llvm-project/llvm/lib/CodeGen/BasicBlockSections.cpp

//===-- BasicBlockSections.cpp ---=========--------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// BasicBlockSections implementation.
//
// The purpose of this pass is to assign sections to basic blocks when
// -fbasic-block-sections= option is used. Further, with profile information
// only the subset of basic blocks with profiles are placed in separate sections
// and the rest are grouped in a cold section. The exception handling blocks are
// treated specially to ensure they are all in one seciton.
//
// This pass also implements machine function splitting
// (-function-splitting=all). Functions without a basic block sections profile
// are split using PGO/AutoFDO profiles: cold blocks are moved into a separate
// section with the symbol `foo.cold`.
//
// Basic Block Sections
// ====================
//
// With option, -fbasic-block-sections=list, every function may be split into
// clusters of basic blocks. Every cluster will be emitted into a separate
// section with its basic blocks sequenced in the given order. To get the
// optimized performance, the clusters must form an optimal BB layout for the
// function. We insert a symbol at the beginning of every cluster's section to
// allow the linker to reorder the sections in any arbitrary sequence. A global
// order of these sections would encapsulate the function layout.
// For example, consider the following clusters for a function foo (consisting
// of 6 basic blocks 0, 1, ..., 5).
//
// 0 2
// 1 3 5
//
// * Basic blocks 0 and 2 are placed in one section with symbol `foo`
//   referencing the beginning of this section.
// * Basic blocks 1, 3, 5 are placed in a separate section. A new symbol
//   `foo.__part.1` will reference the beginning of this section.
// * Basic block 4 (note that it is not referenced in the list) is placed in
//   one section, and a new symbol `foo.cold` will point to it.
//
// There are a couple of challenges to be addressed:
//
// 1. The last basic block of every cluster should not have any implicit
//    fallthrough to its next basic block, as it can be reordered by the linker.
//    The compiler should make these fallthroughs explicit by adding
//    unconditional jumps..
//
// 2. All inter-cluster branch targets would now need to be resolved by the
//    linker as they cannot be calculated during compile time. This is done
//    using static relocations. Further, the compiler tries to use short branch
//    instructions on some ISAs for small branch offsets. This is not possible
//    for inter-cluster branches as the offset is not determined at compile
//    time, and therefore, long branch instructions have to be used for those.
//
// 3. Debug Information (DebugInfo) and Call Frame Information (CFI) emission
//    needs special handling with basic block sections. DebugInfo needs to be
//    emitted with more relocations as basic block sections can break a
//    function into potentially several disjoint pieces, and CFI needs to be
//    emitted per cluster. This also bloats the object file and binary sizes.
//
// Basic Block Address Map
// ==================
//
// With -fbasic-block-address-map, we emit the offsets of BB addresses of
// every function into the .llvm_bb_addr_map section. Along with the function
// symbols, this allows for mapping of virtual addresses in PMU profiles back to
// the corresponding basic blocks. This logic is implemented in AsmPrinter. This
// pass only assigns the BBSectionType of every function to ``labels``.
//
// Machine Function Splitting
// ==========================
//
// With -function-splitting=all, functions without a basic block sections
// profile are split using PGO/AutoFDO profiles. Blocks deemed cold are placed
// in a separate section prefixed with ".text.split." (see
// -bbsections-cold-text-prefix), referenced by the symbol `foo.cold`. Grouping
// cold blocks across functions decreases fragmentation and improves icache and
// iTLB utilization. Such functions get the BBSectionType ``Preset``.
//
// For the original RFC of machine function splitting please see
// https://groups.google.com/d/msg/llvm-dev/RUegaMg-iqc/wFAVxa6fCgAJ
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/EHUtils.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/CodeGen/BasicBlockMatchingAndInference.h"
#include "llvm/CodeGen/BasicBlockSectionUtils.h"
#include "llvm/CodeGen/BasicBlockSectionsProfileReader.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachinePostDominators.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/IR/Function.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/UniqueBBID.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Transforms/Utils/CodeLayout.h"
#include <optional>

using namespace llvm;

