diff options
| author | Dimitry Andric <dim@FreeBSD.org> | 2019-12-20 19:53:05 +0000 |
|---|---|---|
| committer | Dimitry Andric <dim@FreeBSD.org> | 2019-12-20 19:53:05 +0000 |
| commit | 0b57cec536236d46e3dba9bd041533462f33dbb7 (patch) | |
| tree | 56229dbdbbf76d18580f72f789003db17246c8d9 /contrib/llvm/lib/CodeGen/MachineBlockPlacement.cpp | |
| parent | 718ef55ec7785aae63f98f8ca05dc07ed399c16d (diff) | |
Notes
Diffstat (limited to 'contrib/llvm/lib/CodeGen/MachineBlockPlacement.cpp')
| -rw-r--r-- | contrib/llvm/lib/CodeGen/MachineBlockPlacement.cpp | 3159 |
1 files changed, 0 insertions, 3159 deletions
diff --git a/contrib/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/contrib/llvm/lib/CodeGen/MachineBlockPlacement.cpp deleted file mode 100644 index 639b588766a1..000000000000 --- a/contrib/llvm/lib/CodeGen/MachineBlockPlacement.cpp +++ /dev/null @@ -1,3159 +0,0 @@ -//===- 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."), - 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)."), - 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; - - unsigned Align = TLI->getPrefLoopAlignment(L); - if (!Align) - 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 = llvm::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); - - if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(), - getAnalysisIfAvailable<MachineModuleInfo>(), 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(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(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; -} |