due to branch correlation duplicating 2 or + // more triangles is profitable, despite the calculations assuming + // independence. + if (Chain.count() < TriangleChainCount) + continue; + MachineBasicBlock *dst = Chain.Edges.back(); + Chain.Edges.pop_back(); + for (MachineBasicBlock *src : reverse(Chain.Edges)) { + LLVM_DEBUG(dbgs() << "Marking edge: " << getBlockName(src) << "->" + << getBlockName(dst) + << " as pre-computed based on triangles.\n"); + + auto InsertResult = ComputedEdges.insert({src, {dst, true}}); + assert(InsertResult.second && "Block seen twice."); + (void)InsertResult; + + dst = src; + } + } +} + +// When profile is not present, return the StaticLikelyProb. +// When profile is available, we need to handle the triangle-shape CFG. +static BranchProbability getLayoutSuccessorProbThreshold( + const MachineBasicBlock *BB) { + if (!BB->getParent()->getFunction().hasProfileData()) + return BranchProbability(StaticLikelyProb, 100); + if (BB->succ_size() == 2) { + const MachineBasicBlock *Succ1 = *BB->succ_begin(); + const MachineBasicBlock *Succ2 = *(BB->succ_begin() + 1); + if (Succ1->isSuccessor(Succ2) || Succ2->isSuccessor(Succ1)) { + /* See case 1 below for the cost analysis. For BB->Succ to + * be taken with smaller cost, the following needs to hold: + * Prob(BB->Succ) > 2 * Prob(BB->Pred) + * So the threshold T in the calculation below + * (1-T) * Prob(BB->Succ) > T * Prob(BB->Pred) + * So T / (1 - T) = 2, Yielding T = 2/3 + * Also adding user specified branch bias, we have + * T = (2/3)*(ProfileLikelyProb/50) + * = (2*ProfileLikelyProb)/150) + */ + return BranchProbability(2 * ProfileLikelyProb, 150); + } + } + return BranchProbability(ProfileLikelyProb, 100); +} + +/// Checks to see if the layout candidate block \p Succ has a better layout +/// predecessor than \c BB. If yes, returns true. +/// \p SuccProb: The probability adjusted for only remaining blocks. +/// Only used for logging +/// \p RealSuccProb: The un-adjusted probability. +/// \p Chain: The chain that BB belongs to and Succ is being considered for. +/// \p BlockFilter: if non-null, the set of blocks that make up the loop being +/// considered +bool MachineBlockPlacement::hasBetterLayoutPredecessor( + const MachineBasicBlock *BB, const MachineBasicBlock *Succ, + const BlockChain &SuccChain, BranchProbability SuccProb, + BranchProbability RealSuccProb, const BlockChain &Chain, + const BlockFilterSet *BlockFilter) { + + // There isn't a better layout when there are no unscheduled predecessors. + if (SuccChain.UnscheduledPredecessors == 0) + return false; + + // There are two basic scenarios here: + // ------------------------------------- + // Case 1: triangular shape CFG (if-then): + // BB + // | \ + // | \ + // | Pred + // | / + // Succ + // In this case, we are evaluating whether to select edge -> Succ, e.g. + // set Succ as the layout successor of BB. Picking Succ as BB's + // successor breaks the CFG constraints (FIXME: define these constraints). + // With this layout, Pred BB + // is forced to be outlined, so the overall cost will be cost of the + // branch taken from BB to Pred, plus the cost of back taken branch + // from Pred to Succ, as well as the additional cost associated + // with the needed unconditional jump instruction from Pred To Succ. + + // The cost of the topological order layout is the taken branch cost + // from BB to Succ, so to make BB->Succ a viable candidate, the following + // must hold: + // 2 * freq(BB->Pred) * taken_branch_cost + unconditional_jump_cost + // < freq(BB->Succ) * taken_branch_cost. + // Ignoring unconditional jump cost, we get + // freq(BB->Succ) > 2 * freq(BB->Pred), i.e., + // prob(BB->Succ) > 2 * prob(BB->Pred) + // + // When real profile data is available, we can precisely compute the + // probability threshold that is needed for edge BB->Succ to be considered. + // Without profile data, the heuristic requires the branch bias to be + // a lot larger to make sure the signal is very strong (e.g. 80% default). + // ----------------------------------------------------------------- + // Case 2: diamond like CFG (if-then-else): + // S + // / \ + // | \ + // BB Pred + // \ / + // Succ + // .. + // + // The current block is BB and edge BB->Succ is now being evaluated. + // Note that edge S->BB was previously already selected because + // prob(S->BB) > prob(S->Pred). + // At this point, 2 blocks can be placed after BB: Pred or Succ. If we + // choose Pred, we will have a topological ordering as shown on the left + // in the picture below. If we choose Succ, we have the solution as shown + // on the right: + // + // topo-order: + // + // S----- ---S + // | | | | + // ---BB | | BB + // | | | | + // | Pred-- | Succ-- + // | | | | + // ---Succ ---Pred-- + // + // cost = freq(S->Pred) + freq(BB->Succ) cost = 2 * freq (S->Pred) + // = freq(S->Pred) + freq(S->BB) + // + // If we have profile data (i.e, branch probabilities can be trusted), the + // cost (number of taken branches) with layout S->BB->Succ->Pred is 2 * + // freq(S->Pred) while the cost of topo order is freq(S->Pred) + freq(S->BB). + // We know Prob(S->BB) > Prob(S->Pred), so freq(S->BB) > freq(S->Pred), which + // means the cost of topological order is greater. + // When profile data is not available, however, we need to be more + // conservative. If the branch prediction is wrong, breaking the topo-order + // will actually yield a layout with large cost. For this reason, we need + // strong biased branch at block S with Prob(S->BB) in order to select + // BB->Succ. This is equivalent to looking the CFG backward with backward + // edge: Prob(Succ->BB) needs to >= HotProb in order to be selected (without + // profile data). + // -------------------------------------------------------------------------- + // Case 3: forked diamond + // S + // / \ + // / \ + // BB Pred + // | \ / | + // | \ / | + // | X | + // | / \ | + // | / \ | + // S1 S2 + // + // The current block is BB and edge BB->S1 is now being evaluated. + // As above S->BB was already selected because + // prob(S->BB) > prob(S->Pred). Assume that prob(BB->S1) >= prob(BB->S2). + // + // topo-order: + // + // S-------| ---S + // | | | | + // ---BB | | BB + // | | | | + // | Pred----| | S1---- + // | | | | + // --(S1 or S2) ---Pred-- + // | + // S2 + // + // topo-cost = freq(S->Pred) + freq(BB->S1) + freq(BB->S2) + // + min(freq(Pred->S1), freq(Pred->S2)) + // Non-topo-order cost: + // non-topo-cost = 2 * freq(S->Pred) + freq(BB->S2). + // To be conservative, we can assume that min(freq(Pred->S1), freq(Pred->S2)) + // is 0. Then the non topo layout is better when + // freq(S->Pred) < freq(BB->S1). + // This is exactly what is checked below. + // Note there are other shapes that apply (Pred may not be a single block, + // but they all fit this general pattern.) + BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB); + + // Make sure that a hot successor doesn't have a globally more + // important predecessor. + BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(BB) * RealSuccProb; + bool BadCFGConflict = false; + + for (MachineBasicBlock *Pred : Succ->predecessors()) { + if (Pred == Succ || BlockToChain[Pred] == &SuccChain || + (BlockFilter && !BlockFilter->count(Pred)) || + BlockToChain[Pred] == &Chain || + // This check is redundant except for look ahead. This function is + // called for lookahead by isProfitableToTailDup when BB hasn't been + // placed yet. + (Pred == BB)) + continue; + // Do backward checking. + // For all cases above, we need a backward checking to filter out edges that + // are not 'strongly' biased. + // BB Pred + // \ / + // Succ + // We select edge BB->Succ if + // freq(BB->Succ) > freq(Succ) * HotProb + // i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) * + // HotProb + // i.e. freq((BB->Succ) * (1 - HotProb) > freq(Pred->Succ) * HotProb + // Case 1 is covered too, because the first equation reduces to: + // prob(BB->Succ) > HotProb. (freq(Succ) = freq(BB) for a triangle) + BlockFrequency PredEdgeFreq = + MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ); + if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) { + BadCFGConflict = true; + break; + } + } + + if (BadCFGConflict) { + LLVM_DEBUG(dbgs() << " Not a candidate: " << getBlockName(Succ) << " -> " + << SuccProb << " (prob) (non-cold CFG conflict)\n"); + return true; + } + + return false; +} + +/// 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, along +/// with a boolean indicating if tail duplication is necessary. +MachineBlockPlacement::BlockAndTailDupResult +MachineBlockPlacement::selectBestSuccessor( + const MachineBasicBlock *BB, const BlockChain &Chain, + const BlockFilterSet *BlockFilter) { + const BranchProbability HotProb(StaticLikelyProb, 100); + + BlockAndTailDupResult BestSucc = { nullptr, false }; + auto BestProb = BranchProbability::getZero(); + + SmallVector<MachineBasicBlock *, 4> Successors; + auto AdjustedSumProb = + collectViableSuccessors(BB, Chain, BlockFilter, Successors); + + LLVM_DEBUG(dbgs() << "Selecting best successor for: " << getBlockName(BB) + << "\n"); + + // if we already precomputed the best successor for BB, return that if still + // applicable. + auto FoundEdge = ComputedEdges.find(BB); + if (FoundEdge != ComputedEdges.end()) { + MachineBasicBlock *Succ = FoundEdge->second.BB; + ComputedEdges.erase(FoundEdge); + BlockChain *SuccChain = BlockToChain[Succ]; + if (BB->isSuccessor(Succ) && (!BlockFilter || BlockFilter->count(Succ)) && + SuccChain != &Chain && Succ == *SuccChain->begin()) + return FoundEdge->second; + } + + // if BB is part of a trellis, Use the trellis to determine the optimal + // fallthrough edges + if (isTrellis(BB, Successors, Chain, BlockFilter)) + return getBestTrellisSuccessor(BB, Successors, AdjustedSumProb, Chain, + BlockFilter); + + // For blocks with CFG violations, we may be able to lay them out anyway with + // tail-duplication. We keep this vector so we can perform the probability + // calculations the minimum number of times. + SmallVector<std::tuple<BranchProbability, MachineBasicBlock *>, 4> + DupCandidates; + for (MachineBasicBlock *Succ : Successors) { + auto RealSuccProb = MBPI->getEdgeProbability(BB, Succ); + BranchProbability SuccProb = + getAdjustedProbability(RealSuccProb, AdjustedSumProb); + + BlockChain &SuccChain = *BlockToChain[Succ]; + // Skip the edge \c BB->Succ if block \c Succ has a better layout + // predecessor that yields lower global cost. + if (hasBetterLayoutPredecessor(BB, Succ, SuccChain, SuccProb, RealSuccProb, + Chain, BlockFilter)) { + // If tail duplication would make Succ profitable, place it. + if (allowTailDupPlacement() && shouldTailDuplicate(Succ)) + DupCandidates.push_back(std::make_tuple(SuccProb, Succ)); + continue; + } + + LLVM_DEBUG( + dbgs() << " Candidate: " << getBlockName(Succ) + << ", probability: " << SuccProb + << (SuccChain.UnscheduledPredecessors != 0 ? " (CFG break)" : "") + << "\n"); + + if (BestSucc.BB && BestProb >= SuccProb) { + LLVM_DEBUG(dbgs() << " Not the best candidate, continuing\n"); + continue; + } + + LLVM_DEBUG(dbgs() << " Setting it as best candidate\n"); + BestSucc.BB = Succ; + BestProb = SuccProb; + } + // Handle the tail duplication candidates in order of decreasing probability. + // Stop at the first one that is profitable. Also stop if they are less + // profitable than BestSucc. Position is important because we preserve it and + // prefer first best match. Here we aren't comparing in order, so we capture + // the position instead. + llvm::stable_sort(DupCandidates, + [](std::tuple<BranchProbability, MachineBasicBlock *> L, + std::tuple<BranchProbability, MachineBasicBlock *> R) { + return std::get<0>(L) > std::get<0>(R); + }); + for (auto &Tup : DupCandidates) { + BranchProbability DupProb; + MachineBasicBlock *Succ; + std::tie(DupProb, Succ) = Tup; + if (DupProb < BestProb) + break; + if (canTailDuplicateUnplacedPreds(BB, Succ, Chain, BlockFilter) + && (isProfitableToTailDup(BB, Succ, BestProb, Chain, BlockFilter))) { + LLVM_DEBUG(dbgs() << " Candidate: " << getBlockName(Succ) + << ", probability: " << DupProb + << " (Tail Duplicate)\n"); + BestSucc.BB = Succ; + BestSucc.ShouldTailDup = true; + break; + } + } + + if (BestSucc.BB) + LLVM_DEBUG(dbgs() << " Selected: " << getBlockName(BestSucc.BB) << "\n"); + + return BestSucc; +} + +/// 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 i-cache behavior. +/// +/// \returns The best block found, or null if none are viable. +MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock( + const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList) { + // 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(llvm::remove_if(WorkList, + [&](MachineBasicBlock *BB) { + return BlockToChain.lookup(BB) == &Chain; + }), + WorkList.end()); + + if (WorkList.empty()) + return nullptr; + + bool IsEHPad = WorkList[0]->isEHPad(); + + MachineBasicBlock *BestBlock = nullptr; + BlockFrequency BestFreq; + for (MachineBasicBlock *MBB : WorkList) { + assert(MBB->isEHPad() == IsEHPad && + "EHPad mismatch between block and work list."); + + BlockChain &SuccChain = *BlockToChain[MBB]; + if (&SuccChain == &Chain) + continue; + + assert(SuccChain.UnscheduledPredecessors == 0 && + "Found CFG-violating block"); + + BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB); + LLVM_DEBUG(dbgs() << " " << getBlockName(MBB) << " -> "; + MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n"); + + // For ehpad, we layout the least probable first as to avoid jumping back + // from least probable landingpads to more probable ones. + // + // FIXME: Using probability is probably (!) not the best way to achieve + // this. We should probably have a more principled approach to layout + // cleanup code. + // + // The goal is to get: + // + // +--------------------------+ + // | V + // InnerLp -> InnerCleanup OuterLp -> OuterCleanup -> Resume + // + // Rather than: + // + // +-------------------------------------+ + // V | + // OuterLp -> OuterCleanup -> Resume InnerLp -> InnerCleanup + if (BestBlock && (IsEHPad ^ (BestFreq >= CandidateFreq))) + continue; + + BestBlock = MBB; + BestFreq = CandidateFreq; + } + + return BestBlock; +} + +/// 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 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, + 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 nullptr; +} + +void MachineBlockPlacement::fillWorkLists( + const MachineBasicBlock *MBB, + SmallPtrSetImpl<BlockChain *> &UpdatedPreds, + const BlockFilterSet *BlockFilter = nullptr) { + BlockChain &Chain = *BlockToChain[MBB]; + if (!UpdatedPreds.insert(&Chain).second) + return; + + assert( + Chain.UnscheduledPredecessors == 0 && + "Attempting to place block with unscheduled predecessors in worklist."); + for (MachineBasicBlock *ChainBB : Chain) { + assert(BlockToChain[ChainBB] == &Chain && + "Block in chain doesn't match BlockToChain map."); + for (MachineBasicBlock *Pred : ChainBB->predecessors()) { + if (BlockFilter && !BlockFilter->count(Pred)) + continue; + if (BlockToChain[Pred] == &Chain) + continue; + ++Chain.UnscheduledPredecessors; + } + } + + if (Chain.UnscheduledPredecessors != 0) + return; + + MachineBasicBlock *BB = *Chain.begin(); + if (BB->isEHPad()) + EHPadWorkList.push_back(BB); + else + BlockWorkList.push_back(BB); +} + +void MachineBlockPlacement::buildChain( + const MachineBasicBlock *HeadBB, BlockChain &Chain, + BlockFilterSet *BlockFilter) { + assert(HeadBB && "BB must not be null.\n"); + assert(BlockToChain[HeadBB] == &Chain && "BlockToChainMap mis-match.\n"); + MachineFunction::iterator PrevUnplacedBlockIt = F->begin(); + + const MachineBasicBlock *LoopHeaderBB = HeadBB; + markChainSuccessors(Chain, LoopHeaderBB, BlockFilter); + MachineBasicBlock *BB = *std::prev(Chain.end()); + while (true) { + assert(BB && "null block found at end of chain in loop."); + assert(BlockToChain[BB] == &Chain && "BlockToChainMap mis-match in loop."); + assert(*std::prev(Chain.end()) == BB && "BB Not found at end of chain."); + + + // Look for the best viable successor if there is one to place immediately + // after this block. + auto Result = selectBestSuccessor(BB, Chain, BlockFilter); + MachineBasicBlock* BestSucc = Result.BB; + bool ShouldTailDup = Result.ShouldTailDup; + if (allowTailDupPlacement()) + ShouldTailDup |= (BestSucc && shouldTailDuplicate(BestSucc)); + + // 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); + if (!BestSucc) + BestSucc = selectBestCandidateBlock(Chain, EHPadWorkList); + + if (!BestSucc) { + BestSucc = getFirstUnplacedBlock(Chain, PrevUnplacedBlockIt, BlockFilter); + if (!BestSucc) + break; + + LLVM_DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the " + "layout successor until the CFG reduces\n"); + } + + // Placement may have changed tail duplication opportunities. + // Check for that now. + if (allowTailDupPlacement() && BestSucc && ShouldTailDup) { + // If the chosen successor was duplicated into all its predecessors, + // don't bother laying it out, just go round the loop again with BB as + // the chain end. + if (repeatedlyTailDuplicateBlock(BestSucc, BB, LoopHeaderBB, Chain, + BlockFilter, PrevUnplacedBlockIt)) + continue; + } + + // Place