// Placing the cold clusters in a separate section mitigates against poor
// profiles and allows optimizations such as hugepage mapping to be applied at a
// section granularity. Defaults to ".text.split." which is recognized by lld
// via the `-z keep-text-section-prefix` flag.
cl::opt<std::string> llvm::BBSectionsColdTextPrefix(
    "bbsections-cold-text-prefix",
    cl::desc("The text prefix to use for cold basic block clusters"),
    cl::init(".text.split."), cl::Hidden);

static cl::opt<bool> BBSectionsDetectSourceDrift(
    "bbsections-detect-source-drift",
    cl::desc("This checks if there is a fdo instr. profile hash "
             "mismatch for this function"),
    cl::init(true), cl::Hidden);

// FIXME: This cutoff value is CPU dependent and should be moved to
// TargetTransformInfo once we consider enabling this on other platforms.
// The value is expressed as a ProfileSummaryInfo integer percentile cutoff.
// Defaults to 999950, i.e. all blocks colder than 99.995 percentile are split.
// The default was empirically determined to be optimal when considering cutoff
// values between 99%-ile to 100%-ile with respect to iTLB and icache metrics on
// Intel CPUs.
static cl::opt<unsigned>
    PercentileCutoff("mfs-psi-cutoff",
                     cl::desc("Percentile profile summary cutoff used to "
                              "determine cold blocks. Unused if set to zero."),
                     cl::init(999950), cl::Hidden);

static cl::opt<unsigned> ColdCountThreshold(
    "mfs-count-threshold",
    cl::desc(
        "Minimum number of times a block must be executed to be retained."),
    cl::init(1), cl::Hidden);

static cl::opt<bool> SplitAllEHCode(
    "mfs-split-ehcode",
    cl::desc("Splits all EH code and it's descendants by default."),
    cl::init(false), cl::Hidden);

namespace {

class BasicBlockSections : public MachineFunctionPass {
public:
  static char ID;

  explicit BasicBlockSections(bool SplitWithPGO = false)
      : MachineFunctionPass(ID), SplitWithPGO(SplitWithPGO) {
    initializeBasicBlockSectionsPass(*PassRegistry::getPassRegistry());
  }

  StringRef getPassName() const override {
    return "Basic Block Sections Transformation";
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override;

  /// Identify basic blocks that need separate sections and prepare to emit them
  /// accordingly.
  bool runOnMachineFunction(MachineFunction &MF) override;

private:
  bool handleBBSections(MachineFunction &MF);
  bool handleBBAddrMap(MachineFunction &MF);
  bool shouldSplitWithPGO(MachineFunction &MF);
  void assignColdSectionWithPGO(MachineFunction &MF);

  // Whether functions without a basic block sections profile are split using
  // PGO/AutoFDO profiles (-function-splitting=all).
  const bool SplitWithPGO;
};

} // end anonymous namespace

char BasicBlockSections::ID = 0;
INITIALIZE_PASS_BEGIN(
    BasicBlockSections, "bbsections-prepare",
    "Prepares for basic block sections, by splitting functions "
    "into clusters of basic blocks.",
    false, false)
INITIALIZE_PASS_DEPENDENCY(BasicBlockSectionsProfileReaderWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
INITIALIZE_PASS_END(BasicBlockSections, "bbsections-prepare",
                    "Prepares for basic block sections, by splitting functions "
                    "into clusters of basic blocks.",
                    false, false)

// This function updates and optimizes the branching instructions of every basic
// block in a given function to account for changes in the layout.
static void
updateBranches(MachineFunction &MF,
               const SmallVector<MachineBasicBlock *> &PreLayoutFallThroughs) {
  const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
  SmallVector<MachineOperand, 4> Cond;
  for (auto &MBB : MF) {
    auto NextMBBI = std::next(MBB.getIterator());
    auto *FTMBB = PreLayoutFallThroughs[MBB.getNumber()];
    // If this block had a fallthrough before we need an explicit unconditional
    // branch to that block if either
    //     1- the block ends a section, which means its next block may be
    //        reorderd by the linker, or
    //     2- the fallthrough block is not adjacent to the block in the new
    //        order.
    if (FTMBB && (MBB.isEndSection() || &*NextMBBI != FTMBB))
      TII->insertUnconditionalBranch(MBB, FTMBB, MBB.findBranchDebugLoc());

