// structure and branch probability estimates.
//
// The pass strives to preserve the structure of the CFG (that is, retain
-// a topological ordering of basic blocks) in the absense of a *strong* signal
+// 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.
//
//===----------------------------------------------------------------------===//
-#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/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
-#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/Allocator.h"
+#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
-#include "llvm/Support/ErrorHandling.h"
-#include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/PostOrderIterator.h"
-#include "llvm/ADT/SCCIterator.h"
-#include "llvm/ADT/SmallPtrSet.h"
-#include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
+#include "llvm/Target/TargetSubtargetInfo.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-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);
+
+// 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);
+
+static cl::opt<bool> OutlineOptionalBranches(
+ "outline-optional-branches",
+ cl::desc("Put completely optional branches, i.e. branches with a common "
+ "post dominator, out of line."),
+ cl::init(false), cl::Hidden);
+
+static cl::opt<unsigned> OutlineOptionalThreshold(
+ "outline-optional-threshold",
+ cl::desc("Don't outline optional branches that are a single block with an "
+ "instruction count below this threshold"),
+ cl::init(4), cl::Hidden);
namespace {
class BlockChain;
///
/// This is the datastructure representing a chain of consecutive blocks that
/// are profitable to layout together in order to maximize fallthrough
-/// probabilities. We also can use a block chain to represent a sequence of
-/// basic blocks which have some external (correctness) requirement for
-/// sequential layout.
-///
-/// Eventually, the block chains will form a directed graph over the function.
-/// We provide an SCC-supporting-iterator in order to quicky build and walk the
-/// SCCs of block chains within a function.
+/// 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.
///
-/// 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.
+/// 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.
///
/// 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) {
+ : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
assert(BB && "Cannot create a chain with a null basic block");
BlockToChain[BB] = this;
}
/// \brief Iterator over blocks within the chain.
- typedef SmallVectorImpl<MachineBasicBlock *>::const_iterator iterator;
+ typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
/// \brief Beginning of blocks within the chain.
- iterator begin() const { return Blocks.begin(); }
+ iterator begin() { return Blocks.begin(); }
/// \brief End of blocks within the chain.
- iterator end() const { return Blocks.end(); }
+ iterator end() { return Blocks.end(); }
/// \brief Merge a block chain into this one.
///
void merge(MachineBasicBlock *BB, BlockChain *Chain) {
assert(BB);
assert(!Blocks.empty());
- assert(Blocks.back()->isSuccessor(BB));
// Fast path in case we don't have a chain already.
if (!Chain) {
// Update the incoming blocks to point to this chain, and add them to the
// chain structure.
- for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
- BI != BE; ++BI) {
- Blocks.push_back(*BI);
- assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
- BlockToChain[*BI] = this;
+ for (MachineBasicBlock *ChainBB : *Chain) {
+ Blocks.push_back(ChainBB);
+ assert(BlockToChain[ChainBB] == Chain && "Incoming blocks not in chain");
+ BlockToChain[ChainBB] = this;
}
}
+
+#ifndef NDEBUG
+ /// \brief Dump the blocks in this chain.
+ LLVM_DUMP_METHOD void dump() {
+ for (MachineBasicBlock *MBB : *this)
+ MBB->dump();
+ }
+#endif // NDEBUG
+
+ /// \brief Count of predecessors within the loop currently being processed.
+ ///
+ /// This count is updated at each loop we process to represent the number of
+ /// in-loop predecessors of this chain.
+ unsigned LoopPredecessors;
};
}
const TargetInstrInfo *TII;
/// \brief A handle to the target's lowering info.
- const TargetLowering *TLI;
+ const TargetLoweringBase *TLI;
+
+ /// \brief A handle to the post dominator tree.
+ MachineDominatorTree *MDT;
+
+ /// \brief A set of blocks that are unavoidably execute, i.e. they dominate
+ /// all terminators of the MachineFunction.
