//
//===----------------------------------------------------------------------===//
//
-// This file implements basic block placement transformations using branch
-// probability estimates. It is based around "Algo2" from Profile Guided Code
-// Positioning [http://portal.acm.org/citation.cfm?id=989433].
+// This file implements basic block placement transformations using the CFG
+// structure and branch probability estimates.
//
-// We combine the BlockFrequencyInfo with BranchProbabilityInfo to simulate
-// measured edge-weights. The BlockFrequencyInfo effectively summarizes the
-// probability of starting from any particular block, and the
-// BranchProbabilityInfo the probability of exiting the block via a particular
-// edge. Combined they form a function-wide ordering of the edges.
+// 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.
//
//===----------------------------------------------------------------------===//
-#define DEBUG_TYPE "block-placement2"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
-#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/Allocator.h"
-#include "llvm/Support/ErrorHandling.h"
-#include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/SCCIterator.h"
-#include "llvm/ADT/SmallPtrSet.h"
-#include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include <algorithm>
using namespace llvm;
-namespace {
-/// \brief A structure for storing a weighted edge.
-///
-/// This stores an edge and its weight, computed as the product of the
-/// frequency that the starting block is entered with the probability of
-/// a particular exit block.
-struct WeightedEdge {
- BlockFrequency EdgeFrequency;
- MachineBasicBlock *From, *To;
-
- bool operator<(const WeightedEdge &RHS) const {
- return EdgeFrequency < RHS.EdgeFrequency;
- }
-};
-}
+#define DEBUG_TYPE "block-placement2"
+
+STATISTIC(NumCondBranches, "Number of conditional branches");
+STATISTIC(NumUncondBranches, "Number of uncondittional branches");
+STATISTIC(CondBranchTakenFreq,
+ "Potential frequency of taking conditional branches");
+STATISTIC(UncondBranchTakenFreq,
+ "Potential frequency of taking unconditional branches");
+
+static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
+ cl::desc("Force the alignment of all "
+ "blocks in the function."),
+ 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);
namespace {
-struct BlockChain;
+class BlockChain;
/// \brief Type for our function-wide basic block -> block chain mapping.
typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
}
///
/// This is the datastructure representing a chain of consecutive blocks that
/// are profitable to layout together in order to maximize fallthrough
-/// probabilities. We also can use a block chain to represent a sequence of
-/// basic blocks which have some external (correctness) requirement for
-/// sequential layout.
+/// 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.
///
-/// Eventually, the block chains will form a directed graph over the function.
-/// We provide an SCC-supporting-iterator in order to quicky build and walk the
-/// SCCs of block chains within a function.
-///
-/// The block chains also have support for calculating and caching probability
-/// information related to the chain itself versus other chains. This is used
-/// for ranking during the final layout of block chains.
-struct BlockChain {
- class SuccIterator;
-
- /// \brief The first and last basic block that from this chain.
+/// 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 {
+ /// \brief The sequence of blocks belonging to this chain.
///
- /// The chain is stored within the existing function ilist of basic blocks.
- /// When merging chains or otherwise manipulating them, we splice the blocks
- /// within this ilist, giving us very cheap storage here and constant time
- /// merge operations.
- ///
- /// It is extremely important to note that LastBB is the iterator pointing
- /// *at* the last basic block in the chain. That is, the chain consists of
- /// the *closed* range [FirstBB, LastBB]. We cannot use half-open ranges
- /// because the next basic block may get relocated to a different part of the
- /// function at any time during the run of this pass.
- MachineFunction::iterator FirstBB, LastBB;
+ /// This is the sequence of blocks for a particular chain. These will be laid
+ /// out in-order within the function.
+ SmallVector<MachineBasicBlock *, 4> Blocks;
/// \brief A handle to the function-wide basic block to block chain mapping.
///
/// structure.
BlockToChainMapType &BlockToChain;
- /// \brief The weight used to rank two block chains in the same SCC.
- ///
- /// This is used during SCC layout of block chains to cache and rank the
- /// chains. It is supposed to represent the expected frequency with which
- /// control reaches a block within this chain, has the option of branching to
- /// a block in some other chain participating in the SCC, but instead
- /// continues within this chain. The higher this is, the more costly we
- /// expect mis-predicted branches between this chain and other chains within
- /// the SCC to be. Thus, since we expect branches between chains to be
- /// predicted when backwards and not predicted when forwards, the higher this
- /// is the more important that this chain is laid out first among those
- /// chains in the same SCC as it.
- BlockFrequency InChainEdgeFrequency;
-
+public:
/// \brief Construct a new BlockChain.
///
/// This builds a new block chain representing a single basic block in the
/// function. It also registers itself as the chain that block participates
/// in with the BlockToChain mapping.
BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
- : FirstBB(BB), LastBB(BB), BlockToChain(BlockToChain) {
+ : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
assert(BB && "Cannot create a chain with a null basic block");
BlockToChain[BB] = this;
}
- /// \brief Merge another block chain into this one.
+ /// \brief Iterator over blocks within the chain.
+ typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
+
+ /// \brief Beginning of blocks within the chain.
+ iterator begin() { return Blocks.begin(); }
+
+ /// \brief End of blocks within the chain.
+ iterator end() { return Blocks.end(); }
+
+ /// \brief Merge a block chain into this one.
///
/// This routine merges a block chain into this one. It takes care of forming
/// a contiguous sequence of basic blocks, updating the edge list, and
/// updating the block -> chain mapping. It does not free or tear down the
/// old chain, but the old chain's block list is no longer valid.
- void merge(BlockChain *Chain) {
- assert(Chain && "Cannot merge a null chain");
- MachineFunction::iterator EndBB = llvm::next(LastBB);
- MachineFunction::iterator ChainEndBB = llvm::next(Chain->LastBB);
+ void merge(MachineBasicBlock *BB, BlockChain *Chain) {
+ assert(BB);
+ assert(!Blocks.empty());
+
+ // Fast path in case we don't have a chain already.
+ if (!Chain) {
+ assert(!BlockToChain[BB]);
+ Blocks.push_back(BB);
+ BlockToChain[BB] = this;
+ return;
+ }
+
+ assert(BB == *Chain->begin());
+ assert(Chain->begin() != Chain->end());
- // Update the incoming blocks to point to this chain.
- for (MachineFunction::iterator BI = Chain->FirstBB, BE = ChainEndBB;
+ // Update the incoming blocks to point to this chain, and add them to the
+ // chain structure.