this block, updating the datastructures to reflect its placement. + BlockChain &SuccChain = *BlockToChain[BestSucc]; + // Zero out UnscheduledPredecessors for the successor we're about to merge in case + // we selected a successor that didn't fit naturally into the CFG. + SuccChain.UnscheduledPredecessors = 0; + LLVM_DEBUG(dbgs() << "Merging from " << getBlockName(BB) << " to " + << getBlockName(BestSucc) << "\n"); + markChainSuccessors(SuccChain, LoopHeaderBB, BlockFilter); + Chain.merge(BestSucc, &SuccChain); + BB = *std::prev(Chain.end()); + } + + LLVM_DEBUG(dbgs() << "Finished forming chain for header block " + << getBlockName(*Chain.begin()) << "\n"); +} + +// If bottom of block BB has only one successor OldTop, in most cases it is +// profitable to move it before OldTop, except the following case: +// +// -->OldTop<- +// | . | +// | . | +// | . | +// ---Pred | +// | | +// BB----- +// +// If BB is moved before OldTop, Pred needs a taken branch to BB, and it can't +// layout the other successor below it, so it can't reduce taken branch. +// In this case we keep its original layout. +bool +MachineBlockPlacement::canMoveBottomBlockToTop( + const MachineBasicBlock *BottomBlock, + const MachineBasicBlock *OldTop) { + if (BottomBlock->pred_size() != 1) + return true; + MachineBasicBlock *Pred = *BottomBlock->pred_begin(); + if (Pred->succ_size() != 2) + return true; + + MachineBasicBlock *OtherBB = *Pred->succ_begin(); + if (OtherBB == BottomBlock) + OtherBB = *Pred->succ_rbegin(); + if (OtherBB == OldTop) + return false; + + return true; +} + +// Find out the possible fall through frequence to the top of a loop. +BlockFrequency +MachineBlockPlacement::TopFallThroughFreq( + const MachineBasicBlock *Top, + const BlockFilterSet &LoopBlockSet) { + BlockFrequency MaxFreq = 0; + for (MachineBasicBlock *Pred : Top->predecessors()) { + BlockChain *PredChain = BlockToChain[Pred]; + if (!LoopBlockSet.count(Pred) && + (!PredChain || Pred == *std::prev(PredChain->end()))) { + // Found a Pred block can be placed before Top. + // Check if Top is the best successor of Pred. + auto TopProb = MBPI->getEdgeProbability(Pred, Top); + bool TopOK = true; + for (MachineBasicBlock *Succ : Pred->successors()) { + auto SuccProb = MBPI->getEdgeProbability(Pred, Succ); + BlockChain *SuccChain = BlockToChain[Succ]; + // Check if Succ can be placed after Pred. + // Succ should not be in any chain, or it is the head of some chain. + if (!LoopBlockSet.count(Succ) && (SuccProb > TopProb) && + (!SuccChain || Succ == *SuccChain->begin())) { + TopOK = false; + break; + } + } + if (TopOK) { + BlockFrequency EdgeFreq = MBFI->getBlockFreq(Pred) * + MBPI->getEdgeProbability(Pred, Top); + if (EdgeFreq > MaxFreq) + MaxFreq = EdgeFreq; + } + } + } + return MaxFreq; +} + +// Compute the fall through gains when move NewTop before OldTop. +// +// In following diagram, edges marked as "-" are reduced fallthrough, edges +// marked as "+" are increased fallthrough, this function computes +// +// SUM(increased fallthrough) - SUM(decreased fallthrough) +// +// | +// | - +// V +// --->OldTop +// | . +// | . +// +| . + +// | Pred ---> +// | |- +// | V +// --- NewTop <--- +// |- +// V +// +BlockFrequency +MachineBlockPlacement::FallThroughGains( + const MachineBasicBlock *NewTop, + const MachineBasicBlock *OldTop, + const MachineBasicBlock *ExitBB, + const BlockFilterSet &LoopBlockSet) { + BlockFrequency FallThrough2Top = TopFallThroughFreq(OldTop, LoopBlockSet); + BlockFrequency FallThrough2Exit = 0; + if (ExitBB) + FallThrough2Exit = MBFI->getBlockFreq(NewTop) * + MBPI->getEdgeProbability(NewTop, ExitBB); + BlockFrequency BackEdgeFreq = MBFI->getBlockFreq(NewTop) * + MBPI->getEdgeProbability(NewTop, OldTop); + + // Find the best Pred of NewTop. + MachineBasicBlock *BestPred = nullptr; + BlockFrequency FallThroughFromPred = 0; + for (MachineBasicBlock *Pred : NewTop->predecessors()) { + if (!LoopBlockSet.count(Pred)) + continue; + BlockChain *PredChain = BlockToChain[Pred]; + if (!PredChain || Pred == *std::prev(PredChain->end())) { + BlockFrequency EdgeFreq = MBFI->getBlockFreq(Pred) * + MBPI->getEdgeProbability(Pred, NewTop); + if (EdgeFreq > FallThroughFromPred) { + FallThroughFromPred = EdgeFreq; + BestPred = Pred; + } + } + } + + // If NewTop is not placed after Pred, another successor can be placed + // after Pred. + BlockFrequency NewFreq = 0; + if (BestPred) { + for (MachineBasicBlock *Succ : BestPred->successors()) { + if ((Succ == NewTop) || (Succ == BestPred) || !LoopBlockSet.count(Succ)) + continue; + if (ComputedEdges.find(Succ) != ComputedEdges.end()) + continue; + BlockChain *SuccChain = BlockToChain[Succ]; + if ((SuccChain && (Succ != *SuccChain->begin())) || + (SuccChain == BlockToChain[BestPred])) + continue; + BlockFrequency EdgeFreq = MBFI->getBlockFreq(BestPred) * + MBPI->getEdgeProbability(BestPred, Succ); + if (EdgeFreq > NewFreq) + NewFreq = EdgeFreq; + } + BlockFrequency OrigEdgeFreq = MBFI->getBlockFreq(BestPred) * + MBPI->getEdgeProbability(BestPred, NewTop); + if (NewFreq > OrigEdgeFreq) { + // If NewTop is not the best successor of Pred, then Pred doesn't + // fallthrough to NewTop. So there is no FallThroughFromPred and + // NewFreq. + NewFreq = 0; + FallThroughFromPred = 0; + } + } + + BlockFrequency Result = 0; + BlockFrequency Gains = BackEdgeFreq + NewFreq; + BlockFrequency Lost = FallThrough2Top + FallThrough2Exit + + FallThroughFromPred; + if (Gains > Lost) + Result = Gains - Lost; + return Result; +} + +/// Helper function of findBestLoopTop. Find the best loop top block +/// from predecessors of old top. +/// +/// Look for a block which is strictly better than the old top for laying +/// out before the old top of the loop. This looks for only two patterns: +/// +/// 1. a block has only one successor, the old loop top +/// +/// Because such a block will always result in an unconditional jump, +/// rotating it in front of the old top is always profitable. +/// +/// 2. a block has two successors, one is old top, another is exit +/// and it has more than one predecessors +/// +/// If it is below one of its predecessors P, only P can fall through to +/// it, all other predecessors need a jump to it, and another conditional +/// jump to loop header. If it is moved before loop header, all its +/// predecessors jump to it, then fall through to loop header. So all its +/// predecessors except P can reduce one taken branch. +/// At the same time, move it before old top increases the taken branch +/// to loop exit block, so the reduced taken branch will be compared with +/// the increased taken branch to the loop exit block. +MachineBasicBlock * +MachineBlockPlacement::findBestLoopTopHelper( + MachineBasicBlock *OldTop, + const MachineLoop &L, + const BlockFilterSet &LoopBlockSet) { + // Check that the header hasn't been fused with a preheader block due to + // crazy branches. If it has, we need to start with the header at the top to + // prevent pulling the preheader into the loop body. + BlockChain &HeaderChain = *BlockToChain[OldTop]; + if (!LoopBlockSet.count(*HeaderChain.begin())) + return OldTop; + + LLVM_DEBUG(dbgs() << "Finding best loop top for: " << getBlockName(OldTop) + << "\n"); + + BlockFrequency BestGains = 0; + MachineBasicBlock *BestPred = nullptr; + for (MachineBasicBlock *Pred : OldTop->predecessors()) { + if (!LoopBlockSet.count(Pred)) + continue; + if (Pred == L.getHeader()) + continue; + LLVM_DEBUG(dbgs() << " old top pred: " << getBlockName(Pred) << ", has " + << Pred->succ_size() << " successors, "; + MBFI->printBlockFreq(dbgs(), Pred) << " freq\n"); + if (Pred->succ_size() > 2) + continue; + + MachineBasicBlock *OtherBB = nullptr; + if (Pred->succ_size() == 2) { + OtherBB = *Pred->succ_begin(); + if (OtherBB == OldTop) + OtherBB = *Pred->succ_rbegin(); + } + + if (!canMoveBottomBlockToTop(Pred, OldTop)) + continue; + + BlockFrequency Gains = FallThroughGains(Pred, OldTop, OtherBB, + LoopBlockSet); + if ((Gains > 0) && (Gains > BestGains || + ((Gains == BestGains) && Pred->isLayoutSuccessor(OldTop)))) { + BestPred = Pred; + BestGains = Gains; + } + } + + // If no direct predecessor is fine, just use the loop header. + if (!BestPred) { + LLVM_DEBUG(dbgs() << " final top unchanged\n"); + return OldTop; + } + + // Walk backwards through any straight line of predecessors. + while (BestPred->pred_size() == 1 && + (*BestPred->pred_begin())->succ_size() == 1 && + *BestPred->pred_begin() != L.getHeader()) + BestPred = *BestPred->pred_begin(); + + LLVM_DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n"); + return