    // We do not optimize branches for machine basic blocks ending sections, as
    // their adjacent block might be reordered by the linker.
    if (MBB.isEndSection())
      continue;

    // It might be possible to optimize branches by flipping the branch
    // condition.
    Cond.clear();
    MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
    if (TII->analyzeBranch(MBB, TBB, FBB, Cond))
      continue;
    MBB.updateTerminator(FTMBB);
  }
}

// This function generates the machine basic block clusters of "hot" blocks.
// Currently, only support one cluster creation.
// TODO: Support multi-cluster creation.
static SmallVector<BBClusterInfo>
createBBClusterInfoForFunction(MachineFunction &MF,
                               const BasicBlockMatchingAndInference &BMI) {
  SmallVector<BBClusterInfo> BBClusterInfos;
  auto OptWeightInfo = BMI.getWeightInfo(MF.getName());
  if (!OptWeightInfo)
    return BBClusterInfos;
  auto BlockWeights = OptWeightInfo->BlockWeights;
  auto EdgeWeights = OptWeightInfo->EdgeWeights;

  SmallVector<const MachineBasicBlock *, 4> HotMBBs;
  if (MF.size() <= 2) {
    for (auto &MBB : MF) {
      if (MBB.isEntryBlock() || BlockWeights[&MBB] > 0) {
        HotMBBs.push_back(&MBB);
      }
    }
  } else {
    SmallVector<uint64_t, 0> BlockSizes(MF.size());
    SmallVector<uint64_t, 0> BlockCounts(MF.size());
    std::vector<const MachineBasicBlock *> OrigOrder;
    OrigOrder.reserve(MF.size());
    SmallVector<codelayout::EdgeCount, 0> JumpCounts;

    // Renumber blocks for running the layout algorithm.
    MF.RenumberBlocks();

    // Init the MBB size and count.
    for (auto &MBB : MF) {
      auto NonDbgInsts =
          instructionsWithoutDebug(MBB.instr_begin(), MBB.instr_end());
      int NumInsts = std::distance(NonDbgInsts.begin(), NonDbgInsts.end());
      BlockSizes[MBB.getNumber()] = 4 * NumInsts;
      BlockCounts[MBB.getNumber()] = BlockWeights[&MBB];
      OrigOrder.push_back(&MBB);
    }

    // Init the edge count.
    for (auto &MBB : MF) {
      for (auto *Succ : MBB.successors()) {
        auto EdgeWeight = EdgeWeights[std::make_pair(&MBB, Succ)];
        JumpCounts.push_back({static_cast<uint64_t>(MBB.getNumber()),
                              static_cast<uint64_t>(Succ->getNumber()),
                              EdgeWeight});
      }
    }

    // Run the layout algorithm.
    auto Result = computeExtTspLayout(BlockSizes, BlockCounts, JumpCounts);
    for (uint64_t R : Result) {
      auto Block = OrigOrder[R];
      if (Block->isEntryBlock() || BlockWeights[Block] > 0)
        HotMBBs.push_back(Block);
    }
  }

  // Generate the "hot" basic block cluster.
  if (!HotMBBs.empty()) {
    unsigned CurrentPosition = 0;
    for (auto &MBB : HotMBBs)
      BBClusterInfos.push_back({*MBB->getBBID(), 0, CurrentPosition++});
  }
  return BBClusterInfos;
}