+ for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
BI != BE; ++BI) {
- assert(BlockToChain[BI] == Chain && "Incoming blocks not in chain");
- BlockToChain[BI] = this;
+ Blocks.push_back(*BI);
+ assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
+ BlockToChain[*BI] = this;
}
-
- // We splice the blocks together within the function (unless they already
- // are adjacent) so we can represent the new chain with a pair of pointers
- // to basic blocks within the function. This is also useful as each chain
- // of blocks will end up being laid out contiguously within the function.
- if (EndBB != Chain->FirstBB)
- FirstBB->getParent()->splice(EndBB, Chain->FirstBB, ChainEndBB);
- LastBB = Chain->LastBB;
}
-};
-}
-
-namespace {
-/// \brief Successor iterator for BlockChains.
-///
-/// This is an iterator that walks over the successor block chains by looking
-/// through its blocks successors and mapping those back to block chains. This
-/// iterator is not a fully-functioning iterator, it is designed specifically
-/// to support the interface required by SCCIterator when forming and walking
-/// SCCs of BlockChains.
-///
-/// Note that this iterator cannot be used while the chains are still being
-/// formed and/or merged. Unlike the chains themselves, it does store end
-/// iterators which could be moved if the chains are re-ordered. Once we begin
-/// forming and iterating over an SCC of chains, the order of blocks within the
-/// function must not change until we finish using the SCC iterators.
-class BlockChain::SuccIterator
- : public std::iterator<std::forward_iterator_tag,
- BlockChain *, ptrdiff_t> {
- BlockChain *Chain;
- MachineFunction::iterator BI, BE;
- MachineBasicBlock::succ_iterator SI;
-public:
- explicit SuccIterator(BlockChain *Chain)
- : Chain(Chain), BI(Chain->FirstBB), BE(llvm::next(Chain->LastBB)),
- SI(BI->succ_begin()) {
- while (BI != BE && BI->succ_begin() == BI->succ_end())
- ++BI;
- if (BI != BE)
- SI = BI->succ_begin();
+#ifndef NDEBUG
+ /// \brief Dump the blocks in this chain.
+ LLVM_DUMP_METHOD void dump() {
+ for (iterator I = begin(), E = end(); I != E; ++I)
+ (*I)->dump();
}
+#endif // NDEBUG
- /// \brief Helper function to create an end iterator for a particular chain.
+ /// \brief Count of predecessors within the loop currently being processed.
///
- /// The "end" state is extremely arbitrary. We chose to have BI == BE, and SI
- /// == Chain->FirstBB->succ_begin(). The value of SI doesn't really make any
- /// sense, but rather than try to rationalize SI and our increment, when we
- /// detect an "end" state, we just immediately call this function to build
- /// the canonical end iterator.
- static SuccIterator CreateEnd(BlockChain *Chain) {
- SuccIterator It(Chain);
- It.BI = It.BE;
- return It;
- }
-
- bool operator==(const SuccIterator &RHS) const {
- return (Chain == RHS.Chain && BI == RHS.BI && SI == RHS.SI);
- }
- bool operator!=(const SuccIterator &RHS) const {
- return !operator==(RHS);
- }
-
- SuccIterator& operator++() {
- assert(*this != CreateEnd(Chain) && "Cannot increment the end iterator");
- // There may be null successor pointers, skip over them.
- // FIXME: I don't understand *why* there are null successor pointers.
- do {
- ++SI;
- if (SI != BI->succ_end() && *SI)
- return *this;
-
- // There may be a basic block without successors. Skip over them.
- do {
- ++BI;
- if (BI == BE)
- return *this = CreateEnd(Chain);
- } while (BI->succ_begin() == BI->succ_end());
- SI = BI->succ_begin();
- } while (!*SI);
- return *this;
- }
- SuccIterator operator++(int) {
- SuccIterator tmp = *this;
- ++*this;
- return tmp;
- }
-
- BlockChain *operator*() const {
- assert(Chain->BlockToChain.lookup(*SI) && "Missing chain");
- return Chain->BlockToChain.lookup(*SI);
- }
-};
-}
-
-namespace {
-/// \brief Sorter used with containers of BlockChain pointers.
-///
-/// Sorts based on the \see BlockChain::InChainEdgeFrequency -- see its
-/// comments for details on what this ordering represents.
-struct ChainPtrPrioritySorter {
- bool operator()(const BlockChain *LHS, const BlockChain *RHS) const {
- assert(LHS && RHS && "Null chain entry");
- return LHS->InChainEdgeFrequency < RHS->InChainEdgeFrequency;
- }
+ /// This count is updated at each loop we process to represent the number of
+ /// in-loop predecessors of this chain.
+ unsigned LoopPredecessors;
};
}
namespace {
class MachineBlockPlacement : public MachineFunctionPass {
+ /// \brief A typedef for a block filter set.
+ typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
+
/// \brief A handle to the branch probability pass.
const MachineBranchProbabilityInfo *MBPI;
const TargetInstrInfo *TII;
/// \brief A handle to the target's lowering info.
- const TargetLowering *TLI;
-
- /// \brief A prioritized list of edges in the BB-graph.
- ///
- /// For each function, we insert all control flow edges between BBs, along
- /// with their "global" frequency. The Frequency of an edge being taken is
- /// defined as the frequency of entering the source BB (from MBFI) times the
- /// probability of taking a particular branch out of that block (from MBPI).
- ///
- /// Once built, this list is sorted in ascending frequency, making the last
- /// edge the hottest one in the function.
- SmallVector<WeightedEdge, 64> Edges;
+ const TargetLoweringBase *TLI;
/// \brief Allocator and owner of BlockChain structures.
///
- /// We build BlockChains lazily by merging together high probability BB
- /// sequences acording to the "Algo2" in the paper mentioned at the top of
- /// the file. To reduce malloc traffic, we allocate them using this slab-like
- /// allocator, and destroy them after the pass completes.
+ /// 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;
/// \brief Function wide BasicBlock to BlockChain mapping.
/// between basic blocks.
DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
- /// \brief A prioritized sequence of chains.
- ///
- /// We build up the ideal sequence of basic block chains in reverse order
- /// here, and then walk backwards to arrange the final function ordering.
- SmallVector<BlockChain *, 16> PChains;
-
-#ifndef NDEBUG
- /// \brief A set of active chains used to sanity-check the pass algorithm.
- ///
- /// All operations on this member should be wrapped in an assert or NDEBUG.