+ SmallPtrSet<MachineBasicBlock *, 4> UnavoidableBlocks;
/// \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;
- BlockChain *CreateChain(MachineBasicBlock *BB);
- void mergeSuccessor(MachineBasicBlock *BB, BlockChain *Chain,
- BlockFilterSet *Filter = 0);
+ 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);
- void placeChainsTopologically(MachineFunction &F);
- void AlignLoops(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<MachineDominatorTree>();
AU.addRequired<MachineLoopInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
-
- const char *getPassName() const { return "Block Placement"; }
};
}
char MachineBlockPlacement::ID = 0;
-INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
+char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
+INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement",
"Branch Probability Basic Block Placement", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
+INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
-INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
+INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement",
"Branch Probability Basic Block Placement", false, false)
-FunctionPass *llvm::createMachineBlockPlacementPass() {
- return new MachineBlockPlacement();
-}
-
#ifndef NDEBUG
/// \brief Helper to print the name of a MBB.
///
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 << "BB#" << BB->getNumber();
+ OS << " (derived from LLVM BB '" << BB->getName() << "')";
OS.flush();
return Result;
}
}
#endif
-/// \brief Helper to create a new chain for a single BB.
+/// \brief Mark a chain's successors as having one fewer preds.
///
-/// 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);
- return Chain;
+/// 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 (MachineBasicBlock *MBB : Chain) {
+ // 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.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
+ BlockWorkList.push_back(*SuccChain.begin());
+ }
+ }
}
-/// \brief Merge a chain with any viable successor.
+/// \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.
///
-/// This routine walks the predecessors of the current block, looking for
-/// viable merge candidates. It has strict rules it uses to determine when
-/// a predecessor can be merged with the current block, which center around
-/// preserving the CFG structure. It performs the merge if any viable candidate
-/// is found.
-void MachineBlockPlacement::mergeSuccessor(MachineBasicBlock *BB,
- BlockChain *Chain,
- BlockFilterSet *Filter) {
+/// \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 : BB->successors()) {
+ if (BlockFilter && !BlockFilter->count(Succ))
+ continue;
+ BlockChain &SuccChain = *BlockToChain[Succ];
+ if (&SuccChain == &Chain) {
+ DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Already merged!\n");
+ continue;
+ }
+ if (Succ != *SuccChain.begin()) {
+ DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Mid chain!\n");
+ continue;
+ }
+
+ uint32_t SuccWeight = MBPI->getEdgeWeight(BB, Succ);
+ BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
+
+ // If we outline optional branches, look whether Succ is unavoidable, i.e.
+ // dominates all terminators of the MachineFunction. If it does, other
+ // successors must be optional. Don't do this for cold branches.
+ if (OutlineOptionalBranches && SuccProb > HotProb.getCompl() &&
+ UnavoidableBlocks.count(Succ) > 0) {
+ auto HasShortOptionalBranch = [&]() {
+ for (MachineBasicBlock *Pred : Succ->predecessors()) {
+ // Check whether there is an unplaced optional branch.
+ if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
+ BlockToChain[Pred] == &Chain)
+ continue;
+ // Check whether the optional branch has exactly one BB.
+ if (Pred->pred_size() > 1 || *Pred->pred_begin() != BB)
+ continue;
+ // Check whether the optional branch is small.
+ if (Pred->size() < OutlineOptionalThreshold)
+ return true;
+ }
+ return false;
+ };
+ if (!HasShortOptionalBranch())
+ return Succ;
+ }
+
+ // Only consider successors which are either "hot", or wouldn't violate
+ // any CFG constraints.
