xref: /llvm-project-15.0.7/llvm/lib/Transforms/IPO/OpenMPOpt.cpp (revision 4bee2afc)

//===-- IPO/OpenMPOpt.cpp - Collection of OpenMP specific optimizations ---===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// OpenMP specific optimizations:
//
// - Deduplication of runtime calls, e.g., omp_get_thread_num.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/IPO/OpenMPOpt.h"

#include "llvm/ADT/EnumeratedArray.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/CallGraphSCCPass.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Frontend/OpenMP/OMPConstants.h"
#include "llvm/Frontend/OpenMP/OMPIRBuilder.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/CallGraphUpdater.h"

using namespace llvm;
using namespace omp;
using namespace types;

#define DEBUG_TYPE "openmp-opt"

static cl::opt<bool> DisableOpenMPOptimizations(
    "openmp-opt-disable", cl::ZeroOrMore,
    cl::desc("Disable OpenMP specific optimizations."), cl::Hidden,
    cl::init(false));

STATISTIC(NumOpenMPRuntimeCallsDeduplicated,
          "Number of OpenMP runtime calls deduplicated");
STATISTIC(NumOpenMPRuntimeFunctionsIdentified,
          "Number of OpenMP runtime functions identified");
STATISTIC(NumOpenMPRuntimeFunctionUsesIdentified,
          "Number of OpenMP runtime function uses identified");

#if !defined(NDEBUG)
static constexpr auto TAG = "[" DEBUG_TYPE "]";
#endif

namespace {
struct OpenMPOpt {

  using OptimizationRemarkGetter =
      function_ref<OptimizationRemarkEmitter &(Function *)>;

  OpenMPOpt(SmallVectorImpl<Function *> &SCC,
            SmallPtrSetImpl<Function *> &ModuleSlice,
            CallGraphUpdater &CGUpdater, OptimizationRemarkGetter OREGetter)
      : M(*(*SCC.begin())->getParent()), SCC(SCC), ModuleSlice(ModuleSlice),
        OMPBuilder(M), CGUpdater(CGUpdater), OREGetter(OREGetter) {
    initializeTypes(M);
    initializeRuntimeFunctions();
    OMPBuilder.initialize();
  }

  /// Generic information that describes a runtime function
  struct RuntimeFunctionInfo {

    /// The kind, as described by the RuntimeFunction enum.
    RuntimeFunction Kind;

    /// The name of the function.
    StringRef Name;

    /// Flag to indicate a variadic function.
    bool IsVarArg;

    /// The return type of the function.
    Type *ReturnType;

    /// The argument types of the function.
    SmallVector<Type *, 8> ArgumentTypes;

    /// The declaration if available.
    Function *Declaration = nullptr;

    /// Uses of this runtime function per function containing the use.
    using UseVector = SmallVector<Use *, 16>;

    /// Return the vector of uses in function \p F.
    UseVector &getOrCreateUseVector(Function *F) {
      std::unique_ptr<UseVector> &UV = UsesMap[F];
      if (!UV)
        UV = std::make_unique<UseVector>();
      return *UV;
    }

    /// Return the vector of uses in function \p F or `nullptr` if there are
    /// none.
    const UseVector *getUseVector(Function &F) const {
      auto I = UsesMap.find(&F);
      if (I != UsesMap.end())
        return I->second.get();
      return nullptr;
    }

    /// Return how many functions contain uses of this runtime function.
    size_t getNumFunctionsWithUses() const { return UsesMap.size(); }

    /// Return the number of arguments (or the minimal number for variadic
    /// functions).
    size_t getNumArgs() const { return ArgumentTypes.size(); }

    /// Run the callback \p CB on each use and forget the use if the result is
    /// true. The callback will be fed the function in which the use was
    /// encountered as second argument.
    void foreachUse(function_ref<bool(Use &, Function &)> CB) {
      SmallVector<unsigned, 8> ToBeDeleted;
      for (auto &It : UsesMap) {
        ToBeDeleted.clear();
        unsigned Idx = 0;
        UseVector &UV = *It.second;
        for (Use *U : UV) {
          if (CB(*U, *It.first))
            ToBeDeleted.push_back(Idx);
          ++Idx;
        }

        // Remove the to-be-deleted indices in reverse order as prior
        // modifcations will not modify the smaller indices.
        while (!ToBeDeleted.empty()) {
          unsigned Idx = ToBeDeleted.pop_back_val();
          UV[Idx] = UV.back();
          UV.pop_back();
        }
      }
    }

  private:
    /// Map from functions to all uses of this runtime function contained in
    /// them.
    DenseMap<Function *, std::unique_ptr<UseVector>> UsesMap;
  };

  /// Run all OpenMP optimizations on the underlying SCC/ModuleSlice.
  bool run() {
    bool Changed = false;

    LLVM_DEBUG(dbgs() << TAG << "Run on SCC with " << SCC.size()
                      << " functions in a slice with " << ModuleSlice.size()
                      << " functions\n");