// This function sorts basic blocks according to the cluster's information.
// All explicitly specified clusters of basic blocks will be ordered
// accordingly. All non-specified BBs go into a separate "Cold" section.
// Additionally, if exception handling landing pads end up in more than one
// clusters, they are moved into a single "Exception" section. Eventually,
// clusters are ordered in increasing order of their IDs, with the "Exception"
// and "Cold" succeeding all other clusters. With -function-splitting=none, all
// basic blocks stay in the default section instead.
// FuncClusterInfo represents the cluster information for basic blocks. It
// maps from BBID of basic blocks to their cluster information.
static void
assignSections(MachineFunction &MF,
               const DenseMap<UniqueBBID, BBClusterInfo> &FuncClusterInfo) {
  assert(MF.hasBBSections() && "BB Sections is not set for function.");
  // With function splitting disabled, the profile only decides the layout and
  // the function is emitted as a single section.
  if (MF.getTarget().getBBSectionsType() == BasicBlockSection::List &&
      MF.getTarget().Options.FunctionSplitting == FunctionSplittingMode::None)
    return;
  // This variable stores the section ID of the cluster containing eh_pads (if
  // all eh_pads are one cluster). If more than one cluster contain eh_pads, we
  // set it equal to ExceptionSectionID.
  std::optional<MBBSectionID> EHPadsSectionID;

  for (auto &MBB : MF) {
    // With the 'all' option, every basic block is placed in a unique section.
    // With the 'list' option, every basic block is placed in a section
    // associated with its cluster.
    if (MF.getTarget().getBBSectionsType() == llvm::BasicBlockSection::All) {
      // If unique sections are desired for all basic blocks of the function, we
      // set every basic block's section ID equal to its original position in
      // the layout (which is equal to its number). This ensures that basic
      // blocks are ordered canonically.
      MBB.setSectionID(MBB.getNumber());
    } else {
      auto I = FuncClusterInfo.find(*MBB.getBBID());
      if (I != FuncClusterInfo.end()) {
        MBB.setSectionID(I->second.ClusterID);
      } else {
        const TargetInstrInfo &TII =
            *MBB.getParent()->getSubtarget().getInstrInfo();

        if (TII.isMBBSafeToSplitToCold(MBB)) {
          // BB goes into the special cold section if it is not specified in the
          // cluster info map. If it is not safe to split, it is kept in the
          // default section.
          MBB.setSectionID(MBBSectionID::ColdSectionID);
        }
      }
    }

    if (MBB.isEHPad() && EHPadsSectionID != MBB.getSectionID() &&
        EHPadsSectionID != MBBSectionID::ExceptionSectionID) {
      // If we already have one cluster containing eh_pads, this must be updated
      // to ExceptionSectionID. Otherwise, we set it equal to the current
      // section ID.
      EHPadsSectionID = EHPadsSectionID ? MBBSectionID::ExceptionSectionID
                                        : MBB.getSectionID();
    }
  }

  // If EHPads are in more than one section, this places all of them in the
  // special exception section.
  if (EHPadsSectionID == MBBSectionID::ExceptionSectionID)
    for (auto &MBB : MF)
      if (MBB.isEHPad())
        MBB.setSectionID(*EHPadsSectionID);
}

void llvm::sortBasicBlocksAndUpdateBranches(
    MachineFunction &MF, MachineBasicBlockComparator MBBCmp) {
  [[maybe_unused]] const MachineBasicBlock *EntryBlock = &MF.front();
  SmallVector<MachineBasicBlock *> PreLayoutFallThroughs(MF.getNumBlockIDs());
  for (auto &MBB : MF)
    PreLayoutFallThroughs[MBB.getNumber()] =
        MBB.getFallThrough(/*JumpToFallThrough=*/false);

  MF.sort(MBBCmp);
  assert(&MF.front() == EntryBlock &&
         "Entry block should not be displaced by basic block sections");

  // Set IsBeginSection and IsEndSection according to the assigned section IDs.
  MF.assignBeginEndSections();

  // After reordering basic blocks, we must update basic block branches to
  // insert explicit fallthrough branches when required and optimize branches
  // when possible.
  updateBranches(MF, PreLayoutFallThroughs);
}

