BasicBlockSections.cpp

Go to the documentation of this file.

//===-- 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.
//
// 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 Labels
// ==================
//
// With -fbasic-block-sections=labels, 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``.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CodeGen/BasicBlockSectionUtils.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/Error.h"
#include "llvm/Support/LineIterator.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Target/TargetMachine.h"
 
using llvm::SmallSet;
using llvm::SmallVector;
using llvm::StringMap;
using llvm::StringRef;
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);
 
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);
 
namespace {
 
// This struct represents the cluster information for a machine basic block.
struct BBClusterInfo {
  // MachineBasicBlock ID.
  unsigned MBBNumber;
  // Cluster ID this basic block belongs to.
  unsigned ClusterID;
  // Position of basic block within the cluster.
  unsigned PositionInCluster;
};
 
using ProgramBBClusterInfoMapTy = StringMap<SmallVector<BBClusterInfo, 4>>;
 
class BasicBlockSections : public MachineFunctionPass {
public:
  static char ID;
 
  // This contains the basic-block-sections profile.
  const MemoryBuffer *MBuf = nullptr;
 
  // This encapsulates the BB cluster information for the whole program.
  //
  // For every function name, it contains the cluster information for (all or
  // some of) its basic blocks. The cluster information for every basic block
  // includes its cluster ID along with the position of the basic block in that
  // cluster.
  ProgramBBClusterInfoMapTy ProgramBBClusterInfo;
 
  // Some functions have alias names. We use this map to find the main alias
  // name for which we have mapping in ProgramBBClusterInfo.
  StringMap<StringRef> FuncAliasMap;
 
  BasicBlockSections(const MemoryBuffer *Buf)
      : MachineFunctionPass(ID), MBuf(Buf) {
    initializeBasicBlockSectionsPass(*PassRegistry::getPassRegistry());
  };
 
  BasicBlockSections() : MachineFunctionPass(ID) {
    initializeBasicBlockSectionsPass(*PassRegistry::getPassRegistry());
  }
 
  StringRef getPassName() const override {
    return "Basic Block Sections Analysis";
  }
 
  void getAnalysisUsage(AnalysisUsage &AU) const override;
 
  /// Read profiles of basic blocks if available here.
  bool doInitialization(Module &M) override;
 
  /// Identify basic blocks that need separate sections and prepare to emit them
  /// accordingly.
  bool runOnMachineFunction(MachineFunction &MF) override;
};
 
} // end anonymous namespace
 
char BasicBlockSections::ID = 0;
INITIALIZE_PASS(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 *, 4> &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 provides the BBCluster information associated with a function.
// Returns true if a valid association exists and false otherwise.
static bool getBBClusterInfoForFunction(
    const MachineFunction &MF, const StringMap<StringRef> FuncAliasMap,
    const ProgramBBClusterInfoMapTy &ProgramBBClusterInfo,
    std::vector<Optional<BBClusterInfo>> &V) {
  // Get the main alias name for the function.
  auto FuncName = MF.getName();
  auto R = FuncAliasMap.find(FuncName);
  StringRef AliasName = R == FuncAliasMap.end() ? FuncName : R->second;
 
  // Find the assoicated cluster information.
  auto P = ProgramBBClusterInfo.find(AliasName);
  if (P == ProgramBBClusterInfo.end())
    return false;
 
  if (P->second.empty()) {
    // This indicates that sections are desired for all basic blocks of this
    // function. We clear the BBClusterInfo vector to denote this.
    V.clear();
    return true;
  }
 
  V.resize(MF.getNumBlockIDs());
  for (auto bbClusterInfo : P->second) {
    // Bail out if the cluster information contains invalid MBB numbers.
    if (bbClusterInfo.MBBNumber >= MF.getNumBlockIDs())
      return false;
    V[bbClusterInfo.MBBNumber] = bbClusterInfo;
  }
  return true;
}
 