BestPred; +} + +/// Find the best loop top block for layout. +/// +/// This function iteratively calls findBestLoopTopHelper, until no new better +/// BB can be found. +MachineBasicBlock * +MachineBlockPlacement::findBestLoopTop(const MachineLoop &L, + const BlockFilterSet &LoopBlockSet) { + // Placing the latch block before the header may introduce an extra branch + // that skips this block the first time the loop is executed, which we want + // to avoid when optimising for size. + // FIXME: in theory there is a case that does not introduce a new branch, + // i.e. when the layout predecessor does not fallthrough to the loop header. + // In practice this never happens though: there always seems to be a preheader + // that can fallthrough and that is also placed before the header. + if (F->getFunction().hasOptSize()) + return L.getHeader(); + + MachineBasicBlock *OldTop = nullptr; + MachineBasicBlock *NewTop = L.getHeader(); + while (NewTop != OldTop) { + OldTop = NewTop; + NewTop = findBestLoopTopHelper(OldTop, L, LoopBlockSet); + if (NewTop != OldTop) + ComputedEdges[NewTop] = { OldTop, false }; + } + return NewTop; +} + +/// Find the best loop exiting block for layout. +/// +/// This routine implements the logic to analyze the loop looking for the best +/// block to layout at the top of the loop. Typically this is done to maximize +/// fallthrough opportunities. +MachineBasicBlock * +MachineBlockPlacement::findBestLoopExit(const MachineLoop &L, + const BlockFilterSet &LoopBlockSet, + BlockFrequency &ExitFreq) { + // We don't want to layout the loop linearly in all cases. If the loop header + // is just a normal basic block in the loop, we want to look for what block + // within the loop is the best one to layout at the top. However, if the loop + // header has be pre-merged into a chain due to predecessors not having + // analyzable branches, *and* the predecessor it is merged with is *not* part + // of the loop, rotating the header into the middle of the loop will create + // a non-contiguous range of blocks which is Very Bad. So start with the + // header and only rotate if safe. + BlockChain &HeaderChain = *BlockToChain[L.getHeader()]; + if (!LoopBlockSet.count(*HeaderChain.begin())) + return nullptr; + + BlockFrequency BestExitEdgeFreq; + unsigned BestExitLoopDepth = 0; + MachineBasicBlock *ExitingBB = nullptr; + // If there are exits to outer loops, loop rotation can severely limit + // fallthrough opportunities unless it selects such an exit. Keep a set of + // blocks where rotating to exit with that block will reach an outer loop. + SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop; + + LLVM_DEBUG(dbgs() << "Finding best loop exit for: " + << getBlockName(L.getHeader()) << "\n"); + for (MachineBasicBlock *MBB : L.getBlocks()) { + BlockChain &Chain = *BlockToChain[MBB]; + // Ensure that this block is at the end of a chain; otherwise it could be + // mid-way through an inner loop or a successor of an unanalyzable branch. + if (MBB != *std::prev(Chain.end())) + continue; + + // Now walk the successors. We need to establish whether this has a viable + // exiting successor and whether it has a viable non-exiting successor. + // We store the old exiting state and restore it if a viable looping + // successor isn't found. + MachineBasicBlock *OldExitingBB = ExitingBB; + BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq; + bool HasLoopingSucc = false; + for (MachineBasicBlock *Succ : MBB->successors()) { + if (Succ->isEHPad()) + continue; + if (Succ == MBB) + continue; + BlockChain &SuccChain = *BlockToChain[Succ]; + // Don't split chains, either this chain or the successor's chain. + if (&Chain == &SuccChain) { + LLVM_DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> " + << getBlockName(Succ) << " (chain conflict)\n"); + continue; + } + + auto SuccProb = MBPI->getEdgeProbability(MBB, Succ); + if (LoopBlockSet.count(Succ)) { + LLVM_DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> " + << getBlockName(Succ) << " (" << SuccProb << ")\n"); + HasLoopingSucc = true; + continue; + } + + unsigned SuccLoopDepth = 0; + if (MachineLoop *ExitLoop = MLI->getLoopFor(Succ)) { + SuccLoopDepth = ExitLoop->getLoopDepth(); + if (ExitLoop->contains(&L)) + BlocksExitingToOuterLoop.insert(MBB); + } + + BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(MBB) * SuccProb; + LLVM_DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> " + << getBlockName(Succ) << " [L:" << SuccLoopDepth + << "] ("; + MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n"); + // Note that we bias this toward an existing layout successor to retain + // incoming order in the absence of better information. The exit must have + // a frequency higher than the current exit before we consider breaking + // the layout. + BranchProbability Bias(100 - ExitBlockBias, 100); + if (!ExitingBB || SuccLoopDepth > BestExitLoopDepth || + ExitEdgeFreq > BestExitEdgeFreq || + (MBB->isLayoutSuccessor(Succ) && + !(ExitEdgeFreq < BestExitEdgeFreq * Bias))) { + BestExitEdgeFreq = ExitEdgeFreq; + ExitingBB = MBB; + } + } + + if (!HasLoopingSucc) { + // Restore the old exiting state, no viable looping successor was found. + ExitingBB = OldExitingBB; + BestExitEdgeFreq = OldBestExitEdgeFreq; + } + } + // Without a candidate exiting block or with only a single block in the + // loop, just use the loop header to layout the loop. + if (!ExitingBB) { + LLVM_DEBUG( + dbgs() << " No other candidate exit blocks, using loop header\n"); + return nullptr; + } + if (L.getNumBlocks() == 1) { + LLVM_DEBUG(dbgs() << " Loop has 1 block, using loop header as exit\n"); + return nullptr; + } + + // Also, if we have exit blocks which lead to outer loops but didn't select + // one of them as the exiting block we are rotating toward, disable loop + // rotation altogether. + if (!BlocksExitingToOuterLoop.empty() && + !BlocksExitingToOuterLoop.count(ExitingBB)) + return nullptr; + + LLVM_DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) + << "\n"); + ExitFreq = BestExitEdgeFreq; + return ExitingBB; +} + +/// Check if there is a fallthrough to loop header Top. +/// +/// 1. Look for a Pred that can be layout before Top. +/// 2. Check if Top is the most possible successor of Pred. +bool +MachineBlockPlacement::hasViableTopFallthrough( + const MachineBasicBlock *Top, + const BlockFilterSet &LoopBlockSet) { + for (MachineBasicBlock *Pred : Top->predecessors()) { + BlockChain *PredChain = BlockToChain[Pred]; + if (!LoopBlockSet.count(Pred) && + (!PredChain || Pred == *std::prev(PredChain->end()))) { + // Found a Pred block can be placed before Top. + // Check if Top is the best successor of Pred. + auto TopProb = MBPI->getEdgeProbability(Pred, Top); + bool TopOK = true; + for (MachineBasicBlock *Succ : Pred->successors()) { + auto SuccProb = MBPI->getEdgeProbability(Pred, Succ); + BlockChain *SuccChain = BlockToChain[Succ]; + // Check if Succ can be placed after Pred. + // Succ should not be in any chain, or it is the head of some chain. + if ((!SuccChain || Succ == *SuccChain->begin()) && SuccProb > TopProb) { + TopOK = false; + break; + } + } + if (TopOK) + return true; + } + } + return false; +} + +/// Attempt to rotate an exiting block to the bottom of the loop. +/// +/// Once we have built a chain, try to rotate it to line up the hot exit block +/// with fallthrough out of the loop if doing so doesn't introduce unnecessary +/// branches. For example, if the loop has fallthrough into its header and out +/// of its bottom already, don't rotate it. +void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain, + const MachineBasicBlock *ExitingBB, + BlockFrequency ExitFreq, + const BlockFilterSet &LoopBlockSet) { + if (!ExitingBB) + return; + + MachineBasicBlock *Top = *LoopChain.begin(); + MachineBasicBlock *Bottom = *std::prev(LoopChain.end()); + + // If ExitingBB is already the last one in a chain then nothing to do. + if (Bottom == ExitingBB) + return; + + bool ViableTopFallthrough = hasViableTopFallthrough(Top, LoopBlockSet); + + // If the header has viable fallthrough, check whether the current loop + // bottom is a viable exiting block. If so, bail out as rotating will + // introduce an unnecessary branch. + if (ViableTopFallthrough) { + for (MachineBasicBlock *Succ : Bottom->successors()) { + BlockChain *SuccChain = BlockToChain[Succ]; + if (!LoopBlockSet.count(Succ) && + (!SuccChain || Succ == *SuccChain->begin())) + return; + } + + // Rotate will destroy the top fallthrough, we need to ensure the new exit + // frequency is larger than top fallthrough. + BlockFrequency FallThrough2Top = TopFallThroughFreq(Top, LoopBlockSet); + if (FallThrough2Top >= ExitFreq) + return; + } + + BlockChain::iterator ExitIt = llvm::find(LoopChain, ExitingBB); + if (ExitIt == LoopChain.end()) + return; + + // Rotating a loop exit to the bottom when there is a fallthrough to top + // trades the entry fallthrough for an exit fallthrough. + // If there is no bottom->top edge, but the chosen exit block does have + // a fallthrough, we break that fallthrough for nothing in return. + + // Let's consider an example. We have a built chain of basic blocks + // B1, B2, ..., Bn, where Bk is a ExitingBB - chosen exit block. + // By doing a rotation we get + // Bk+1, ..., Bn, B1, ..., Bk + // Break of fallthrough to B1 is compensated by a fallthrough from Bk. + // If we had a fallthrough Bk -> Bk+1 it is broken now. + // It might be compensated by fallthrough Bn -> B1. + // So we have a condition to avoid creation of extra branch by loop rotation. + // All below must be true to avoid loop rotation: + // If there is a fallthrough to top (B1) + // There was fallthrough from chosen exit block (Bk) to next one (Bk+1) + // There is no fallthrough from bottom (Bn) to top (B1). + // Please note that there is no exit fallthrough from Bn because we checked it + // above. + if (ViableTopFallthrough) { + assert(std::next(ExitIt) != LoopChain.end() && + "Exit should not be last BB"); + MachineBasicBlock *NextBlockInChain = *std::next(ExitIt); + if (ExitingBB->isSuccessor(NextBlockInChain)) + if (!Bottom->isSuccessor(Top)) + return; + } + + LLVM_DEBUG(dbgs() << "Rotating loop to put exit " << getBlockName(ExitingBB) + << " at bottom\n"); + std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end()); +} + +/// Attempt to rotate a loop based on profile data to reduce branch cost. +/// +/// With profile data, we can determine the cost in terms of missed fall through +/// opportunities when rotating a loop chain and select the best rotation. +/// Basically, there are three kinds of cost to consider for each rotation: +/// 1. The possibly missed fall through edge (if it exists) from BB out of +/// the loop to the loop header. +/// 2. The possibly missed fall through edges (if they exist) from the loop +/// exits to BB out of the loop. +/// 3. The missed fall through edge (if it exists) from the last BB to the +/// first BB in the loop chain. +/// Therefore, the cost for a given rotation is the sum of costs listed above. +/// We select the best rotation with the smallest cost. +void MachineBlockPlacement::rotateLoopWithProfile( + BlockChain &LoopChain, const MachineLoop &L, + const BlockFilterSet &LoopBlockSet) { + auto RotationPos = LoopChain.end(); + + BlockFrequency SmallestRotationCost = BlockFrequency::getMaxFrequency(); + + // A utility lambda that scales up a block frequency by dividing it by a + // branch probability which is the reciprocal of the scale. + auto ScaleBlockFrequency = [](BlockFrequency Freq, + unsigned Scale) -> BlockFrequency { + if (Scale == 0) + return 0; + // Use operator / between BlockFrequency and BranchProbability to implement + // saturating multiplication. + return Freq / BranchProbability(1, Scale); + }; + + // Compute the cost of the missed fall-through edge to the loop header if the + // chain head is not the loop header. As we only consider natural loops with + // single header, this computation can be done only once. + BlockFrequency HeaderFallThroughCost(0); + MachineBasicBlock *ChainHeaderBB = *LoopChain.begin(); + for (auto *Pred : ChainHeaderBB->predecessors()) { + BlockChain *PredChain = BlockToChain[Pred]; + if (!LoopBlockSet.count(Pred) && + (!PredChain || Pred == *std::prev(PredChain->end()))) { + auto EdgeFreq = MBFI->getBlockFreq(Pred) * + MBPI->getEdgeProbability(Pred, ChainHeaderBB); + auto FallThruCost = ScaleBlockFrequency(EdgeFreq, MisfetchCost); + // If the predecessor has only an unconditional jump to the header, we + // need to consider the cost of this jump. + if (Pred->succ_size() == 1) + FallThruCost += ScaleBlockFrequency(EdgeFreq, JumpInstCost); + HeaderFallThroughCost = std::max(HeaderFallThroughCost, FallThruCost); + } + } + + // Here we collect all exit blocks in the loop, and for each exit we find out + // its hottest exit edge. For each loop rotation, we define the loop exit cost + // as the sum of frequencies of exit edges we collect here, excluding the exit + // edge from the tail of the loop chain. + SmallVector<std::pair<MachineBasicBlock *, BlockFrequency>, 4> ExitsWithFreq; + for (auto BB : LoopChain) { + auto LargestExitEdgeProb = BranchProbability::getZero(); + for (auto *Succ : BB->successors()) { + BlockChain *SuccChain = BlockToChain[Succ]; + if (!LoopBlockSet.count(Succ) && + (!SuccChain || Succ == *SuccChain->begin())) { + auto SuccProb = MBPI->getEdgeProbability(BB, Succ); + LargestExitEdgeProb = std::max(LargestExitEdgeProb, SuccProb); + } + } + if (LargestExitEdgeProb > BranchProbability::getZero()) { + auto ExitFreq = MBFI->getBlockFreq(BB) * LargestExitEdgeProb; + ExitsWithFreq.emplace_back(BB, ExitFreq); + } + } + + // In this loop we iterate every block in the loop chain and calculate the + // cost assuming the block is the head of the loop chain. When the loop ends, + // we should have found the best candidate as the loop chain's head. + for (auto Iter = LoopChain.begin(), TailIter = std::prev(LoopChain.end()), + EndIter = LoopChain.end(); + Iter != EndIter; Iter++, TailIter++) { + // TailIter is used to track the tail of the loop chain if the block we are + // checking (pointed by Iter) is the head of the chain. + if (TailIter == LoopChain.end()) + TailIter = LoopChain.begin(); + + auto TailBB = *TailIter; + + // Calculate the cost by putting this BB to the top. + BlockFrequency Cost = 0; + + // If the current BB is the loop header, we need to take into account the + // cost of the missed fall through edge from outside of the loop to the + // header. + if (Iter != LoopChain.begin()) + Cost += HeaderFallThroughCost; + + // Collect the loop exit cost by summing up frequencies of all exit edges + // except the one from the chain tail. + for (auto &ExitWithFreq : ExitsWithFreq) + if (TailBB != ExitWithFreq.first) + Cost += ExitWithFreq.second; + + // The cost of breaking the once fall-through edge from the tail to the top + // of the loop chain. Here we need to consider three cases: + // 1. If the tail node has only one successor, then we will get an + // additional jmp instruction. So the cost here is (MisfetchCost + + // JumpInstCost) * tail node frequency. + // 2. If the tail node has two successors, then we may still get an + // additional jmp instruction if the layout successor after the loop + // chain is not its CFG successor. Note that the more frequently executed + // jmp instruction will be put ahead of the other one. Assume the + // frequency of those two branches are x and y, where x is the frequency + // of the edge to the chain head, then the cost will be + // (x * MisfetechCost + min(x, y) * JumpInstCost) * tail node frequency. + // 3. If the tail node has more than two successors (this rarely happens), + // we won't consider any additional cost. + if (TailBB->isSuccessor(*Iter)) { + auto TailBBFreq = MBFI->getBlockFreq(TailBB); + if (TailBB->succ_size() == 1) + Cost += ScaleBlockFrequency(TailBBFreq.getFrequency(), + MisfetchCost + JumpInstCost); + else if (TailBB->succ_size() == 2) { + auto TailToHeadProb = MBPI->getEdgeProbability(TailBB, *Iter); + auto TailToHeadFreq = TailBBFreq * TailToHeadProb; + auto ColderEdgeFreq = TailToHeadProb > BranchProbability(1, 2) + ? TailBBFreq * TailToHeadProb.getCompl() + : TailToHeadFreq; + Cost += ScaleBlockFrequency(TailToHeadFreq, MisfetchCost) + + ScaleBlockFrequency(ColderEdgeFreq, JumpInstCost); + } + } + + LLVM_DEBUG(dbgs() << "The cost of loop rotation by making " + << getBlockName(*Iter) + << " to the top: " << Cost.getFrequency() << "\n"); + + if (Cost < SmallestRotationCost) { + SmallestRotationCost = Cost; + RotationPos = Iter; + } + } + + if (RotationPos != LoopChain.end()) { + LLVM_DEBUG(dbgs() << "Rotate loop by making " << getBlockName(*RotationPos) + << " to the top\n"); + std::rotate(LoopChain.begin(), RotationPos, LoopChain.end()); + } +} + +/// Collect blocks in the given loop that are to be placed. +/// +/// When profile data is available, exclude cold blocks from the returned set; +/// otherwise, collect all blocks in the loop. +MachineBlockPlacement::BlockFilterSet +MachineBlockPlacement::collectLoopBlockSet(const