// If the exception section begins with a landing pad, that landing pad will
// assume a zero offset (relative to @LPStart) in the LSDA. However, a value of
// zero implies "no landing pad." This function inserts a NOP just before the EH
// pad label to ensure a nonzero offset.
void llvm::avoidZeroOffsetLandingPad(MachineFunction &MF) {
  std::optional<MBBSectionID> CurrentSection;
  auto IsFirstNonEmptyBBInSection = [&](const MachineBasicBlock &MBB) {
    if (MBB.getFirstNonDebugInstr() == MBB.end() ||
        MBB.getSectionID() == CurrentSection)
      return false;
    CurrentSection = MBB.getSectionID();
    return true;
  };

  for (auto &MBB : MF) {
    if (IsFirstNonEmptyBBInSection(MBB) && MBB.isEHPad()) {
      MachineBasicBlock::iterator MI = MBB.begin();
      while (!MI->isEHLabel())
        ++MI;
      MF.getSubtarget().getInstrInfo()->insertNoop(MBB, MI);
    }
  }
}

bool llvm::hasInstrProfHashMismatch(MachineFunction &MF) {
  if (!BBSectionsDetectSourceDrift)
    return false;

  const char MetadataName[] = "instr_prof_hash_mismatch";
  auto *Existing = MF.getFunction().getMetadata(LLVMContext::MD_annotation);
  if (Existing) {
    MDTuple *Tuple = cast<MDTuple>(Existing);
    for (const auto &N : Tuple->operands())
      if (N.equalsStr(MetadataName))
        return true;
  }

  return false;
}

/// setDescendantEHBlocksCold - This splits all EH pads and blocks reachable
/// only by EH pad as cold. This will help mark EH pads statically cold
/// instead of relying on profile data.
static void setDescendantEHBlocksCold(MachineFunction &MF) {
  DenseSet<MachineBasicBlock *> EHBlocks;
  computeEHOnlyBlocks(MF, EHBlocks);
  for (auto Block : EHBlocks) {
    Block->setSectionID(MBBSectionID::ColdSectionID);
  }
}

static bool isColdBlock(const MachineBasicBlock &MBB,
                        const MachineBlockFrequencyInfo *MBFI,
                        ProfileSummaryInfo *PSI) {
  std::optional<uint64_t> Count = MBFI->getBlockProfileCount(&MBB);
  // For instrumentation profiles and sample profiles, we use different ways
  // to judge whether a block is cold and should be split.
  if (PSI->hasInstrumentationProfile() || PSI->hasCSInstrumentationProfile()) {
    // If using instrument profile, which is deemed "accurate", no count means
    // cold.
    if (!Count)
      return true;
    if (PercentileCutoff > 0)
      return PSI->isColdCountNthPercentile(PercentileCutoff, *Count);
    // Fallthrough to end of function.
  } else if (PSI->hasSampleProfile()) {
    // For sample profile, no count means "do not judege coldness".
    if (!Count)
      return false;
  }

  return (*Count < ColdCountThreshold);
}

// Returns whether a function which does not have a basic block sections profile
// should be split using its PGO/AutoFDO profile.
bool BasicBlockSections::shouldSplitWithPGO(MachineFunction &MF) {
  if (!SplitWithPGO || skipFunction(MF.getFunction()))
    return false;

  // We target functions with profile data. Static information in the form
  // of exception handling code may be split to cold if user passes the
  // mfs-split-ehcode flag.
  if (!MF.getFunction().hasProfileData() && !SplitAllEHCode)
    return false;

  const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
  if (!TII.isFunctionSafeToSplit(MF))
    return false;

  // Do not split functions which are in the basic block sections profile, even
  // if their profile is not used due to source drift.
  if (MF.getTarget().getBBSectionsType() == BasicBlockSection::List &&
      getAnalysis<BasicBlockSectionsProfileReaderWrapperPass>()
          .getBBSPR()
          .isFunctionHot(MF.getName()))
    return false;

  return true;
}

// Splits a function using its PGO/AutoFDO profile: assigns the cold section to
// cold blocks, and the default section to all the other blocks.
void BasicBlockSections::assignColdSectionWithPGO(MachineFunction &MF) {
  MF.setBBSectionsType(BasicBlockSection::Preset);

  const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
  bool UseProfileData = MF.getFunction().hasProfileData();
  MachineBlockFrequencyInfo *MBFI = nullptr;
  ProfileSummaryInfo *PSI = nullptr;
  if (UseProfileData) {
    MBFI = &getAnalysis<MachineBlockFrequencyInfoWrapperPass>().getMBFI();
    PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
    // If we don't have a good profile (sample profile is not deemed
    // as a "good profile") and the function is not hot, then early
    // return. (Because we can only trust hot functions when profile
    // quality is not good.)
    if (PSI->hasSampleProfile() && !PSI->isFunctionHotInCallGraph(&MF, *MBFI)) {
      // Split all EH code and it's descendant statically by default.
      if (SplitAllEHCode)
        setDescendantEHBlocksCold(MF);
      return;
    }
  }