+ if (SuccChain.LoopPredecessors != 0) {
+ if (SuccProb < HotProb) {
+ DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << 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 : Succ->predecessors()) {
+ if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
+ BlockToChain[Pred] == &Chain)
+ continue;
+ BlockFrequency PredEdgeFreq =
+ MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
+ if (PredEdgeFreq >= CandidateEdgeFreq) {
+ BadCFGConflict = true;
+ break;
+ }
+ }
+ if (BadCFGConflict) {
+ DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
+ << " (prob) (non-cold CFG conflict)\n");
+ continue;
+ }
+ }
+
+ DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
+ << " (prob)"
+ << (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
+ << "\n");
+ if (BestSucc && BestWeight >= SuccWeight)
+ continue;
+ BestSucc = Succ;
+ BestWeight = SuccWeight;
+ }
+ return BestSucc;
+}
+
+/// \brief Select the best block from a worklist.
+///
+/// This looks through the provided worklist as a list of candidate basic
+/// blocks and select the most profitable one to place. The definition of
+/// profitable only really makes sense in the context of a loop. This returns
+/// the most frequently visited block in the worklist, which in the case of
+/// a loop, is the one most desirable to be physically close to the rest of the
+/// loop body in order to improve icache behavior.
+///
+/// \returns The best block found, or null if none are viable.
+MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
+ BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
+ const BlockFilterSet *BlockFilter) {
+ // Once we need to walk the worklist looking for a candidate, cleanup the
+ // worklist of already placed entries.
+ // FIXME: If this shows up on profiles, it could be folded (at the cost of
+ // some code complexity) into the loop below.
+ WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
+ [&](MachineBasicBlock *BB) {
+ return BlockToChain.lookup(BB) == &Chain;
+ }),
+ WorkList.end());
+
+ MachineBasicBlock *BestBlock = nullptr;
+ BlockFrequency BestFreq;
+ for (MachineBasicBlock *MBB : WorkList) {
+ BlockChain &SuccChain = *BlockToChain[MBB];
+ if (&SuccChain == &Chain) {
+ DEBUG(dbgs() << " " << getBlockName(MBB) << " -> Already merged!\n");
+ continue;
+ }
+ assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
+
+ BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB);
+ DEBUG(dbgs() << " " << getBlockName(MBB) << " -> ";
+ MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
+ if (BestBlock && BestFreq >= CandidateFreq)
+ continue;
+ BestBlock = MBB;
+ BestFreq = CandidateFreq;
+ }
+ return BestBlock;
+}
+
+/// \brief Retrieve the first unplaced basic block.
+///
+/// This routine is called when we are unable to use the CFG to walk through
+/// all of the basic blocks and form a chain due to unnatural loops in the CFG.
+/// We walk through the 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;
+}
+
+void MachineBlockPlacement::buildChain(
+ MachineBasicBlock *BB, BlockChain &Chain,
+ SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
+ const BlockFilterSet *BlockFilter) {
assert(BB);
- assert(Chain);
+ assert(BlockToChain[BB] == &Chain);
+ MachineFunction &F = *BB->getParent();
+ MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
- // If this block is not at the end of its chain, it cannot merge with any
- // other chain.
- if (Chain && *llvm::prior(Chain->end()) != BB)
- return;
+ 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);
- // Walk through the successors looking for the highest probability edge.
- MachineBasicBlock *Successor = 0;
- BranchProbability BestProb = BranchProbability::getZero();
- DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
- for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
- SE = BB->succ_end();
- SI != SE; ++SI) {
- if (BB == *SI || (Filter && !Filter->count(*SI)))
+ // 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());
+ }
+
+ DEBUG(dbgs() << "Finished forming chain for header block "
+ << getBlockNum(*Chain.begin()) << "\n");
+}
+
+/// \brief Find the best loop top block for layout.