MachineLoop &L) { + BlockFilterSet LoopBlockSet; + + // Filter cold blocks off from LoopBlockSet when profile data is available. + // Collect the sum of frequencies of incoming edges to the loop header from + // outside. If we treat the loop as a super block, this is the frequency of + // the loop. Then for each block in the loop, we calculate the ratio between + // its frequency and the frequency of the loop block. When it is too small, + // don't add it to the loop chain. If there are outer loops, then this block + // will be merged into the first outer loop chain for which this block is not + // cold anymore. This needs precise profile data and we only do this when + // profile data is available. + if (F->getFunction().hasProfileData() || ForceLoopColdBlock) { + BlockFrequency LoopFreq(0); + for (auto LoopPred : L.getHeader()->predecessors()) + if (!L.contains(LoopPred)) + LoopFreq += MBFI->getBlockFreq(LoopPred) * + MBPI->getEdgeProbability(LoopPred, L.getHeader()); + + for (MachineBasicBlock *LoopBB : L.getBlocks()) { + auto Freq = MBFI->getBlockFreq(LoopBB).getFrequency(); + if (Freq == 0 || LoopFreq.getFrequency() / Freq > LoopToColdBlockRatio) + continue; + LoopBlockSet.insert(LoopBB); + } + } else + LoopBlockSet.insert(L.block_begin(), L.block_end()); + + return LoopBlockSet; +} + +/// 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(const MachineLoop &L) { + // First recurse through any nested loops, building chains for those inner + // loops. + for (const MachineLoop *InnerLoop : L) + buildLoopChains(*InnerLoop); + + assert(BlockWorkList.empty() && + "BlockWorkList not empty when starting to build loop chains."); + assert(EHPadWorkList.empty() && + "EHPadWorkList not empty when starting to build loop chains."); + BlockFilterSet LoopBlockSet = collectLoopBlockSet(L); + + // Check if we have profile data for this function. If yes, we will rotate + // this loop by modeling costs more precisely which requires the profile data + // for better layout. + bool RotateLoopWithProfile = + ForcePreciseRotationCost || + (PreciseRotationCost && F->getFunction().hasProfileData()); + + // First check to see if there is an obviously preferable top block for the + // loop. This will default to the header, but may end up as one of the + // predecessors to the header if there is one which will result in strictly + // fewer branches in the loop body. + MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet); + + // If we selected just the header for the loop top, look for a potentially + // profitable exit block in the event that rotating the loop can eliminate + // branches by placing an exit edge at the bottom. + // + // Loops are processed innermost to uttermost, make sure we clear + // PreferredLoopExit before processing a new loop. + PreferredLoopExit = nullptr; + BlockFrequency ExitFreq; + if (!RotateLoopWithProfile && LoopTop == L.getHeader()) + PreferredLoopExit = findBestLoopExit(L, LoopBlockSet, ExitFreq); + + BlockChain &LoopChain = *BlockToChain[LoopTop]; + + // 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; + assert(LoopChain.UnscheduledPredecessors == 0 && + "LoopChain should not have unscheduled predecessors."); + UpdatedPreds.insert(&LoopChain); + + for (const MachineBasicBlock *LoopBB : LoopBlockSet) + fillWorkLists(LoopBB, UpdatedPreds, &LoopBlockSet); + + buildChain(LoopTop, LoopChain, &LoopBlockSet); + + if (RotateLoopWithProfile) + rotateLoopWithProfile(LoopChain, L, LoopBlockSet); + else + rotateLoop(LoopChain, PreferredLoopExit, ExitFreq, LoopBlockSet); + + LLVM_DEBUG({ + // Crash at the end so we get all of the debugging output first. + bool BadLoop = false; + if (LoopChain.UnscheduledPredecessors) { + 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 (MachineBasicBlock *ChainBB : LoopChain) { + dbgs() << " ... " << getBlockName(ChainBB) << "\n"; + if (!LoopBlockSet.remove(ChainBB)) { + // 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" + << " Bad block: " << getBlockName(ChainBB) << "\n"; + } + } + + if (!LoopBlockSet.empty()) { + BadLoop = true; + for (const MachineBasicBlock *LoopBB : LoopBlockSet) + 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(LoopBB) << "\n"; + } + assert(!BadLoop && "Detected problems with the placement of this loop."); + }); + + BlockWorkList.clear(); + EHPadWorkList.clear(); +} + +void MachineBlockPlacement::buildCFGChains() { + // Ensure that every BB in the function has an associated chain to simplify + // the assumptions of the remaining algorithm. + SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch. + for (MachineFunction::iterator FI = F->begin(), FE = F->end(); FI != FE; + ++FI) { + MachineBasicBlock *BB = &*FI; + BlockChain *Chain = + new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB); + // Also, merge any blocks which we cannot reason about and must preserve + // the exact fallthrough behavior for. + while (true) { + Cond.clear(); + MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch. + if (!TII->analyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough()) + break; + + MachineFunction::iterator NextFI = std::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."); + LLVM_DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: " + << getBlockName(BB) << " -> " << getBlockName(NextBB) + << "\n"); + Chain->merge(NextBB, nullptr); +#ifndef NDEBUG + BlocksWithUnanalyzableExits.insert(&*BB); +#endif + FI = NextFI; + BB = NextBB; + } + } + + // Build any loop-based chains. + PreferredLoopExit = nullptr; + for (MachineLoop *L : *MLI) + buildLoopChains(*L); + + assert(BlockWorkList.empty() && + "BlockWorkList should be empty before building final chain."); + assert(EHPadWorkList.empty() && + "EHPadWorkList should be empty before building final chain."); + + SmallPtrSet<BlockChain *, 4> UpdatedPreds; + for (MachineBasicBlock &MBB : *F) + fillWorkLists(&MBB, UpdatedPreds); + + BlockChain &FunctionChain = *BlockToChain[&F->front()]; + buildChain(&F->front(), FunctionChain); + +#ifndef NDEBUG + using FunctionBlockSetType = SmallPtrSet<MachineBasicBlock *, 16>; +#endif + LLVM_DEBUG({ + // Crash at the end so we get all of the debugging output first. + bool BadFunc = false; + FunctionBlockSetType FunctionBlockSet; + for (MachineBasicBlock &MBB : *F) + FunctionBlockSet.insert(&MBB); + + for (MachineBasicBlock *ChainBB : FunctionChain) + if (!FunctionBlockSet.erase(ChainBB)) { + BadFunc = true; + dbgs() << "Function chain contains a block not in the function!\n" + << " Bad block: " << getBlockName(ChainBB) << "\n"; + } + + if (!FunctionBlockSet.empty()) { + BadFunc = true; + for (MachineBasicBlock *RemainingBB : FunctionBlockSet) + dbgs() << "Function contains blocks never placed into a chain!\n" + << " Bad block: " << getBlockName(RemainingBB) << "\n"; + } + assert(!BadFunc && "Detected problems with the block placement."); + }); + + // Splice the blocks into place. + MachineFunction::iterator InsertPos = F->begin(); + LLVM_DEBUG(dbgs() << "[MBP] Function: " << F->getName() << "\n"); + for (MachineBasicBlock *ChainBB : FunctionChain) { + LLVM_DEBUG(dbgs() << (ChainBB == *FunctionChain.begin() ? "Placing chain " + : " ... ") + << getBlockName(ChainBB) << "\n"); + if (InsertPos != MachineFunction::iterator(ChainBB)) + F->splice(InsertPos, ChainBB); + else + ++InsertPos; + + // Update the terminator of the previous block. + if (ChainBB == *FunctionChain.begin()) + continue; + MachineBasicBlock *PrevBB = &*std::prev(MachineFunction::iterator(ChainBB)); + + // 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 = nullptr, *FBB = nullptr; // For AnalyzeBranch. + +#ifndef NDEBUG + if (!BlocksWithUnanalyzableExits.count(PrevBB)) { + // Given the exact block placement we chose, we may actually not _need_ to + // be able to edit PrevBB's terminator sequence, but not being _able_ to + // do that at this point is a bug. + assert((!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) || + !PrevBB->canFallThrough()) && + "Unexpected block with un-analyzable fallthrough!"); + Cond.clear(); + TBB = FBB = nullptr; + } +#endif + + // The "PrevBB" is not yet updated to reflect current code layout, so, + // o. it may fall-through to a block without explicit "goto" instruction + // before layout, and no longer fall-through it after layout; or + // o. just opposite. + // + // analyzeBranch() may return erroneous value for FBB when these two + // situations take place. For the first scenario FBB is mistakenly set NULL; + // for the 2nd scenario, the FBB, which is expected to be NULL, is + // mistakenly pointing to "*BI". + // Thus, if the future change needs to use FBB before the layout is set, it + // has to correct FBB first by using the code similar to the following: + // + // if (!Cond.empty() && (!FBB || FBB == ChainBB)) { + // PrevBB->updateTerminator(); + // Cond.clear(); + // TBB = FBB = nullptr; + // if (TII->analyzeBranch(*PrevBB, TBB, FBB, Cond)) { + // // FIXME: This should never take place. + // TBB = FBB = nullptr; + // } + // } + if (!