Implement a block placement pass based on the branch probability and

block frequency analyses. This differs substantially from the existing
block-placement pass in LLVM:

1) It operates on the Machine-IR in the CodeGen layer. This exposes much
   more (and more precise) information and opportunities. Also, the
   results are more stable due to fewer transforms ocurring after the
   pass runs.
2) It uses the generalized probability and frequency analyses. These can
   model static heuristics, code annotation derived heuristics as well
   as eventual profile loading. By basing the optimization on the
   analysis interface it can work from any (or a combination) of these
   inputs.
3) It uses a more aggressive algorithm, both building chains from tho
   bottom up to maximize benefit, and using an SCC-based walk to layout
   chains of blocks in a profitable ordering without O(N^2) iterations
   which the old pass involves.

The pass is currently gated behind a flag, and not enabled by default
because it still needs to grow some important features. Most notably, it
needs to support loop aligning and careful layout of loop structures
much as done by hand currently in CodePlacementOpt. Once it supports
these, and has sufficient testing and quality tuning, it should replace
both of these passes.

Thanks to Nick Lewycky and Richard Smith for help authoring & debugging
this, and to Jakob, Andy, Eric, Jim, and probably a few others I'm
forgetting for reviewing and answering all my questions. Writing
a backend pass is *sooo* much better now than it used to be. =D

llvm-svn: 142641

llvm/include/llvm/CodeGen/Passes.h

@@ -155,6 +155,10 @@ namespace llvm {
   /// IfConverter Pass - This pass performs machine code if conversion.
   FunctionPass *createIfConverterPass();
 
+  /// MachineBlockPlacement Pass - This pass places basic blocks based on branch
+  /// probabilities.
+  FunctionPass *createMachineBlockPlacementPass();
+
   /// Code Placement Pass - This pass optimize code placement and aligns loop
   /// headers to target specific alignment boundary.
   FunctionPass *createCodePlacementOptPass();

llvm/include/llvm/InitializePasses.h

@@ -146,6 +146,7 @@ void initializeLowerIntrinsicsPass(PassRegistry&);
 void initializeLowerInvokePass(PassRegistry&);
 void initializeLowerSwitchPass(PassRegistry&);
 void initializeMachineBlockFrequencyInfoPass(PassRegistry&);
+void initializeMachineBlockPlacementPass(PassRegistry&);
 void initializeMachineBranchProbabilityInfoPass(PassRegistry&);
 void initializeMachineCSEPass(PassRegistry&);
 void initializeMachineDominatorTreePass(PassRegistry&);

llvm/lib/CodeGen/CMakeLists.txt

@@ -37,6 +37,7 @@ add_llvm_library(LLVMCodeGen
   LocalStackSlotAllocation.cpp
   MachineBasicBlock.cpp
   MachineBlockFrequencyInfo.cpp
+  MachineBlockPlacement.cpp
   MachineBranchProbabilityInfo.cpp
   MachineCSE.cpp
   MachineDominators.cpp

llvm/lib/CodeGen/CodeGen.cpp

@@ -28,6 +28,7 @@ void llvm::initializeCodeGen(PassRegistry &Registry) {
   initializeLiveStacksPass(Registry);
   initializeLiveVariablesPass(Registry);
   initializeMachineBlockFrequencyInfoPass(Registry);
+  initializeMachineBlockPlacementPass(Registry);
   initializeMachineCSEPass(Registry);
   initializeMachineDominatorTreePass(Registry);
   initializeMachineLICMPass(Registry);