- SmallPtrSet<BlockChain *, 16> ActiveChains;
-#endif
-
- BlockChain *CreateChain(MachineBasicBlock *BB);
- void PrioritizeEdges(MachineFunction &F);
- void BuildBlockChains();
- void PrioritizeChains(MachineFunction &F);
- void PlaceBlockChains(MachineFunction &F);
- void AlignLoops(MachineFunction &F);
+ void markChainSuccessors(BlockChain &Chain,
+ MachineBasicBlock *LoopHeaderBB,
+ SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
+ const BlockFilterSet *BlockFilter = nullptr);
+ MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
+ BlockChain &Chain,
+ const BlockFilterSet *BlockFilter);
+ MachineBasicBlock *selectBestCandidateBlock(
+ BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
+ const BlockFilterSet *BlockFilter);
+ MachineBasicBlock *getFirstUnplacedBlock(
+ MachineFunction &F,
+ const BlockChain &PlacedChain,
+ MachineFunction::iterator &PrevUnplacedBlockIt,
+ const BlockFilterSet *BlockFilter);
+ void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
+ SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
+ const BlockFilterSet *BlockFilter = nullptr);
+ MachineBasicBlock *findBestLoopTop(MachineLoop &L,
+ const BlockFilterSet &LoopBlockSet);
+ MachineBasicBlock *findBestLoopExit(MachineFunction &F,
+ MachineLoop &L,
+ const BlockFilterSet &LoopBlockSet);
+ void buildLoopChains(MachineFunction &F, MachineLoop &L);
+ void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
+ const BlockFilterSet &LoopBlockSet);
+ void buildCFGChains(MachineFunction &F);
public:
static char ID; // Pass identification, replacement for typeid
initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
}
- bool runOnMachineFunction(MachineFunction &F);
+ bool runOnMachineFunction(MachineFunction &F) override;
- void getAnalysisUsage(AnalysisUsage &AU) const {
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<MachineBranchProbabilityInfo>();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addRequired<MachineLoopInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
-
- const char *getPassName() const { return "Block Placement"; }
};
}
char MachineBlockPlacement::ID = 0;
+char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
"Branch Probability Basic Block Placement", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
"Branch Probability Basic Block Placement", false, false)
-FunctionPass *llvm::createMachineBlockPlacementPass() {
- return new MachineBlockPlacement();
+#ifndef NDEBUG
+/// \brief Helper to print the name of a MBB.
+///
+/// Only used by debug logging.
+static std::string getBlockName(MachineBasicBlock *BB) {
+ std::string Result;
+ raw_string_ostream OS(Result);
+ OS << "BB#" << BB->getNumber()
+ << " (derived from LLVM BB '" << BB->getName() << "')";
+ OS.flush();
+ return Result;
}
-namespace llvm {
-/// \brief GraphTraits specialization for our BlockChain graph.
-template <> struct GraphTraits<BlockChain *> {
- typedef BlockChain NodeType;
- typedef BlockChain::SuccIterator ChildIteratorType;
+/// \brief Helper to print the number of a MBB.
+///
+/// Only used by debug logging.
+static std::string getBlockNum(MachineBasicBlock *BB) {
+ std::string Result;
+ raw_string_ostream OS(Result);
+ OS << "BB#" << BB->getNumber();
+ OS.flush();
+ return Result;
+}
+#endif
- static NodeType *getEntryNode(NodeType *N) { return N; }
- static BlockChain::SuccIterator child_begin(NodeType *N) {
- return BlockChain::SuccIterator(N);
+/// \brief Mark a chain's successors as having one fewer preds.
+///
+/// When a chain is being merged into the "placed" chain, this routine will
+/// quickly walk the successors of each block in the chain and mark them as
+/// having one fewer active predecessor. It also adds any successors of this
+/// chain which reach the zero-predecessor state to the worklist passed in.
+void MachineBlockPlacement::markChainSuccessors(
+ BlockChain &Chain,
+ MachineBasicBlock *LoopHeaderBB,
+ SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
+ const BlockFilterSet *BlockFilter) {
+ // Walk all the blocks in this chain, marking their successors as having
+ // a predecessor placed.
+ for (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
+ CBI != CBE; ++CBI) {
+ // Add any successors for which this is the only un-placed in-loop
+ // predecessor to the worklist as a viable candidate for CFG-neutral
+ // placement. No subsequent placement of this block will violate the CFG
+ // shape, so we get to use heuristics to choose a favorable placement.
+ for (MachineBasicBlock::succ_iterator SI = (*CBI)->succ_begin(),
+ SE = (*CBI)->succ_end();
+ SI != SE; ++SI) {
+ if (BlockFilter && !BlockFilter->count(*SI))
+ continue;
+ BlockChain &SuccChain = *BlockToChain[*SI];
+ // Disregard edges within a fixed chain, or edges to the loop header.
+ if (&Chain == &SuccChain || *SI == LoopHeaderBB)
+ continue;
+
+ // This is a cross-chain edge that is within the loop, so decrement the
+ // loop predecessor count of the destination chain.
+ if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
+ BlockWorkList.push_back(*SuccChain.begin());
+ }
}
- static BlockChain::SuccIterator child_end(NodeType *N) {
- return BlockChain::SuccIterator::CreateEnd(N);
+}
+
+/// \brief Select the best successor for a block.
+///
+/// This looks across all successors of a particular block and attempts to
+/// select the "best" one to be the layout successor. It only considers direct
+/// successors which also pass the block filter. It will attempt to avoid
+/// breaking CFG structure, but cave and break such structures in the case of
+/// very hot successor edges.
+///
+/// \returns The best successor block found, or null if none are viable.
+MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
+ MachineBasicBlock *BB, BlockChain &Chain,
+ const BlockFilterSet *BlockFilter) {
+ const BranchProbability HotProb(4, 5); // 80%
+
+ MachineBasicBlock *BestSucc = nullptr;
+ // FIXME: Due to the performance of the probability and weight routines in
+ // the MBPI analysis, we manually compute probabilities using the edge
+ // weights. This is suboptimal as it means that the somewhat subtle
+ // definition of edge weight semantics is encoded here as well. We should
+ // improve the MBPI interface to efficiently support query patterns such as
+ // this.
+ uint32_t BestWeight = 0;
+ uint32_t WeightScale = 0;
+ uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
+ DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
+ for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
+ SE = BB->succ_end();
+ SI != SE; ++SI) {
+ if (BlockFilter && !BlockFilter->count(*SI))
+ continue;
+ BlockChain &SuccChain = *BlockToChain[*SI];
+ if (&SuccChain == &Chain) {
+ DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
+ continue;
+ }
+ if (*SI != *SuccChain.begin()) {
+ DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
+ continue;
+ }
+
+ uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
+ BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
+
+ // Only consider successors which are either "hot", or wouldn't violate
+ // any CFG constraints.