+///
+/// 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 : L.getHeader()->predecessors()) {
+ 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;
- BranchProbability SuccProb = MBPI->getEdgeProbability(BB, *SI);
- DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb << "\n");
- if (!Successor || SuccProb > BestProb || (!(SuccProb < BestProb) &&
- BB->isLayoutSuccessor(*SI))) {
- Successor = *SI;
- BestProb = SuccProb;
+ BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
+ if (!BestPred || PredFreq > BestPredFreq ||
+ (!(PredFreq < BestPredFreq) &&
+ Pred->isLayoutSuccessor(L.getHeader()))) {
+ BestPred = Pred;
+ BestPredFreq = PredFreq;
}
}
- if (!Successor)
- return;
- // Grab a chain if it exists already for this successor and make sure the
- // successor is at the start of the chain as we can't merge mid-chain. Also,
- // if the successor chain is the same as our chain, we're already merged.
- BlockChain *SuccChain = BlockToChain[Successor];
- if (SuccChain && (SuccChain == Chain || Successor != *SuccChain->begin()))
+ // If no direct predecessor is fine, just use the loop header.
+ if (!BestPred)
+ return L.getHeader();
+
+ // 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;
+}
+
+/// \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 (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;
+ // 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(MBB, WeightScale);
+ 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) {
+ DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
+ << getBlockName(Succ) << " (chain conflict)\n");
+ continue;
+ }
+
+ uint32_t SuccWeight = MBPI->getEdgeWeight(MBB, Succ);
+ if (LoopBlockSet.count(Succ)) {
+ DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> "
+ << getBlockName(Succ) << " (" << SuccWeight << ")\n");
+ HasLoopingSucc = true;
+ continue;
+ }
+
+ unsigned SuccLoopDepth = 0;
+ if (MachineLoop *ExitLoop = MLI->getLoopFor(Succ)) {
+ SuccLoopDepth = ExitLoop->getLoopDepth();
+ if (ExitLoop->contains(&L))
+ BlocksExitingToOuterLoop.insert(MBB);
+ }
+
+ BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
+ BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(MBB) * SuccProb;
+ 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;
+ 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 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,
+ MachineBasicBlock *ExitingBB,
+ const BlockFilterSet &LoopBlockSet) {
+ if (!ExitingBB)
return;
- // We only merge chains across a CFG merge when the desired merge path is
- // significantly hotter than the incoming edge. We define a hot edge more
- // strictly than the BranchProbabilityInfo does, as the two predecessor
- // blocks may have dramatically different incoming probabilities we need to
- // account for. Therefor we use the "global" edge weight which is the
- // branch's probability times the block frequency of the predecessor.
- BlockFrequency MergeWeight = MBFI->getBlockFreq(BB);
- MergeWeight *= MBPI->getEdgeProbability(BB, Successor);
- // We only want to consider breaking the CFG when the merge weight is much
- // higher (80% vs. 20%), so multiply it by 1/4. This will require the merged
- // edge to be 4x more likely before we disrupt the CFG. This number matches
- // the definition of "hot" in BranchProbabilityAnalysis (80% vs. 20%).
- MergeWeight *= BranchProbability(1, 4);
- for (MachineBasicBlock::pred_iterator PI = Successor->pred_begin(),
- PE = Successor->pred_end();
- PI != PE; ++PI) {
- if (BB == *PI || Successor == *PI) continue;
- BlockFrequency PredWeight = MBFI->getBlockFreq(*PI);
- PredWeight *= MBPI->getEdgeProbability(*PI, Successor);
-
- // Return on the first predecessor we find which outstrips our merge weight.
- if (MergeWeight < PredWeight)
- return;
- DEBUG(dbgs() << "Breaking CFG edge!\n"
- << " Edge from " << getBlockNum(BB) << " to "
- << getBlockNum(Successor) << ": " << MergeWeight << "\n"
- << " vs. " << getBlockNum(BB) << " to "
- << getBlockNum(*PI) << ": " << PredWeight << "\n");
+ MachineBasicBlock *Top = *LoopChain.begin();
+ bool ViableTopFallthrough = false;
+ for (MachineBasicBlock *Pred : Top->predecessors()) {
+ BlockChain *PredChain = BlockToChain[Pred];
+ if (!LoopBlockSet.count(Pred) &&
+ (!PredChain || Pred == *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 : Bottom->successors()) {
+ BlockChain *SuccChain = BlockToChain[Succ];
+ if (!LoopBlockSet.count(Succ) &&
+ (!SuccChain || Succ == *SuccChain->begin()))
+ return;
+ }
}
- DEBUG(dbgs() << "Merging from " << getBlockNum(BB) << " to "
- << getBlockNum(Successor) << "\n");
- Chain->merge(Successor, SuccChain);
+ 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.