  SmallVector<MachineBasicBlock *, 2> LandingPads;
  for (auto &MBB : MF) {
    if (MBB.isEntryBlock())
      continue;

    if (MBB.isEHPad())
      LandingPads.push_back(&MBB);
    else if (UseProfileData && isColdBlock(MBB, MBFI, PSI) &&
             TII.isMBBSafeToSplitToCold(MBB) && !SplitAllEHCode)
      MBB.setSectionID(MBBSectionID::ColdSectionID);
  }

  // Split all EH code and it's descendant statically by default.
  if (SplitAllEHCode)
    setDescendantEHBlocksCold(MF);
  // We only split out eh pads if all of them are cold.
  else {
    // Here we have UseProfileData == true.
    bool HasHotLandingPads = false;
    for (const MachineBasicBlock *LP : LandingPads) {
      if (!isColdBlock(*LP, MBFI, PSI) || !TII.isMBBSafeToSplitToCold(*LP))
        HasHotLandingPads = true;
    }
    if (!HasHotLandingPads) {
      for (MachineBasicBlock *LP : LandingPads)
        LP->setSectionID(MBBSectionID::ColdSectionID);
    }
  }
}

// Identify, arrange, and modify basic blocks which need separate sections
// according to the specification provided by the -fbasic-block-sections flag,
// or according to the PGO/AutoFDO profile with -function-splitting=all.
bool BasicBlockSections::handleBBSections(MachineFunction &MF) {
  auto BBSectionsType = MF.getTarget().getBBSectionsType();

  DenseMap<UniqueBBID, BBClusterInfo> FuncClusterInfo;
  // Check for source drift. If the source has changed since the profiles
  // were obtained, optimizing basic blocks might be sub-optimal.
  // This only applies to BasicBlockSection::List as it creates
  // clusters of basic blocks using basic block ids. Source drift can
  // invalidate these groupings leading to sub-optimal code generation with
  // regards to performance.
  if (BBSectionsType == BasicBlockSection::List &&
      !hasInstrProfHashMismatch(MF)) {
    SmallVector<BBClusterInfo> ClusterInfo;
    if (auto *BMI = getAnalysisIfAvailable<BasicBlockMatchingAndInference>()) {
      ClusterInfo = createBBClusterInfoForFunction(MF, *BMI);
    } else {
      ClusterInfo = getAnalysis<BasicBlockSectionsProfileReaderWrapperPass>()
                        .getClusterInfoForFunction(MF.getName());
    }
    for (auto &BBClusterInfo : ClusterInfo) {
      FuncClusterInfo.try_emplace(BBClusterInfo.BBID, BBClusterInfo);
    }
  }

  // A function without a basic block sections profile is split using its
  // PGO/AutoFDO profile, if enabled.
  const bool UsePGO =
      BBSectionsType == BasicBlockSection::None ||
      (BBSectionsType == BasicBlockSection::List && FuncClusterInfo.empty());
  if (UsePGO && !shouldSplitWithPGO(MF))
    return false;

  // Renumber blocks before assigning sections and sorting them. This is useful
  // for accessing the original layout positions and finding the original
  // fallthroughs. Blocks which are not in any cluster keep their original
  // order, which retains decisions made by prior passes such as
  // MachineBlockPlacement.
  MF.RenumberBlocks();

  if (UsePGO) {
    assignColdSectionWithPGO(MF);
  } else {
    MF.setBBSectionsType(BBSectionsType);
    assignSections(MF, FuncClusterInfo);
  }

  const MachineBasicBlock &EntryBB = MF.front();
  auto EntryBBSectionID = EntryBB.getSectionID();