+ if (SuccChain.LoopPredecessors != 0) {
+ if (SuccProb < HotProb) {
+ DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
+ << " (prob) (CFG conflict)\n");
+ continue;
+ }
+
+ // Make sure that a hot successor doesn't have a globally more important
+ // predecessor.
+ BlockFrequency CandidateEdgeFreq
+ = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
+ bool BadCFGConflict = false;
+ for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
+ PE = (*SI)->pred_end();
+ PI != PE; ++PI) {
+ if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
+ BlockToChain[*PI] == &Chain)
+ continue;
+ BlockFrequency PredEdgeFreq
+ = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
+ if (PredEdgeFreq >= CandidateEdgeFreq) {
+ BadCFGConflict = true;
+ break;
+ }
+ }
+ if (BadCFGConflict) {
+ DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
+ << " (prob) (non-cold CFG conflict)\n");
+ continue;
+ }
+ }
+
+ DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
+ << " (prob)"
+ << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
+ << "\n");
+ if (BestSucc && BestWeight >= SuccWeight)
+ continue;
+ BestSucc = *SI;
+ BestWeight = SuccWeight;
}
-};
+ return BestSucc;
}
-/// \brief Helper to create a new chain for a single BB.
+/// \brief Select the best block from a worklist.
+///
+/// This looks through the provided worklist as a list of candidate basic
+/// blocks and select the most profitable one to place. The definition of
+/// profitable only really makes sense in the context of a loop. This returns
+/// the most frequently visited block in the worklist, which in the case of
+/// a loop, is the one most desirable to be physically close to the rest of the
+/// loop body in order to improve icache behavior.
///
-/// Takes care of growing the Chains, setting up the BlockChain object, and any
-/// debug checking logic.
-/// \returns A pointer to the new BlockChain.
-BlockChain *MachineBlockPlacement::CreateChain(MachineBasicBlock *BB) {
- BlockChain *Chain =
- new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
- assert(ActiveChains.insert(Chain));
- return Chain;
+/// \returns The best block found, or null if none are viable.
+MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
+ BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
+ const BlockFilterSet *BlockFilter) {
+ // Once we need to walk the worklist looking for a candidate, cleanup the
+ // worklist of already placed entries.
+ // FIXME: If this shows up on profiles, it could be folded (at the cost of
+ // some code complexity) into the loop below.
+ WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
+ [&](MachineBasicBlock *BB) {
+ return BlockToChain.lookup(BB) == &Chain;
+ }),
+ WorkList.end());
+
+ MachineBasicBlock *BestBlock = nullptr;
+ BlockFrequency BestFreq;
+ for (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
+ WBE = WorkList.end();
+ WBI != WBE; ++WBI) {
+ BlockChain &SuccChain = *BlockToChain[*WBI];
+ if (&SuccChain == &Chain) {
+ DEBUG(dbgs() << " " << getBlockName(*WBI)
+ << " -> Already merged!\n");
+ continue;
+ }
+ assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
+
+ BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
+ DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> ";
+ MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
+ if (BestBlock && BestFreq >= CandidateFreq)
+ continue;
+ BestBlock = *WBI;
+ BestFreq = CandidateFreq;
+ }
+ return BestBlock;
}
-/// \brief Build a prioritized list of edges.
+/// \brief Retrieve the first unplaced basic block.
///
-/// The priority is determined by the product of the block frequency (how
-/// likely it is to arrive at a particular block) times the probability of
-/// taking this particular edge out of the block. This provides a function-wide
-/// ordering of the edges.
-void MachineBlockPlacement::PrioritizeEdges(MachineFunction &F) {
- assert(Edges.empty() && "Already have an edge list");
- SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
- BlockChain *RequiredChain = 0;
- for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
- MachineBasicBlock *From = &*FI;
- // We only consider MBBs with analyzable branches. Even if the analysis
- // fails, if there is no fallthrough, we can still work with the MBB.
- MachineBasicBlock *TBB = 0, *FBB = 0;
- Cond.clear();
- if (TII->AnalyzeBranch(*From, TBB, FBB, Cond) && From->canFallThrough()) {
- // We push all unanalyzed blocks onto a chain eagerly to prevent them
- // from being split later. Create the chain if needed, otherwise just
- // keep track that these blocks reside on it.
- if (!RequiredChain)
- RequiredChain = CreateChain(From);
- else
- BlockToChain[From] = RequiredChain;
- } else {
- // As soon as we find an analyzable branch, add that block to and
- // finalize any required chain that has been started. The required chain
- // is only modeling potentially inexplicable fallthrough, so the first
- // block to have analyzable fallthrough is a known-safe stopping point.
- if (RequiredChain) {
- BlockToChain[From] = RequiredChain;
- RequiredChain->LastBB = FI;
- RequiredChain = 0;
- }
+/// 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(
+ MachineFunction &F, const BlockChain &PlacedChain,
+ MachineFunction::iterator &PrevUnplacedBlockIt,
+ const BlockFilterSet *BlockFilter) {
+ for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F.end(); I != E;
+ ++I) {
+ if (BlockFilter && !BlockFilter->count(I))
+ continue;
+ if (BlockToChain[I] != &PlacedChain) {
+ PrevUnplacedBlockIt = I;
+ // Now select the head of the chain to which the unplaced block belongs
+ // as the block to place. This will force the entire chain to be placed,
+ // and satisfies the requirements of merging chains.
+ return *BlockToChain[I]->begin();
}
+ }
+ return nullptr;
+}
- BlockFrequency BaseFrequency = MBFI->getBlockFreq(From);
- for (MachineBasicBlock::succ_iterator SI = From->succ_begin(),
- SE = From->succ_end();
- SI != SE; ++SI) {
- MachineBasicBlock *To = *SI;
- WeightedEdge WE = { BaseFrequency * MBPI->getEdgeProbability(From, To),
- From, To };
- Edges.push_back(WE);
+void MachineBlockPlacement::buildChain(
+ MachineBasicBlock *BB,
+ BlockChain &Chain,
+ SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
+ const BlockFilterSet *BlockFilter) {
+ assert(BB);
+ assert(BlockToChain[BB] == &Chain);
+ MachineFunction &F = *BB->getParent();
+ MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
+
+ MachineBasicBlock *LoopHeaderBB = BB;
+ markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
+ BB = *std::prev(Chain.end());
+ for (;;) {
+ assert(BB);
+ assert(BlockToChain[BB] == &Chain);
+ assert(*std::prev(Chain.end()) == BB);
+
+ // Look for the best viable successor if there is one to place immediately
+ // after this block.
+ MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
+
+ // If an immediate successor isn't available, look for the best viable
+ // block among those we've identified as not violating the loop's CFG at
+ // this point. This won't be a fallthrough, but it will increase locality.
+ if (!BestSucc)
+ BestSucc = selectBestCandidateBlock(Chain, BlockWorkList, BlockFilter);
+
+ if (!BestSucc) {
+ BestSucc = getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt,
+ BlockFilter);
+ if (!BestSucc)
+ break;
+
+ DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
+ "layout successor until the CFG reduces\n");
}
+
+ // Place this block, updating the datastructures to reflect its placement.
+ BlockChain &SuccChain = *BlockToChain[BestSucc];
+ // Zero out LoopPredecessors for the successor we're about to merge in case
+ // we selected a successor that didn't fit naturally into the CFG.
+ SuccChain.LoopPredecessors = 0;
+ DEBUG(dbgs() << "Merging from " << getBlockNum(BB)
+ << " to " << getBlockNum(BestSucc) << "\n");
+ markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
+ Chain.merge(BestSucc, &SuccChain);
+ BB = *std::prev(Chain.end());
}
- assert(!RequiredChain && "Never found a terminator for a required chain");
- std::stable_sort(Edges.begin(), Edges.end());
+
+ DEBUG(dbgs() << "Finished forming chain for header block "
+ << getBlockNum(*Chain.begin()) << "\n");
}
-/// \brief Build chains of basic blocks along hot paths.
+/// \brief Find the best loop top block for layout.
///
-/// Build chains by trying to merge each pair of blocks from the mostly costly
-/// edge first. This is essentially "Algo2" from the Profile Guided Code
-/// Placement paper. While each node is considered a chain of one block, this
-/// routine lazily build the chain objects themselves so that when possible it
-/// can just merge a block into an existing chain.
-void MachineBlockPlacement::BuildBlockChains() {
- for (SmallVectorImpl<WeightedEdge>::reverse_iterator EI = Edges.rbegin(),
- EE = Edges.rend();
- EI != EE; ++EI) {
- MachineBasicBlock *SourceB = EI->From, *DestB = EI->To;
- if (SourceB == DestB) continue;
-
- BlockChain *SourceChain = BlockToChain.lookup(SourceB);
- if (!SourceChain) SourceChain = CreateChain(SourceB);
- BlockChain *DestChain = BlockToChain.lookup(DestB);
- if (!DestChain) DestChain = CreateChain(DestB);
- if (SourceChain == DestChain)
+/// Look for a block which is strictly better than the loop header for laying
+/// out at the top of the loop. This looks for one and only one pattern:
+/// a latch block with no conditional exit. This block will cause a conditional
+/// jump around it or will be the bottom of the loop if we lay it out in place,
+/// but if it it doesn't end up at the bottom of the loop for any reason,
+/// rotation alone won't fix it. Because such a block will always result in an
+/// unconditional jump (for the backedge) rotating it in front of the loop
+/// header is always profitable.
+MachineBasicBlock *
+MachineBlockPlacement::findBestLoopTop(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[L.getHeader()];
+ if (!LoopBlockSet.count(*HeaderChain.begin()))
+ return L.getHeader();
+
+ DEBUG(dbgs() << "Finding best loop top for: "
+ << getBlockName(L.getHeader()) << "\n");
+
+ BlockFrequency BestPredFreq;
+ MachineBasicBlock *BestPred = nullptr;
+ for (MachineBasicBlock::pred_iterator PI = L.getHeader()->pred_begin(),
+ PE = L.getHeader()->pred_end();
+ PI != PE; ++PI) {
+ MachineBasicBlock *Pred = *PI;
+ if (!LoopBlockSet.count(Pred))
continue;
+ DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", "
+ << Pred->succ_size() << " successors, ";
+ MBFI->printBlockFreq(dbgs(), Pred) << " freq\n");
+ if (Pred->succ_size() > 1)
+ continue;
+
+ BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
+ if (!BestPred || PredFreq > BestPredFreq ||
+ (!(PredFreq < BestPredFreq) &&
+ Pred->isLayoutSuccessor(L.getHeader()))) {
+ BestPred = Pred;
+ BestPredFreq = PredFreq;
+ }
+ }
+
+ // If no direct predecessor is fine, just use the loop header.
+ if (!BestPred)
+ return L.getHeader();
- bool IsSourceTail =
- SourceChain->LastBB == MachineFunction::iterator(SourceB);
- bool IsDestHead =
- DestChain->FirstBB == MachineFunction::iterator(DestB);
+ // 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();
+
+ DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
+ return BestPred;
+}
- if (!IsSourceTail || !IsDestHead)
+
+/// \brief 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(MachineFunction &F,
+ MachineLoop &L,
+ const BlockFilterSet &LoopBlockSet) {
+ // 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 opportunites 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;
+
+ DEBUG(dbgs() << "Finding best loop exit for: "
+ << getBlockName(L.getHeader()) << "\n");
+ for (MachineLoop::block_iterator I = L.block_begin(),
+ E = L.block_end();
+ I != E; ++I) {
+ BlockChain &Chain = *BlockToChain[*I];
+ // 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 analyzable branch.
+ if (*I != *std::prev(Chain.end()))
continue;
- SourceChain->merge(DestChain);
- assert(ActiveChains.erase(DestChain));
+ // 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;
+ // FIXME: Due to the performance of the probability and weight routines in
+ // the MBPI analysis, we use the internal weights and manually compute the
+ // probabilities to avoid quadratic behavior.
+ uint32_t WeightScale = 0;
+ uint32_t SumWeight = MBPI->getSumForBlock(*I, WeightScale);
+ for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
+ SE = (*I)->succ_end();
+ SI != SE; ++SI) {
+ if ((*SI)->isLandingPad())
+ continue;
+ if (*SI == *I)
+ continue;
+ BlockChain &SuccChain = *BlockToChain[*SI];
+ // Don't split chains, either this chain or the successor's chain.
+ if (&Chain == &SuccChain) {
+ DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
+ << getBlockName(*SI) << " (chain conflict)\n");
+ continue;
+ }
+
+ uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
+ if (LoopBlockSet.count(*SI)) {
+ DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
+ << getBlockName(*SI) << " (" << SuccWeight << ")\n");
+ HasLoopingSucc = true;
+ continue;
+ }
+
+ unsigned SuccLoopDepth = 0;
+ if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI)) {
+ SuccLoopDepth = ExitLoop->getLoopDepth();
+ if (ExitLoop->contains(&L))
+ BlocksExitingToOuterLoop.insert(*I);
+ }
+
+ BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
+ BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
+ DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
+ << getBlockName(*SI) << " [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 || BestExitLoopDepth < SuccLoopDepth ||
+ ExitEdgeFreq > BestExitEdgeFreq ||
+ ((*I)->isLayoutSuccessor(*SI) &&
+ !(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
+ BestExitEdgeFreq = ExitEdgeFreq;
+ ExitingBB = *I;
+ }
+ }
+
+ // Restore the old exiting state, no viable looping successor was found.