llvm/lib/CodeGen/LLVMTargetMachine.cpp

@@ -53,6 +53,8 @@ static cl::opt<bool> DisableTailDuplicate("disable-tail-duplicate", cl::Hidden,
     cl::desc("Disable tail duplication"));
 static cl::opt<bool> DisableEarlyTailDup("disable-early-taildup", cl::Hidden,
     cl::desc("Disable pre-register allocation tail duplication"));
+static cl::opt<bool> EnableBlockPlacement("enable-block-placement",
+    cl::Hidden, cl::desc("Enable probability-driven block placement"));
 static cl::opt<bool> DisableCodePlace("disable-code-place", cl::Hidden,
     cl::desc("Disable code placement"));
 static cl::opt<bool> DisableSSC("disable-ssc", cl::Hidden,
@@ -486,8 +488,16 @@ bool LLVMTargetMachine::addCommonCodeGenPasses(PassManagerBase &PM,
     PM.add(createGCInfoPrinter(dbgs()));
 
   if (OptLevel != CodeGenOpt::None && !DisableCodePlace) {
-    PM.add(createCodePlacementOptPass());
-    printNoVerify(PM, "After CodePlacementOpt");
+    if (EnableBlockPlacement) {
+      // MachineBlockPlacement is an experimental pass which is disabled by
+      // default currently. Eventually it should subsume CodePlacementOpt, so
+      // when enabled, the other is disabled.
+      PM.add(createMachineBlockPlacementPass());
+      printNoVerify(PM, "After MachineBlockPlacement");
+    } else {
+      PM.add(createCodePlacementOptPass());
+      printNoVerify(PM, "After CodePlacementOpt");
+    }
   }
 
   if (addPreEmitPass(PM, OptLevel))