  // Helper function for ordering BB sections as follows:
  //   * Entry section (section including the entry block).
  //   * Regular sections (in increasing order of their Number).
  //     ...
  //   * Exception section
  //   * Cold section
  auto MBBSectionOrder = [EntryBBSectionID](const MBBSectionID &LHS,
                                            const MBBSectionID &RHS) {
    // We make sure that the section containing the entry block precedes all the
    // other sections.
    if (LHS == EntryBBSectionID || RHS == EntryBBSectionID)
      return LHS == EntryBBSectionID;
    return LHS.Type == RHS.Type ? LHS.Number < RHS.Number : LHS.Type < RHS.Type;
  };

  // We sort all basic blocks to make sure the basic blocks of every cluster are
  // contiguous and ordered accordingly. Furthermore, clusters are ordered in
  // increasing order of their section IDs, with the exception and the
  // cold section placed at the end of the function.
  // Also, we force the entry block of the function to be placed at the
  // beginning of the function, regardless of the requested order.
  auto Comparator = [&](const MachineBasicBlock &X,
                        const MachineBasicBlock &Y) {
    auto XSectionID = X.getSectionID();
    auto YSectionID = Y.getSectionID();
    if (XSectionID != YSectionID)
      return MBBSectionOrder(XSectionID, YSectionID);
    // Make sure that the entry block is placed at the beginning.
    if (&X == &EntryBB || &Y == &EntryBB)
      return &X == &EntryBB;
    // If the two basic blocks are in the same section, the order is decided by
    // their cluster and their position within the cluster. A section holds more
    // than one cluster only with -function-splitting=none. Basic blocks which
    // are not in any cluster come after all the profiled basic blocks of the
    // section, in their original order. Without a basic block sections profile,
    // all basic blocks keep their original order (they might not have BBIDs).
    if (XSectionID.Type == MBBSectionID::SectionType::Default &&
        !FuncClusterInfo.empty()) {
      auto XI = FuncClusterInfo.find(*X.getBBID());
      auto YI = FuncClusterInfo.find(*Y.getBBID());
      bool XInCluster = XI != FuncClusterInfo.end();
      bool YInCluster = YI != FuncClusterInfo.end();
      if (XInCluster != YInCluster)
        return XInCluster;
      if (XInCluster)
        return std::tie(XI->second.ClusterID, XI->second.PositionInCluster) <
               std::tie(YI->second.ClusterID, YI->second.PositionInCluster);
    }
    return X.getNumber() < Y.getNumber();
  };

  sortBasicBlocksAndUpdateBranches(MF, Comparator);
  avoidZeroOffsetLandingPad(MF);
  return true;
}

// When the BB address map needs to be generated, this renumbers basic blocks to
// make them appear in increasing order of their IDs in the function. This
// avoids the need to store basic block IDs in the BB address map section, since
// they can be determined implicitly.
bool BasicBlockSections::handleBBAddrMap(MachineFunction &MF) {
  if (!MF.getTarget().Options.BBAddrMap)
    return false;
  MF.RenumberBlocks();
  return true;
}

bool BasicBlockSections::runOnMachineFunction(MachineFunction &MF) {
  // First handle the basic block sections.
  auto R1 = handleBBSections(MF);
  // Handle basic block address map after basic block sections are finalized.
  auto R2 = handleBBAddrMap(MF);
  return R1 || R2;
}

void BasicBlockSections::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesAll();
  AU.addRequired<BasicBlockSectionsProfileReaderWrapperPass>();
  if (SplitWithPGO) {
    AU.addRequired<MachineBlockFrequencyInfoWrapperPass>();
    AU.addRequired<ProfileSummaryInfoWrapperPass>();
  }
  AU.addUsedIfAvailable<BasicBlockMatchingAndInference>();
  AU.addUsedIfAvailable<MachineDominatorTreeWrapperPass>();
  AU.addUsedIfAvailable<MachinePostDominatorTreeWrapperPass>();
  MachineFunctionPass::getAnalysisUsage(AU);
}

MachineFunctionPass *llvm::createBasicBlockSectionsPass(bool SplitWithPGO) {
  return new BasicBlockSections(SplitWithPGO);
}