+ if (!HasLoopingSucc) {
+ ExitingBB = OldExitingBB;
+ BestExitEdgeFreq = OldBestExitEdgeFreq;
+ continue;
+ }
}
+ // 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 || L.getNumBlocks() == 1)
+ 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;
+
+ DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
+ return ExitingBB;
}
-/// \brief Prioritize the chains to minimize back-edges between chains.
-///
-/// This is the trickiest part of the placement algorithm. Each chain is
-/// a hot-path through a sequence of basic blocks, but there are conditional
-/// branches away from this hot path, and to some other chain. Hardware branch
-/// predictors favor back edges over forward edges, and so it is desirable to
-/// arrange the targets of branches away from a hot path and to some other
-/// chain to come later in the function, making them forward branches, and
-/// helping the branch predictor to predict fallthrough.
+/// \brief Attempt to rotate an exiting block to the bottom of the loop.
///
-/// In some cases, this is easy. simply topologically walking from the entry
-/// chain through its successors in order would work if there were no cycles
-/// between the chains of blocks, but often there are. In such a case, we first
-/// need to identify the participants in the cycle, and then rank them so that
-/// the linearizing of the chains has the lowest *probability* of causing
-/// a mispredicted branch. To compute the correct rank for a chain, we take the
-/// complement of the branch probability for each branch leading away from the
-/// chain and multiply it by the frequency of the source block for that branch.
-/// This gives us the probability of that particular branch *not* being taken
-/// in this function. The sum of these probabilities for each chain is used as
-/// a rank, so that we order the chain with the highest such sum first.
-/// FIXME: This seems like a good approximation, but there is probably a known
-/// technique for ordering of an SCC given edge weights. It would be good to
-/// use that, or even use its code if possible.
+/// 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,
+ MachineBasicBlock *ExitingBB,
+ const BlockFilterSet &LoopBlockSet) {
+ if (!ExitingBB)
+ return;
+
+ MachineBasicBlock *Top = *LoopChain.begin();
+ bool ViableTopFallthrough = false;
+ for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(),
+ PE = Top->pred_end();
+ PI != PE; ++PI) {
+ BlockChain *PredChain = BlockToChain[*PI];
+ if (!LoopBlockSet.count(*PI) &&
+ (!PredChain || *PI == *std::prev(PredChain->end()))) {
+ ViableTopFallthrough = true;
+ break;
+ }
+ }
+
+ // 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) {
+ MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
+ for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(),
+ SE = Bottom->succ_end();
+ SI != SE; ++SI) {
+ BlockChain *SuccChain = BlockToChain[*SI];
+ if (!LoopBlockSet.count(*SI) &&
+ (!SuccChain || *SI == *SuccChain->begin()))
+ return;
+ }
+ }
+
+ BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
+ ExitingBB);
+ if (ExitIt == LoopChain.end())
+ return;
+
+ std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
+}
+
+/// \brief Forms basic block chains from the natural loop structures.
///
-/// Also notable is that we prioritize the chains from the bottom up, and so
-/// all of the "first" and "before" relationships end up inverted in the code.
-void MachineBlockPlacement::PrioritizeChains(MachineFunction &F) {
- MachineBasicBlock *EntryB = &F.front();
- BlockChain *EntryChain = BlockToChain[EntryB];
- assert(EntryChain && "Missing chain for entry block");
- assert(EntryChain->FirstBB == F.begin() &&
- "Entry block is not the head of the entry block chain");
-
- // Form an SCC and walk it from the bottom up.
- SmallPtrSet<BlockChain *, 4> IsInSCC;
- for (scc_iterator<BlockChain *> I = scc_begin(EntryChain);
- !I.isAtEnd(); ++I) {
- const std::vector<BlockChain *> &SCC = *I;
- PChains.insert(PChains.end(), SCC.begin(), SCC.end());
-
- // If there is only one chain in the SCC, it's trivially sorted so just
- // bail out early. Sorting the SCC is expensive.
- if (SCC.size() == 1)
+/// These chains are designed to preserve the existing *structure* of the code
+/// as much as possible. We can then stitch the chains together in a way which
+/// both preserves the topological structure and minimizes taken conditional
+/// branches.
+void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
+ MachineLoop &L) {
+ // First recurse through any nested loops, building chains for those inner
+ // loops.
+ for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
+ buildLoopChains(F, **LI);
+
+ SmallVector<MachineBasicBlock *, 16> BlockWorkList;
+ BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
+
+ // 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.
+ MachineBasicBlock *ExitingBB = nullptr;
+ if (LoopTop == L.getHeader())
+ ExitingBB = findBestLoopExit(F, L, LoopBlockSet);
+
+ 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.LoopPredecessors == 0);
+ UpdatedPreds.insert(&LoopChain);
+ for (MachineLoop::block_iterator BI = L.block_begin(),
+ BE = L.block_end();
+ BI != BE; ++BI) {
+ BlockChain &Chain = *BlockToChain[*BI];
+ if (!UpdatedPreds.insert(&Chain))
continue;
- // We work strictly on the PChains range from here on out to maximize
- // locality.
- SmallVectorImpl<BlockChain *>::iterator SCCEnd = PChains.end(),
- SCCBegin = SCCEnd - SCC.size();
- IsInSCC.clear();
- IsInSCC.insert(SCCBegin, SCCEnd);
-
- // Compute the edge frequency of staying in a chain, despite the existency
- // of an edge to some other chain within this SCC.
- for (SmallVectorImpl<BlockChain *>::iterator SCCI = SCCBegin;
- SCCI != SCCEnd; ++SCCI) {
- BlockChain *Chain = *SCCI;
-
- // Special case the entry chain. Regardless of the weights of other
- // chains, the entry chain *must* come first, so move it to the end, and
- // avoid processing that chain at all.
- if (Chain == EntryChain) {
- --SCCEnd;
- if (SCCI == SCCEnd) break;
- Chain = *SCCI = *SCCEnd;
- *SCCEnd = EntryChain;
+ assert(Chain.LoopPredecessors == 0);
+ for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
+ BCI != BCE; ++BCI) {
+ assert(BlockToChain[*BCI] == &Chain);
+ for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
+ PE = (*BCI)->pred_end();
+ PI != PE; ++PI) {
+ if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
+ continue;
+ ++Chain.LoopPredecessors;
}
+ }
- // Walk over every block in this chain looking for out-bound edges to
- // other chains in this SCC.