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -0,0 +1,624 @@
+//===-- MachineBlockPlacement.cpp - Basic Block Code Layout optimization --===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements basic block placement transformations using branch
+// probability estimates. It is based around "Algo2" from Profile Guided Code
+// Positioning [http://portal.acm.org/citation.cfm?id=989433].
+//
+// 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.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "block-placement2"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
+#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunctionPass.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/Target/TargetInstrInfo.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;
+  }
+};
+}
+
+namespace {
+struct BlockChain;
+/// \brief Type for our function-wide basic block -> block chain mapping.
+typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
+}
+
+namespace {
+/// \brief A chain of blocks which will be laid out contiguously.
+///
+/// This is the datastructure representing a chain of consecutive blocks that
+/// are profitable to layout together in order to maximize fallthrough
+/// probabilities. We also can use a block chain to represent a sequence of
+/// basic blocks which have some external (correctness) requirement for
+/// sequential layout.
+///
+/// Eventually, the block chains will form a directed graph over the function.
+/// We provide an SCC-supporting-iterator in order to quicky build and walk the
+/// SCCs of block chains within a function.
+///
+/// The block chains also have support for calculating and caching probability
+/// information related to the chain itself versus other chains. This is used
+/// for ranking during the final layout of block chains.
+struct BlockChain {
+  class SuccIterator;
+
+  /// \brief The first and last basic block that from 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;
+
+  /// \brief A handle to the function-wide basic block to block chain mapping.
+  ///
+  /// This is retained in each block chain to simplify the computation of child
+  /// block chains for SCC-formation and iteration. We store the edges to child
+  /// basic blocks, and map them back to their associated chains using this
+  /// structure.
+  BlockToChainMapType &BlockToChain;
+
+  /// \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;
+
+  /// \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) {
+    assert(BB && "Cannot create a chain with a null basic block");
+    BlockToChain[BB] = this;
+  }
+
+  /// \brief Merge another 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);
+
+    // Update the incoming blocks to point to this chain.
+    for (MachineFunction::iterator BI = Chain->FirstBB, BE = ChainEndBB;
+         BI != BE; ++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();
+  }
+
+  /// \brief Helper function to create an end iterator for a particular chain.
+  ///
+  /// 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;
+  }
+};
+}
+
+namespace {
+class MachineBlockPlacement : 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;
+
+  /// \brief A handle to the target's instruction info.
+  const TargetInstrInfo *TII;
+
+  /// \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;
+
+  /// \brief Allocator and owner of BlockChain structures.
+  ///
+  /// We build BlockChains lazily by merging together high probability BB
+  /// sequences acording to the "Algo2" in the paper mentioned at the top of
+  /// the file. To reduce malloc traffic, we allocate them using this slab-like
+  /// allocator, and destroy them after the pass completes.
+  SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
+
+  /// \brief Function wide BasicBlock to BlockChain mapping.
+  ///
+  /// This mapping allows efficiently moving from any given basic block to the
+  /// BlockChain it participates in, if any. We use it to, among other things,
+  /// allow implicitly defining edges between chains as the existing edges
+  /// between basic blocks.
+  DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
+
+  /// \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);
+
+public:
+  static char ID; // Pass identification, replacement for typeid
+  MachineBlockPlacement() : MachineFunctionPass(ID) {
+    initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnMachineFunction(MachineFunction &F);
+
+  void getAnalysisUsage(AnalysisUsage &AU) const {
+    AU.addRequired<MachineBranchProbabilityInfo>();
+    AU.addRequired<MachineBlockFrequencyInfo>();
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  const char *getPassName() const { return "Block Placement"; }
+};
+}
+
+char MachineBlockPlacement::ID = 0;
+INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
+                      "Branch Probability Basic Block Placement", false, false)
+INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
+INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
+INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
+                    "Branch Probability Basic Block Placement", false, false)
+
+FunctionPass *llvm::createMachineBlockPlacementPass() {
+  return new MachineBlockPlacement();
+}
+
+namespace llvm {
+/// \brief GraphTraits specialization for our BlockChain graph.
+template <> struct GraphTraits<BlockChain *> {
+  typedef BlockChain NodeType;
+  typedef BlockChain::SuccIterator ChildIteratorType;
+
+  static NodeType *getEntryNode(NodeType *N) { return N; }
+  static BlockChain::SuccIterator child_begin(NodeType *N) {
+    return BlockChain::SuccIterator(N);
+  }
+  static BlockChain::SuccIterator child_end(NodeType *N) {
+    return BlockChain::SuccIterator::CreateEnd(N);
+  }
+};
+}
+
+/// \brief Helper to create a new chain for a single BB.
+///
+/// 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;
+}
+
+/// \brief Build a prioritized list of edges.
+///
+/// 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;
+      }
+    }
+
+    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);
+    }
+  }
+  assert(!RequiredChain && "Never found a terminator for a required chain");
+  std::stable_sort(Edges.begin(), Edges.end());
+}
+
+/// \brief Build chains of basic blocks along hot paths.
+///
+/// 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)
+      continue;
+
+    bool IsSourceTail =
+      SourceChain->LastBB == MachineFunction::iterator(SourceB);
+    bool IsDestHead =
+      DestChain->FirstBB == MachineFunction::iterator(DestB);
+
+    if (!IsSourceTail || !IsDestHead)
+      continue;
+
+    SourceChain->merge(DestChain);
+    assert(ActiveChains.erase(DestChain));
+  }
+}
+
+/// \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.
+///
+/// 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.
+///
+/// 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)
+      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;
+      }
+
+      // 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;
+        }
+      }
+    }
+
+    // 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());
+  }
+}
+
+/// \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");
+
+  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));
+  }
+
+  // Note that we can't assert this is empty as there may be unreachable blocks
+  // in the function.
+#ifndef NDEBUG
+  ActiveChains.clear();
+#endif
+
+  // 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();
+  }
+}
+
+bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
+  // Check for single-block functions and skip them.
+  if (llvm::next(F.begin()) == F.end())
+    return false;
+
+  MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
+  MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
+  TII = F.getTarget().getInstrInfo();
+  assert(Edges.empty());
+  assert(BlockToChain.empty());
+  assert(PChains.empty());
+  assert(ActiveChains.empty());
+
+  PrioritizeEdges(F);
+  BuildBlockChains();
+  PrioritizeChains(F);
+  PlaceBlockChains(F);
+
+  Edges.clear();
+  BlockToChain.clear();
+  PChains.clear();
+  ChainAllocator.DestroyAll();
+
+  // We always return true as we have no way to track whether the final order
+  // differs from the original order.
+  return true;
+}