    Changed |= deduplicateRuntimeCalls();
    Changed |= deleteParallelRegions();

    return Changed;
  }

private:
  /// Try to delete parallel regions if possible.
  bool deleteParallelRegions() {
    const unsigned CallbackCalleeOperand = 2;

    RuntimeFunctionInfo &RFI = RFIs[OMPRTL___kmpc_fork_call];
    if (!RFI.Declaration)
      return false;

    bool Changed = false;
    auto DeleteCallCB = [&](Use &U, Function &) {
      CallInst *CI = getCallIfRegularCall(U);
      if (!CI)
        return false;
      auto *Fn = dyn_cast<Function>(
          CI->getArgOperand(CallbackCalleeOperand)->stripPointerCasts());
      if (!Fn)
        return false;
      if (!Fn->onlyReadsMemory())
        return false;
      if (!Fn->hasFnAttribute(Attribute::WillReturn))
        return false;

      LLVM_DEBUG(dbgs() << TAG << "Delete read-only parallel region in "
                        << CI->getCaller()->getName() << "\n");

      auto Remark = [&](OptimizationRemark OR) {
        return OR << "Parallel region in "
                  << ore::NV("OpenMPParallelDelete", CI->getCaller()->getName())
                  << " deleted";
      };
      emitRemark<OptimizationRemark>(CI, "OpenMPParallelRegionDeletion",
                                     Remark);

      CGUpdater.removeCallSite(*CI);
      CI->eraseFromParent();
      Changed = true;
      return true;
    };

    RFI.foreachUse(DeleteCallCB);

    return Changed;
  }

  /// Try to eliminiate runtime calls by reusing existing ones.
  bool deduplicateRuntimeCalls() {
    bool Changed = false;

    RuntimeFunction DeduplicableRuntimeCallIDs[] = {
        OMPRTL_omp_get_num_threads,
        OMPRTL_omp_in_parallel,
        OMPRTL_omp_get_cancellation,
        OMPRTL_omp_get_thread_limit,
        OMPRTL_omp_get_supported_active_levels,
        OMPRTL_omp_get_level,
        OMPRTL_omp_get_ancestor_thread_num,
        OMPRTL_omp_get_team_size,
        OMPRTL_omp_get_active_level,
        OMPRTL_omp_in_final,
        OMPRTL_omp_get_proc_bind,
        OMPRTL_omp_get_num_places,
        OMPRTL_omp_get_num_procs,
        OMPRTL_omp_get_place_num,
        OMPRTL_omp_get_partition_num_places,
        OMPRTL_omp_get_partition_place_nums};

    // Global-tid is handled separatly.
    SmallSetVector<Value *, 16> GTIdArgs;
    collectGlobalThreadIdArguments(GTIdArgs);
    LLVM_DEBUG(dbgs() << TAG << "Found " << GTIdArgs.size()
                      << " global thread ID arguments\n");

    for (Function *F : SCC) {
      for (auto DeduplicableRuntimeCallID : DeduplicableRuntimeCallIDs)
        deduplicateRuntimeCalls(*F, RFIs[DeduplicableRuntimeCallID]);

      // __kmpc_global_thread_num is special as we can replace it with an
      // argument in enough cases to make it worth trying.
      Value *GTIdArg = nullptr;
      for (Argument &Arg : F->args())
        if (GTIdArgs.count(&Arg)) {
          GTIdArg = &Arg;
          break;
        }
      Changed |= deduplicateRuntimeCalls(
          *F, RFIs[OMPRTL___kmpc_global_thread_num], GTIdArg);
    }

    return Changed;
  }

  static Value *combinedIdentStruct(Value *CurrentIdent, Value *NextIdent,
                                    bool GlobalOnly, bool &SingleChoice) {
    if (CurrentIdent == NextIdent)
      return CurrentIdent;

    // TODO: Figure out how to actually combine multiple debug locations. For
    //       now we just keep an existing one if there is a single choice.
    if (!GlobalOnly || isa<GlobalValue>(NextIdent)) {
      SingleChoice = !CurrentIdent;
      return NextIdent;
    }
    return nullptr;
  }

  /// Return an `struct ident_t*` value that represents the ones used in the
  /// calls of \p RFI inside of \p F. If \p GlobalOnly is true, we will not
  /// return a local `struct ident_t*`. For now, if we cannot find a suitable
  /// return value we create one from scratch. We also do not yet combine
  /// information, e.g., the source locations, see combinedIdentStruct.
  Value *getCombinedIdentFromCallUsesIn(RuntimeFunctionInfo &RFI, Function &F,
                                        bool GlobalOnly) {
    bool SingleChoice = true;
    Value *Ident = nullptr;
    auto CombineIdentStruct = [&](Use &U, Function &Caller) {
      CallInst *CI = getCallIfRegularCall(U, &RFI);
      if (!CI || &F != &Caller)
        return false;
      Ident = combinedIdentStruct(Ident, CI->getArgOperand(0),
                                  /* GlobalOnly */ true, SingleChoice);
      return false;
    };
    RFI.foreachUse(CombineIdentStruct);

    if (!Ident || !SingleChoice) {
      // The IRBuilder uses the insertion block to get to the module, this is
      // unfortunate but we work around it for now.
      if (!OMPBuilder.getInsertionPoint().getBlock())
        OMPBuilder.updateToLocation(OpenMPIRBuilder::InsertPointTy(
            &F.getEntryBlock(), F.getEntryBlock().begin()));
      // Create a fallback location if non was found.
      // TODO: Use the debug locations of the calls instead.
      Constant *Loc = OMPBuilder.getOrCreateDefaultSrcLocStr();
      Ident = OMPBuilder.getOrCreateIdent(Loc);
    }
    return Ident;
  }