- for (MachineFunction::iterator BI = Chain->FirstBB,
- BE = llvm::next(Chain->LastBB);
- BI != BE; ++BI) {
- MachineBasicBlock *From = &*BI;
- for (MachineBasicBlock::succ_iterator SI = BI->succ_begin(),
- SE = BI->succ_end();
- SI != SE; ++SI) {
- MachineBasicBlock *To = *SI;
- if (!To || !IsInSCC.count(BlockToChain[To]))
- continue;
- BranchProbability ComplEdgeProb =
- MBPI->getEdgeProbability(From, To).getCompl();
- Chain->InChainEdgeFrequency +=
- MBFI->getBlockFreq(From) * ComplEdgeProb;
- }
+ if (Chain.LoopPredecessors == 0)
+ BlockWorkList.push_back(*Chain.begin());
+ }
+
+ buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
+ rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
+
+ DEBUG({
+ // Crash at the end so we get all of the debugging output first.
+ bool BadLoop = false;
+ if (LoopChain.LoopPredecessors) {
+ BadLoop = true;
+ dbgs() << "Loop chain contains a block without its preds placed!\n"
+ << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
+ << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
+ }
+ for (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
+ BCI != BCE; ++BCI) {
+ dbgs() << " ... " << getBlockName(*BCI) << "\n";
+ if (!LoopBlockSet.erase(*BCI)) {
+ // We don't mark the loop as bad here because there are real situations
+ // where this can occur. For example, with an unanalyzable fallthrough
+ // from a loop block to a non-loop block 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(*BCI) << "\n";
}
}
- // Sort the chains within the SCC according to their edge frequencies,
- // which should make the least costly chain of blocks to mis-place be
- // ordered first in the prioritized sequence.
- std::stable_sort(SCCBegin, SCCEnd, ChainPtrPrioritySorter());
- }
+ if (!LoopBlockSet.empty()) {
+ BadLoop = true;
+ for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
+ LBE = LoopBlockSet.end();
+ LBI != LBE; ++LBI)
+ dbgs() << "Loop contains blocks never placed into a chain!\n"
+ << " Loop header: " << getBlockName(*L.block_begin()) << "\n"
+ << " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
+ << " Bad block: " << getBlockName(*LBI) << "\n";
+ }
+ assert(!BadLoop && "Detected problems with the placement of this loop.");
+ });
}
-/// \brief Splice the function blocks together based on the chain priorities.
-///
-/// Each chain is already represented as a contiguous range of blocks in the
-/// function. Simply walk backwards down the prioritized chains and splice in
-/// any chains out of order. Note that the first chain we visit is necessarily
-/// the entry chain. It has no predecessors and so must be the top of the SCC.
-/// Also, we cannot splice any chain prior to the entry chain as we can't
-/// splice any blocks prior to the entry block.
-void MachineBlockPlacement::PlaceBlockChains(MachineFunction &F) {
- assert(!PChains.empty() && "No chains were prioritized");
- assert(PChains.back() == BlockToChain[&F.front()] &&
- "The entry chain must always be the final chain");
+void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
+ // 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.
+ for (;;) {
+ 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.");
+ DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
+ << getBlockName(BB) << " -> " << getBlockName(NextBB)
+ << "\n");
+ Chain->merge(NextBB, nullptr);
+ FI = NextFI;
+ BB = NextBB;
+ }
+ }
- MachineFunction::iterator InsertPos = F.begin();
- for (SmallVectorImpl<BlockChain *>::reverse_iterator CI = PChains.rbegin(),
- CE = PChains.rend();
- CI != CE; ++CI) {
- BlockChain *Chain = *CI;
- // Check that we process this chain only once for debugging.
- assert(ActiveChains.erase(Chain) && "Processed a chain twice");
-
- // If this chain is already in the right position, just skip past it.
- // Otherwise, splice it into position.
- if (InsertPos == Chain->FirstBB)
- InsertPos = llvm::next(Chain->LastBB);
- else
- F.splice(InsertPos, Chain->FirstBB, llvm::next(Chain->LastBB));
+ // Build any loop-based chains.
+ for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
+ ++LI)
+ buildLoopChains(F, **LI);
+
+ SmallVector<MachineBasicBlock *, 16> BlockWorkList;
+
+ SmallPtrSet<BlockChain *, 4> UpdatedPreds;
+ for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
+ MachineBasicBlock *BB = &*FI;
+ BlockChain &Chain = *BlockToChain[BB];
+ if (!UpdatedPreds.insert(&Chain))
+ continue;
+
+ assert(Chain.LoopPredecessors == 0);
+ for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
+ BCI != BCE; ++BCI) {
+ assert(BlockToChain[*BCI] == &Chain);
+ for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
+ PE = (*BCI)->pred_end();
+ PI != PE; ++PI) {
+ if (BlockToChain[*PI] == &Chain)
+ continue;
+ ++Chain.LoopPredecessors;
+ }
+ }
+
+ if (Chain.LoopPredecessors == 0)
+ BlockWorkList.push_back(*Chain.begin());
}
- // Note that we can't assert this is empty as there may be unreachable blocks
- // in the function.
+ BlockChain &FunctionChain = *BlockToChain[&F.front()];
+ buildChain(&F.front(), FunctionChain, BlockWorkList);
+
#ifndef NDEBUG
- ActiveChains.clear();
+ typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
#endif
+ DEBUG({
+ // Crash at the end so we get all of the debugging output first.
+ bool BadFunc = false;
+ FunctionBlockSetType FunctionBlockSet;
+ for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
+ FunctionBlockSet.insert(FI);
+
+ for (BlockChain::iterator BCI = FunctionChain.begin(),
+ BCE = FunctionChain.end();
+ BCI != BCE; ++BCI)
+ if (!FunctionBlockSet.erase(*BCI)) {
+ BadFunc = true;
+ dbgs() << "Function chain contains a block not in the function!\n"
+ << " Bad block: " << getBlockName(*BCI) << "\n";
+ }
+
+ if (!FunctionBlockSet.empty()) {
+ BadFunc = true;
+ for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
+ FBE = FunctionBlockSet.end();
+ FBI != FBE; ++FBI)
+ dbgs() << "Function contains blocks never placed into a chain!\n"
+ << " Bad block: " << getBlockName(*FBI) << "\n";
+ }
+ assert(!BadFunc && "Detected problems with the block placement.");
+ });
+
+ // Splice the blocks into place.