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond)) + PrevBB->updateTerminator(); + } + + // Fixup the last block. + Cond.clear(); + MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch. + if (!TII->analyzeBranch(F->back(), TBB, FBB, Cond)) + F->back().updateTerminator(); + + BlockWorkList.clear(); + EHPadWorkList.clear(); +} + +void MachineBlockPlacement::optimizeBranches() { + BlockChain &FunctionChain = *BlockToChain[&F->front()]; + SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch. + + // Now that all the basic blocks in the chain have the proper layout, + // make a final call to AnalyzeBranch with AllowModify set. + // Indeed, the target may be able to optimize the branches in a way we + // cannot because all branches may not be analyzable. + // E.g., the target may be able to remove an unconditional branch to + // a fallthrough when it occurs after predicated terminators. + for (MachineBasicBlock *ChainBB : FunctionChain) { + Cond.clear(); + MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch. + if (!TII->analyzeBranch(*ChainBB, TBB, FBB, Cond, /*AllowModify*/ true)) { + // If PrevBB has a two-way branch, try to re-order the branches + // such that we branch to the successor with higher probability first. + if (TBB && !Cond.empty() && FBB && + MBPI->getEdgeProbability(ChainBB, FBB) > + MBPI->getEdgeProbability(ChainBB, TBB) && + !TII->reverseBranchCondition(Cond)) { + LLVM_DEBUG(dbgs() << "Reverse order of the two branches: " + << getBlockName(ChainBB) << "\n"); + LLVM_DEBUG(dbgs() << " Edge probability: " + << MBPI->getEdgeProbability(ChainBB, FBB) << " vs " + << MBPI->getEdgeProbability(ChainBB, TBB) << "\n"); + DebugLoc dl; // FIXME: this is nowhere + TII->removeBranch(*ChainBB); + TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl); + ChainBB->updateTerminator(); + } + } + } +} + +void MachineBlockPlacement::alignBlocks() { + // Walk through the backedges of the function now that we have fully laid out + // the basic blocks and align the destination of each backedge. We don't rely + // exclusively on the loop info here so that we can align backedges in + // unnatural CFGs and backedges that were introduced purely because of the + // loop rotations done during this layout pass. + if (F->getFunction().hasMinSize() || + (F->getFunction().hasOptSize() && !TLI->alignLoopsWithOptSize())) + return; + BlockChain &FunctionChain = *BlockToChain[&F->front()]; + if (FunctionChain.begin() == FunctionChain.end()) + return; // Empty chain. + + const BranchProbability ColdProb(1, 5); // 20% + BlockFrequency EntryFreq = MBFI->getBlockFreq(&F->front()); + BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb; + for (MachineBasicBlock *ChainBB : FunctionChain) { + if (ChainBB == *FunctionChain.begin()) + continue; + + // Don't align non-looping basic blocks. These are unlikely to execute + // enough times to matter in practice. Note that we'll still handle + // unnatural CFGs inside of a natural outer loop (the common case) and + // rotated loops. + MachineLoop *L = MLI->getLoopFor(ChainBB); + if (!L) + continue; + + const Align Align = TLI->getPrefLoopAlignment(L); + if (Align == 1) + continue; // Don't care about loop alignment. + + // If the block is cold relative to the function entry don't waste space + // aligning it. + BlockFrequency Freq = MBFI->getBlockFreq(ChainBB); + if (Freq < WeightedEntryFreq) + continue; + + // If the block is cold relative to its loop header, don't align it + // regardless of what edges into the block exist. + MachineBasicBlock *LoopHeader = L->getHeader(); + BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader); + if (Freq < (LoopHeaderFreq * ColdProb)) + continue; + + // Check for the existence of a non-layout predecessor which would benefit + // from aligning this block. + MachineBasicBlock *LayoutPred = + &*std::prev(MachineFunction::iterator(ChainBB)); + + // Force alignment if all the predecessors are jumps. We already checked + // that the block isn't cold above. + if (!LayoutPred->isSuccessor(ChainBB)) { + ChainBB->setAlignment(Align); + continue; + } + + // Align this block if the layout predecessor's edge into this block is + // cold relative to the block. When this is true, other predecessors make up + // all of the hot entries into the block and thus alignment is likely to be + // important. + BranchProbability LayoutProb = + MBPI->getEdgeProbability(LayoutPred, ChainBB); + BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb; + if (LayoutEdgeFreq <= (Freq * ColdProb)) + ChainBB->setAlignment(Align); + } +} + +/// Tail duplicate \p BB into (some) predecessors if profitable, repeating if +/// it was duplicated into its chain predecessor and removed. +/// \p BB - Basic block that may be duplicated. +/// +/// \p LPred - Chosen layout predecessor of \p BB. +/// Updated to be the chain end if LPred is removed. +/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong. +/// \p BlockFilter - Set of blocks that belong to the loop being laid out. +/// Used to identify which blocks to update predecessor +/// counts. +/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was +/// chosen in the given order due to unnatural CFG +/// only needed if \p BB is removed and +/// \p PrevUnplacedBlockIt pointed to \p BB. +/// @return true if \p BB was removed. +bool MachineBlockPlacement::repeatedlyTailDuplicateBlock( + MachineBasicBlock *BB, MachineBasicBlock *&LPred, + const MachineBasicBlock *LoopHeaderBB, + BlockChain &Chain, BlockFilterSet *BlockFilter, + MachineFunction::iterator &PrevUnplacedBlockIt) { + bool Removed, DuplicatedToLPred; + bool DuplicatedToOriginalLPred; + Removed = maybeTailDuplicateBlock(BB, LPred, Chain, BlockFilter, + PrevUnplacedBlockIt, + DuplicatedToLPred); + if (!Removed) + return false; + DuplicatedToOriginalLPred = DuplicatedToLPred; + // Iteratively try to duplicate again. It can happen that a block that is + // duplicated into is still small enough to be duplicated again. + // No need to call markBlockSuccessors in this case, as the blocks being + // duplicated from here on are already scheduled. + // Note that DuplicatedToLPred always implies Removed. + while (DuplicatedToLPred) { + assert(Removed && "Block must have been removed to be duplicated into its " + "layout predecessor."); + MachineBasicBlock *DupBB, *DupPred; + // The removal callback causes Chain.end() to be updated when a block is + // removed. On the first pass through the loop, the chain end should be the + // same as it was on function entry. On subsequent passes, because we are + // duplicating the block at the end of the chain, if it is removed the + // chain will have shrunk by one block. + BlockChain::iterator ChainEnd = Chain.end(); + DupBB = *(--ChainEnd); + // Now try to duplicate again. + if (ChainEnd == Chain.begin()) + break; + DupPred = *std::prev(ChainEnd); + Removed = maybeTailDuplicateBlock(DupBB, DupPred, Chain, BlockFilter, + PrevUnplacedBlockIt, + DuplicatedToLPred); + } + // If BB was duplicated into LPred, it is now scheduled. But because it was + // removed, markChainSuccessors won't be called for its chain. Instead we + // call markBlockSuccessors for LPred to achieve the same effect. This must go + // at the end because repeating the tail duplication can increase the number + // of unscheduled predecessors. + LPred = *std::prev(Chain.end()); + if (DuplicatedToOriginalLPred) + markBlockSuccessors(Chain, LPred, LoopHeaderBB, BlockFilter); + return true; +} + +/// Tail duplicate \p BB into (some) predecessors if profitable. +/// \p BB - Basic block that may be duplicated +/// \p LPred - Chosen layout predecessor of \p BB +/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong. +/// \p BlockFilter - Set of blocks that belong to the loop being laid out. +/// Used to identify which blocks to update predecessor +/// counts. +/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was +/// chosen in the given order due to unnatural CFG +/// only needed if \p BB is removed and +/// \p PrevUnplacedBlockIt pointed to \p BB. +/// \p DuplicatedToLPred - True if the block was duplicated into LPred. Will +/// only be true if the block was