/// 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) {
+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);
-
- SmallPtrSet<MachineBasicBlock *, 16> LoopBlockSet(L.block_begin(),
- L.block_end());
-
- // Begin building up a set of chains of blocks within this loop which should
- // remain contiguous. Some of the blocks already belong to a chain which
- // represents an inner loop.
- for (MachineLoop::block_iterator BI = L.block_begin(), BE = L.block_end();
- BI != BE; ++BI) {
- MachineBasicBlock *BB = *BI;
- BlockChain *Chain = BlockToChain[BB];
- if (!Chain) Chain = CreateChain(BB);
- mergeSuccessor(BB, Chain, &LoopBlockSet);
+ for (MachineLoop *InnerLoop : L)
+ buildLoopChains(F, *InnerLoop);
+
+ 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 (MachineBasicBlock *LoopBB : L.getBlocks()) {
+ BlockChain &Chain = *BlockToChain[LoopBB];
+ if (!UpdatedPreds.insert(&Chain).second)
+ continue;
+
+ assert(Chain.LoopPredecessors == 0);
+ for (MachineBasicBlock *ChainBB : Chain) {
+ assert(BlockToChain[ChainBB] == &Chain);
+ for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
+ if (BlockToChain[Pred] == &Chain || !LoopBlockSet.count(Pred))
+ continue;
+ ++Chain.LoopPredecessors;
+ }
+ }
+
+ 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 (MachineBasicBlock *ChainBB : LoopChain) {
+ dbgs() << " ... " << getBlockName(ChainBB) << "\n";
+ if (!LoopBlockSet.erase(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 (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.");
+ });
}
void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
- // First build any loop-based chains.
- for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
- ++LI)
- buildLoopChains(F, **LI);
-
- // Now walk the blocks of the function forming chains where they don't
- // violate any CFG structure.
- for (MachineFunction::iterator BI = F.begin(), BE = F.end();
- BI != BE; ++BI) {
- MachineBasicBlock *BB = BI;
- BlockChain *Chain = BlockToChain[BB];
- if (!Chain) Chain = CreateChain(BB);
- mergeSuccessor(BB, Chain);
+ // 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;
+ }
}
-}
-void MachineBlockPlacement::placeChainsTopologically(MachineFunction &F) {
- MachineBasicBlock *EntryB = &F.front();
- BlockChain *EntryChain = BlockToChain[EntryB];
- assert(EntryChain && "Missing chain for entry block");
- assert(*EntryChain->begin() == EntryB &&
- "Entry block is not the head of the entry block chain");
+ if (OutlineOptionalBranches) {
+ // Find the nearest common dominator of all of F's terminators.
+ MachineBasicBlock *Terminator = nullptr;
+ for (MachineBasicBlock &MBB : F) {
+ if (MBB.succ_size() == 0) {
+ if (Terminator == nullptr)
+ Terminator = &MBB;
+ else
+ Terminator = MDT->findNearestCommonDominator(Terminator, &MBB);
+ }
+ }
- // Walk the blocks in RPO, and insert each block for a chain in order the
- // first time we see that chain.