// 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.
// FuncBBClusterInfo represent the cluster information for basic blocks. If this
// is empty, it means unique sections for all basic blocks in the function.
static void
assignSections(MachineFunction &MF,
               const std::vector<Optional<BBClusterInfo>> &FuncBBClusterInfo) {
  assert(MF.hasBBSections() && "BB Sections is not set for function.");
  // 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.
  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, unless we want individual unique sections
    // for every basic block in this function (if FuncBBClusterInfo is empty).
    if (MF.getTarget().getBBSectionsType() == llvm::BasicBlockSection::All ||
        FuncBBClusterInfo.empty()) {
      // If unique sections are desired for all basic blocks of the function, we
      // set every basic block's section ID equal to its number (basic block
      // id). This further ensures that basic blocks are ordered canonically.
      MBB.setSectionID({static_cast<unsigned int>(MBB.getNumber())});
    } else if (FuncBBClusterInfo[MBB.getNumber()].hasValue())
      MBB.setSectionID(FuncBBClusterInfo[MBB.getNumber()]->ClusterID);
    else {
      // BB goes into the special cold section if it is not specified in the
      // cluster info map.
      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.hasValue()
                            ? 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.getValue());
}
 
void llvm::sortBasicBlocksAndUpdateBranches(
    MachineFunction &MF, MachineBasicBlockComparator MBBCmp) {
  SmallVector<MachineBasicBlock *, 4> PreLayoutFallThroughs(
      MF.getNumBlockIDs());
  for (auto &MBB : MF)
    PreLayoutFallThroughs[MBB.getNumber()] = MBB.getFallThrough();
 
  MF.sort(MBBCmp);
 
  // 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. Returns true if padding is not needed.
static bool avoidZeroOffsetLandingPad(MachineFunction &MF) {
  for (auto &MBB : MF) {
    if (MBB.isBeginSection() && MBB.isEHPad()) {
      MachineBasicBlock::iterator MI = MBB.begin();
      while (!MI->isEHLabel())
        ++MI;
      MCInst Nop = MF.getSubtarget().getInstrInfo()->getNop();
      BuildMI(MBB, MI, DebugLoc(),
              MF.getSubtarget().getInstrInfo()->get(Nop.getOpcode()));
      return false;
    }
  }
  return true;
}
 
// This checks if the source of this function has drifted since this binary was
// profiled previously.  For now, we are piggy backing on what PGO does to
// detect this with instrumented profiles.  PGO emits an hash of the IR and
// checks if the hash has changed.  Advanced basic block layout is usually done
// on top of PGO optimized binaries and hence this check works well in practice.
static bool 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 (auto &N : Tuple->operands())
      if (cast<MDString>(N.get())->getString() == MetadataName)
        return true;
  }
 
  return false;
}
 
bool BasicBlockSections::runOnMachineFunction(MachineFunction &MF) {
  auto BBSectionsType = MF.getTarget().getBBSectionsType();
  assert(BBSectionsType != BasicBlockSection::None &&
         "BB Sections not enabled!");
 
  // 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))
    return true;
 
  // Renumber blocks before sorting them for basic block sections.  This is
  // useful during sorting, basic blocks in the same section will retain the
  // default order.  This renumbering should also be done for basic block
  // labels to match the profiles with the correct blocks.
  MF.RenumberBlocks();
 
  if (BBSectionsType == BasicBlockSection::Labels) {
    MF.setBBSectionsType(BBSectionsType);
    return true;
  }
 
  std::vector<Optional<BBClusterInfo>> FuncBBClusterInfo;
  if (BBSectionsType == BasicBlockSection::List &&
      !getBBClusterInfoForFunction(MF, FuncAliasMap, ProgramBBClusterInfo,
                                   FuncBBClusterInfo))
    return true;
  MF.setBBSectionsType(BBSectionsType);
  assignSections(MF, FuncBBClusterInfo);
 
  // We make sure that the cluster including the entry basic block precedes all
  // other clusters.
  auto EntryBBSectionID = MF.front().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.
  auto Comparator = [&](const MachineBasicBlock &X,
                        const MachineBasicBlock &Y) {
    auto XSectionID = X.getSectionID();
    auto YSectionID = Y.getSectionID();
    if (XSectionID != YSectionID)
      return MBBSectionOrder(XSectionID, YSectionID);
    // If the two basic block are in the same section, the order is decided by
    // their position within the section.
    if (XSectionID.Type == MBBSectionID::SectionType::Default)
      return FuncBBClusterInfo[X.getNumber()]->PositionInCluster <
             FuncBBClusterInfo[Y.getNumber()]->PositionInCluster;
    return X.getNumber() < Y.getNumber();
  };
 
  sortBasicBlocksAndUpdateBranches(MF, Comparator);
  avoidZeroOffsetLandingPad(MF);
  return true;
}
 