removed. +/// \return - True if the block was duplicated into all preds and removed. +bool MachineBlockPlacement::maybeTailDuplicateBlock( + MachineBasicBlock *BB, MachineBasicBlock *LPred, + BlockChain &Chain, BlockFilterSet *BlockFilter, + MachineFunction::iterator &PrevUnplacedBlockIt, + bool &DuplicatedToLPred) { + DuplicatedToLPred = false; + if (!shouldTailDuplicate(BB)) + return false; + + LLVM_DEBUG(dbgs() << "Redoing tail duplication for Succ#" << BB->getNumber() + << "\n"); + + // This has to be a callback because none of it can be done after + // BB is deleted. + bool Removed = false; + auto RemovalCallback = + [&](MachineBasicBlock *RemBB) { + // Signal to outer function + Removed = true; + + // Conservative default. + bool InWorkList = true; + // Remove from the Chain and Chain Map + if (BlockToChain.count(RemBB)) { + BlockChain *Chain = BlockToChain[RemBB]; + InWorkList = Chain->UnscheduledPredecessors == 0; + Chain->remove(RemBB); + BlockToChain.erase(RemBB); + } + + // Handle the unplaced block iterator + if (&(*PrevUnplacedBlockIt) == RemBB) { + PrevUnplacedBlockIt++; + } + + // Handle the Work Lists + if (InWorkList) { + SmallVectorImpl<MachineBasicBlock *> &RemoveList = BlockWorkList; + if (RemBB->isEHPad()) + RemoveList = EHPadWorkList; + RemoveList.erase( + llvm::remove_if(RemoveList, + [RemBB](MachineBasicBlock *BB) { + return BB == RemBB; + }), + RemoveList.end()); + } + + // Handle the filter set + if (BlockFilter) { + BlockFilter->remove(RemBB); + } + + // Remove the block from loop info. + MLI->removeBlock(RemBB); + if (RemBB == PreferredLoopExit) + PreferredLoopExit = nullptr; + + LLVM_DEBUG(dbgs() << "TailDuplicator deleted block: " + << getBlockName(RemBB) << "\n"); + }; + auto RemovalCallbackRef = + function_ref<void(MachineBasicBlock*)>(RemovalCallback); + + SmallVector<MachineBasicBlock *, 8> DuplicatedPreds; + bool IsSimple = TailDup.isSimpleBB(BB); + TailDup.tailDuplicateAndUpdate(IsSimple, BB, LPred, + &DuplicatedPreds, &RemovalCallbackRef); + + // Update UnscheduledPredecessors to reflect tail-duplication. + DuplicatedToLPred = false; + for (MachineBasicBlock *Pred : DuplicatedPreds) { + // We're only looking for unscheduled predecessors that match the filter. + BlockChain* PredChain = BlockToChain[Pred]; + if (Pred == LPred) + DuplicatedToLPred = true; + if (Pred == LPred || (BlockFilter && !BlockFilter->count(Pred)) + || PredChain == &Chain) + continue; + for (MachineBasicBlock *NewSucc : Pred->successors()) { + if (BlockFilter && !BlockFilter->count(NewSucc)) + continue; + BlockChain *NewChain = BlockToChain[NewSucc]; + if (NewChain != &Chain && NewChain != PredChain) + NewChain->UnscheduledPredecessors++; + } + } + return Removed; +} + +bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) { + if (skipFunction(MF.getFunction())) + return false; + + // Check for single-block functions and skip them. + if (std::next(MF.begin()) == MF.end()) + return false; + + F = &MF; + MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); + MBFI = std::make_unique<BranchFolder::MBFIWrapper>( + getAnalysis<MachineBlockFrequencyInfo>()); + MLI = &getAnalysis<MachineLoopInfo>(); + TII = MF.getSubtarget().getInstrInfo(); + TLI = MF.getSubtarget().getTargetLowering(); + MPDT = nullptr; + + // Initialize PreferredLoopExit to nullptr here since it may never be set if + // there are no MachineLoops. + PreferredLoopExit = nullptr; + + assert(BlockToChain.empty() && + "BlockToChain map should be empty before starting placement."); + assert(ComputedEdges.empty() && + "Computed Edge map should be empty before starting placement."); + + unsigned TailDupSize = TailDupPlacementThreshold; + // If only the aggressive threshold is explicitly set, use it. + if (TailDupPlacementAggressiveThreshold.getNumOccurrences() != 0 && + TailDupPlacementThreshold.getNumOccurrences() == 0) + TailDupSize = TailDupPlacementAggressiveThreshold; + + TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>(); + // For aggressive optimization, we can adjust some thresholds to be less + // conservative. + if (PassConfig->getOptLevel() >= CodeGenOpt::Aggressive) { + // At O3 we should be more willing to copy blocks for tail duplication. This + // increases size pressure, so we only do it at O3 + // Do this unless only the regular threshold is explicitly set. + if (TailDupPlacementThreshold.getNumOccurrences() == 0 || + TailDupPlacementAggressiveThreshold.getNumOccurrences() != 0) + TailDupSize = TailDupPlacementAggressiveThreshold; + } + + if (allowTailDupPlacement()) { + MPDT = &getAnalysis<MachinePostDominatorTree>(); + if (MF.getFunction().hasOptSize()) + TailDupSize = 1; + bool PreRegAlloc = false; + TailDup.initMF(MF, PreRegAlloc, MBPI, /* LayoutMode */ true, TailDupSize); + precomputeTriangleChains(); + } + + buildCFGChains(); + + // Changing the layout can create new tail merging opportunities. + // TailMerge can create jump into if branches that make CFG irreducible for + // HW that requires structured CFG. + bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() && + PassConfig->getEnableTailMerge() && + BranchFoldPlacement; + // No tail merging opportunities if the block number is less than four. + if (MF.size() > 3 && EnableTailMerge) { + unsigned TailMergeSize = TailDupSize + 1; + BranchFolder BF(/*EnableTailMerge=*/true, /*CommonHoist=*/false, *MBFI, + *MBPI, TailMergeSize); + + auto *MMIWP = getAnalysisIfAvailable<MachineModuleInfoWrapperPass>(); + if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(), + MMIWP ? &MMIWP->getMMI() : nullptr, MLI, + /*AfterPlacement=*/true)) { + // Redo the layout if tail merging creates/removes/moves blocks. + BlockToChain.clear(); + ComputedEdges.clear(); + // Must redo the post-dominator tree if blocks were changed. + if (MPDT) + MPDT->runOnMachineFunction(MF); + ChainAllocator.DestroyAll(); + buildCFGChains(); + } + } + + optimizeBranches(); + alignBlocks(); + + BlockToChain.clear(); + ComputedEdges.clear(); + ChainAllocator.DestroyAll(); + + if (AlignAllBlock) + // Align all of the blocks in the function to a specific alignment. + for (MachineBasicBlock &MBB : MF) + MBB.setAlignment(Align(1ULL << AlignAllBlock)); + else if (AlignAllNonFallThruBlocks) { + // Align all of the blocks that have no fall-through predecessors to a + // specific alignment. + for (auto MBI = std::next(MF.begin()), MBE = MF.end(); MBI != MBE; ++MBI) { + auto LayoutPred = std::prev(MBI); + if (!LayoutPred->isSuccessor(&*MBI)) + MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks)); + } + } + if (ViewBlockLayoutWithBFI != GVDT_None && + (ViewBlockFreqFuncName.empty() || + F->getFunction().getName().equals(ViewBlockFreqFuncName))) { + MBFI->view("MBP." + MF.getName(), false); + } + + + // We always return true as we have no way to track whether the final order + // differs from the original order. + return true; +} + +namespace { + +/// 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 absence of any placement transformations or when using +/// alternative placement strategies. +class MachineBlockPlacementStats : public MachineFunctionPass { + /// A handle to the branch probability pass. + const MachineBranchProbabilityInfo *MBPI; + + /// 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) override; + + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.addRequired<MachineBranchProbabilityInfo>(); + AU.addRequired<MachineBlockFrequencyInfo>(); + AU.setPreservesAll(); + MachineFunctionPass::getAnalysisUsage(AU); + } +}; + +} // end anonymous namespace + +char MachineBlockPlacementStats::ID = 0; + +char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID; + +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) + +bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) { + // Check for single-block functions and skip them. + if (std::next(F.begin()) == F.end()) + return false; + + MBPI = &getAnalysis<MachineBranchProbabilityInfo>(); + MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); + + for (MachineBasicBlock &MBB : F) { + BlockFrequency BlockFreq = MBFI->getBlockFreq(&MBB); + Statistic &NumBranches = + (MBB.succ_size() > 1) ? NumCondBranches : NumUncondBranches; + Statistic &BranchTakenFreq = + (MBB.succ_size() > 1) ? CondBranchTakenFreq : UncondBranchTakenFreq; + for (MachineBasicBlock *Succ : MBB.successors()) { + // Skip if this successor is a fallthrough. + if (MBB.isLayoutSuccessor(Succ)) + continue; + + BlockFrequency EdgeFreq = + BlockFreq * MBPI->getEdgeProbability(&MBB, Succ); + ++NumBranches; + BranchTakenFreq += EdgeFreq.getFrequency(); + } + } + + return false; +} |