- MachineFunction::iterator InsertPos = F.begin();
- SmallPtrSet<BlockChain *, 16> VisitedChains;
- ReversePostOrderTraversal<MachineBasicBlock *> RPOT(EntryB);
- typedef ReversePostOrderTraversal<MachineBasicBlock *>::rpo_iterator
- rpo_iterator;
- for (rpo_iterator I = RPOT.begin(), E = RPOT.end(); I != E; ++I) {
- BlockChain *Chain = BlockToChain[*I];
- assert(Chain);
- if(!VisitedChains.insert(Chain))
+ // MBBs dominating this common dominator are unavoidable.
+ UnavoidableBlocks.clear();
+ for (MachineBasicBlock &MBB : F) {
+ if (MDT->dominates(&MBB, Terminator)) {
+ UnavoidableBlocks.insert(&MBB);
+ }
+ }
+ }
+
+ // Build any loop-based chains.
+ for (MachineLoop *L : *MLI)
+ buildLoopChains(F, *L);
+
+ SmallVector<MachineBasicBlock *, 16> BlockWorkList;
+
+ SmallPtrSet<BlockChain *, 4> UpdatedPreds;
+ for (MachineBasicBlock &MBB : F) {
+ BlockChain &Chain = *BlockToChain[&MBB];
+ if (!UpdatedPreds.insert(&Chain).second)
continue;
- for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end(); BI != BE;
- ++BI) {
- DEBUG(dbgs() << (BI == Chain->begin() ? "Placing chain "
- : " ... ")
- << getBlockName(*BI) << "\n");
- if (InsertPos != MachineFunction::iterator(*BI))
- F.splice(InsertPos, *BI);
- else
- ++InsertPos;
+
+ assert(Chain.LoopPredecessors == 0);
+ for (MachineBasicBlock *ChainBB : Chain) {
+ assert(BlockToChain[ChainBB] == &Chain);
+ for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
+ if (BlockToChain[Pred] == &Chain)
+ continue;
+ ++Chain.LoopPredecessors;
+ }
}
+
+ if (Chain.LoopPredecessors == 0)
+ BlockWorkList.push_back(*Chain.begin());
}
- // 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) {
+ BlockChain &FunctionChain = *BlockToChain[&F.front()];
+ buildChain(&F.front(), FunctionChain, BlockWorkList);
+
+#ifndef NDEBUG
+ 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 (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();
+ for (MachineBasicBlock *ChainBB : FunctionChain) {
+ 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 = 0, *FBB = 0; // For AnalyzeBranch.
- if (!TII->AnalyzeBranch(*FI, TBB, FBB, Cond))
- FI->updateTerminator();
- }
-}
+ 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 == ChainBB)) {
+ 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;
+ }
+ }
-/// \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);
+ // 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();
+ }
+ }
- L->getTopBlock()->setAlignment(Align);
-}
+ // Fixup the last block.
+ Cond.clear();
+ MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
+ if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
+ F.back().updateTerminator();
-/// \brief Align loop headers to target preferred alignments.
-void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
- if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
+ // 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.
+ // FIXME: Use Function::optForSize().
+ if (F.getFunction()->hasFnAttribute(Attribute::OptimizeForSize))
return;
+ 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 (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.
- unsigned Align = TLI->getPrefLoopAlignment();
- if (!Align)
- return; // 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;
+ }
- for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
- AlignLoop(F, *I, Align);
+ // 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);
+ }
}
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>();
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
MLI = &getAnalysis<MachineLoopInfo>();
- TII = F.getTarget().getInstrInfo();
- TLI = F.getTarget().getTargetLowering();
+ TII = F.getSubtarget().getInstrInfo();
+ TLI = F.getSubtarget().getTargetLowering();
+ MDT = &getAnalysis<MachineDominatorTree>();
assert(BlockToChain.empty());
buildCFGChains(F);
- placeChainsTopologically(F);
- AlignLoops(F);
BlockToChain.clear();
+ ChainAllocator.DestroyAll();
+
+ if (AlignAllBlock)
+ // Align all of the blocks in the function to a specific alignment.
+ for (MachineBasicBlock &MBB : F)
+ MBB.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 (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;
+}
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