// Basic Block Sections can be enabled for a subset of machine basic blocks.
// This is done by passing a file containing names of functions for which basic
// block sections are desired.  Additionally, machine basic block ids of the
// functions can also be specified for a finer granularity. Moreover, a cluster
// of basic blocks could be assigned to the same section.
// A file with basic block sections for all of function main and three blocks
// for function foo (of which 1 and 2 are placed in a cluster) looks like this:
// ----------------------------
// list.txt:
// !main
// !foo
// !!1 2
// !!4
static Error getBBClusterInfo(const MemoryBuffer *MBuf,
                              ProgramBBClusterInfoMapTy &ProgramBBClusterInfo,
                              StringMap<StringRef> &FuncAliasMap) {
  assert(MBuf);
  line_iterator LineIt(*MBuf, /*SkipBlanks=*/true, /*CommentMarker=*/'#');
 
  auto invalidProfileError = [&](auto Message) {
    return make_error<StringError>(
        Twine("Invalid profile " + MBuf->getBufferIdentifier() + " at line " +
              Twine(LineIt.line_number()) + ": " + Message),
        inconvertibleErrorCode());
  };
 
  auto FI = ProgramBBClusterInfo.end();
 
  // Current cluster ID corresponding to this function.
  unsigned CurrentCluster = 0;
  // Current position in the current cluster.
  unsigned CurrentPosition = 0;
 
  // Temporary set to ensure every basic block ID appears once in the clusters
  // of a function.
  SmallSet<unsigned, 4> FuncBBIDs;
 
  for (; !LineIt.is_at_eof(); ++LineIt) {
    StringRef S(*LineIt);
    if (S[0] == '@')
      continue;
    // Check for the leading "!"
    if (!S.consume_front("!") || S.empty())
      break;
    // Check for second "!" which indicates a cluster of basic blocks.
    if (S.consume_front("!")) {
      if (FI == ProgramBBClusterInfo.end())
        return invalidProfileError(
            "Cluster list does not follow a function name specifier.");
      SmallVector<StringRef, 4> BBIndexes;
      S.split(BBIndexes, ' ');
      // Reset current cluster position.
      CurrentPosition = 0;
      for (auto BBIndexStr : BBIndexes) {
        unsigned long long BBIndex;
        if (getAsUnsignedInteger(BBIndexStr, 10, BBIndex))
          return invalidProfileError(Twine("Unsigned integer expected: '") +
                                     BBIndexStr + "'.");
        if (!FuncBBIDs.insert(BBIndex).second)
          return invalidProfileError(Twine("Duplicate basic block id found '") +
                                     BBIndexStr + "'.");
        if (!BBIndex && CurrentPosition)
          return invalidProfileError("Entry BB (0) does not begin a cluster.");
 
        FI->second.emplace_back(BBClusterInfo{
            ((unsigned)BBIndex), CurrentCluster, CurrentPosition++});
      }
      CurrentCluster++;
    } else { // This is a function name specifier.
      // Function aliases are separated using '/'. We use the first function
      // name for the cluster info mapping and delegate all other aliases to
      // this one.
      SmallVector<StringRef, 4> Aliases;
      S.split(Aliases, '/');
      for (size_t i = 1; i < Aliases.size(); ++i)
        FuncAliasMap.try_emplace(Aliases[i], Aliases.front());
 
      // Prepare for parsing clusters of this function name.
      // Start a new cluster map for this function name.
      FI = ProgramBBClusterInfo.try_emplace(Aliases.front()).first;
      CurrentCluster = 0;
      FuncBBIDs.clear();
    }
  }
  return Error::success();
}
 
bool BasicBlockSections::doInitialization(Module &M) {
  if (!MBuf)
    return false;
  if (auto Err = getBBClusterInfo(MBuf, ProgramBBClusterInfo, FuncAliasMap))
    report_fatal_error(std::move(Err));
  return false;
}
 
void BasicBlockSections::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesAll();
  MachineFunctionPass::getAnalysisUsage(AU);
}
 
MachineFunctionPass *
llvm::createBasicBlockSectionsPass(const MemoryBuffer *Buf) {
  return new BasicBlockSections(Buf);
}