+ MachineFunction::iterator InsertPos = F.begin();
+ for (BlockChain::iterator BI = FunctionChain.begin(),
+ BE = FunctionChain.end();
+ BI != BE; ++BI) {
+ DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
+ : " ... ")
+ << getBlockName(*BI) << "\n");
+ if (InsertPos != MachineFunction::iterator(*BI))
+ F.splice(InsertPos, *BI);
+ else
+ ++InsertPos;
+
+ // Update the terminator of the previous block.
+ if (BI == FunctionChain.begin())
+ continue;
+ MachineBasicBlock *PrevBB = std::prev(MachineFunction::iterator(*BI));
- // Now that every block is in its final position, update all of the
- // terminators.
- SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
- for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
// FIXME: It would be awesome of updateTerminator would just return rather
// than assert when the branch cannot be analyzed in order to remove this
// boiler plate.
Cond.clear();
- MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
- if (!TII->AnalyzeBranch(*FI, TBB, FBB, Cond))
- FI->updateTerminator();
- }
-}
-
-/// \brief Recursive helper to align a loop and any nested loops.
-static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
- // Recurse through nested loops.
- for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
- AlignLoop(F, *I, Align);
+ MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
+ if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
+ // The "PrevBB" is not yet updated to reflect current code layout, so,
+ // o. it may fall-through to a block without explict "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".
+ //
+ bool needUpdateBr = true;
+ if (!Cond.empty() && (!FBB || FBB == *BI)) {
+ PrevBB->updateTerminator();
+ needUpdateBr = false;
+ Cond.clear();
+ TBB = FBB = nullptr;
+ if (TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
+ // FIXME: This should never take place.
+ TBB = FBB = nullptr;
+ }
+ }
- L->getTopBlock()->setAlignment(Align);
-}
+ // If PrevBB has a two-way branch, try to re-order the branches
+ // such that we branch to the successor with higher weight first.
+ if (TBB && !Cond.empty() && FBB &&
+ MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
+ !TII->ReverseBranchCondition(Cond)) {
+ DEBUG(dbgs() << "Reverse order of the two branches: "
+ << getBlockName(PrevBB) << "\n");
+ DEBUG(dbgs() << " Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
+ << " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
+ DebugLoc dl; // FIXME: this is nowhere
+ TII->RemoveBranch(*PrevBB);
+ TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
+ needUpdateBr = true;
+ }
+ if (needUpdateBr)
+ PrevBB->updateTerminator();
+ }
+ }
-/// \brief Align loop headers to target preferred alignments.
-void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
- if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
+ // Fixup the last block.
+ Cond.clear();
+ MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
+ if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
+ F.back().updateTerminator();
+
+ // 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()->getAttributes().
+ hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize))
return;
-
unsigned Align = TLI->getPrefLoopAlignment();
if (!Align)
return; // Don't care about loop alignment.
+ if (FunctionChain.begin() == FunctionChain.end())
+ return; // Empty chain.
+
+ const BranchProbability ColdProb(1, 5); // 20%
+ BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
+ BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
+ for (BlockChain::iterator BI = std::next(FunctionChain.begin()),
+ BE = FunctionChain.end();
+ BI != BE; ++BI) {
+ // 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(*BI);
+ if (!L)
+ continue;
+
+ // If the block is cold relative to the function entry don't waste space
+ // aligning it.
+ BlockFrequency Freq = MBFI->getBlockFreq(*BI);
+ 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(BI);
- for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
- AlignLoop(F, *I, Align);
+ // Force alignment if all the predecessors are jumps. We already checked
+ // that the block isn't cold above.
+ if (!LayoutPred->isSuccessor(*BI)) {
+ (*BI)->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, *BI);
+ BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
+ if (LayoutEdgeFreq <= (Freq * ColdProb))
+ (*BI)->setAlignment(Align);
+ }
}
bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
// Check for single-block functions and skip them.
- if (llvm::next(F.begin()) == F.end())
+ if (std::next(F.begin()) == F.end())
+ return false;
+
+ if (skipOptnoneFunction(*F.getFunction()))
return false;
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
MLI = &getAnalysis<MachineLoopInfo>();
TII = F.getTarget().getInstrInfo();
TLI = F.getTarget().getTargetLowering();
- assert(Edges.empty());
assert(BlockToChain.empty());
- assert(PChains.empty());
- assert(ActiveChains.empty());
- PrioritizeEdges(F);
- BuildBlockChains();
- PrioritizeChains(F);
- PlaceBlockChains(F);
- AlignLoops(F);
+ buildCFGChains(F);
- Edges.clear();
BlockToChain.clear();
- PChains.clear();
ChainAllocator.DestroyAll();
+ if (AlignAllBlock)
+ // Align all of the blocks in the function to a specific alignment.
+ for (MachineFunction::iterator FI = F.begin(), FE = F.end();
+ FI != FE; ++FI)
+ FI->setAlignment(AlignAllBlock);
+
// We always return true as we have no way to track whether the final order
// differs from the original order.
return true;
}
+
+namespace {
+/// \brief A pass to compute block placement statistics.
+///
+/// A separate pass to compute interesting statistics for evaluating block
+/// placement. This is separate from the actual placement pass so that they can
+/// be computed in the absence of any placement transformations or when using
+/// alternative placement strategies.
+class MachineBlockPlacementStats : public MachineFunctionPass {
+ /// \brief A handle to the branch probability pass.
+ const MachineBranchProbabilityInfo *MBPI;
+
+ /// \brief A handle to the function-wide block frequency pass.
+ const MachineBlockFrequencyInfo *MBFI;
+
+public:
+ static char ID; // Pass identification, replacement for typeid
+ MachineBlockPlacementStats() : MachineFunctionPass(ID) {
+ initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
+ }
+
+ bool runOnMachineFunction(MachineFunction &F) override;
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.addRequired<MachineBranchProbabilityInfo>();
+ AU.addRequired<MachineBlockFrequencyInfo>();
+ AU.setPreservesAll();
+ MachineFunctionPass::getAnalysisUsage(AU);
+ }
+};
+}
+
+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 (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
+ BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
+ Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
+ : NumUncondBranches;
+ Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
+ : UncondBranchTakenFreq;
+ for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
+ SE = I->succ_end();
+ SI != SE; ++SI) {
+ // Skip if this successor is a fallthrough.
+ if (I->isLayoutSuccessor(*SI))
+ continue;
+
+ BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
+ ++NumBranches;
+ BranchTakenFreq += EdgeFreq.getFrequency();
+ }
+ }
+
+ return false;
+}
+
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