  /// Try to eliminiate calls of \p RFI in \p F by reusing an existing one or
  /// \p ReplVal if given.
  bool deduplicateRuntimeCalls(Function &F, RuntimeFunctionInfo &RFI,
                               Value *ReplVal = nullptr) {
    auto *UV = RFI.getUseVector(F);
    if (!UV || UV->size() + (ReplVal != nullptr) < 2)
      return false;

    LLVM_DEBUG(dbgs() << TAG << "Deduplicate " << UV->size() << " uses of "
                      << RFI.Name
                      << (ReplVal ? " with an existing value\n" : "\n")
                      << "\n");
    assert((!ReplVal || (isa<Argument>(ReplVal) &&
                         cast<Argument>(ReplVal)->getParent() == &F)) &&
           "Unexpected replacement value!");

    // TODO: Use dominance to find a good position instead.
    auto CanBeMoved = [](CallBase &CB) {
      unsigned NumArgs = CB.getNumArgOperands();
      if (NumArgs == 0)
        return true;
      if (CB.getArgOperand(0)->getType() != IdentPtr)
        return false;
      for (unsigned u = 1; u < NumArgs; ++u)
        if (isa<Instruction>(CB.getArgOperand(u)))
          return false;
      return true;
    };

    if (!ReplVal) {
      for (Use *U : *UV)
        if (CallInst *CI = getCallIfRegularCall(*U, &RFI)) {
          if (!CanBeMoved(*CI))
            continue;

          auto Remark = [&](OptimizationRemark OR) {
            auto newLoc = &*F.getEntryBlock().getFirstInsertionPt();
            return OR << "OpenMP runtime call "
                      << ore::NV("OpenMPOptRuntime", RFI.Name) << " moved to "
                      << ore::NV("OpenMPRuntimeMoves", newLoc->getDebugLoc());
          };
          emitRemark<OptimizationRemark>(CI, "OpenMPRuntimeCodeMotion", Remark);

          CI->moveBefore(&*F.getEntryBlock().getFirstInsertionPt());
          ReplVal = CI;
          break;
        }
      if (!ReplVal)
        return false;
    }

    // If we use a call as a replacement value we need to make sure the ident is
    // valid at the new location. For now we just pick a global one, either
    // existing and used by one of the calls, or created from scratch.
    if (CallBase *CI = dyn_cast<CallBase>(ReplVal)) {
      if (CI->getNumArgOperands() > 0 &&
          CI->getArgOperand(0)->getType() == IdentPtr) {
        Value *Ident = getCombinedIdentFromCallUsesIn(RFI, F,
                                                      /* GlobalOnly */ true);
        CI->setArgOperand(0, Ident);
      }
    }

    bool Changed = false;
    auto ReplaceAndDeleteCB = [&](Use &U, Function &Caller) {
      CallInst *CI = getCallIfRegularCall(U, &RFI);
      if (!CI || CI == ReplVal || &F != &Caller)
        return false;
      assert(CI->getCaller() == &F && "Unexpected call!");

      auto Remark = [&](OptimizationRemark OR) {
        return OR << "OpenMP runtime call "
                  << ore::NV("OpenMPOptRuntime", RFI.Name) << " deduplicated";
      };
      emitRemark<OptimizationRemark>(CI, "OpenMPRuntimeDeduplicated", Remark);

      CGUpdater.removeCallSite(*CI);
      CI->replaceAllUsesWith(ReplVal);
      CI->eraseFromParent();
      ++NumOpenMPRuntimeCallsDeduplicated;
      Changed = true;
      return true;
    };
    RFI.foreachUse(ReplaceAndDeleteCB);

    return Changed;
  }

  /// Collect arguments that represent the global thread id in \p GTIdArgs.
  void collectGlobalThreadIdArguments(SmallSetVector<Value *, 16> &GTIdArgs) {
    // TODO: Below we basically perform a fixpoint iteration with a pessimistic
    //       initialization. We could define an AbstractAttribute instead and
    //       run the Attributor here once it can be run as an SCC pass.

    // Helper to check the argument \p ArgNo at all call sites of \p F for
    // a GTId.
    auto CallArgOpIsGTId = [&](Function &F, unsigned ArgNo, CallInst &RefCI) {
      if (!F.hasLocalLinkage())
        return false;
      for (Use &U : F.uses()) {
        if (CallInst *CI = getCallIfRegularCall(U)) {
          Value *ArgOp = CI->getArgOperand(ArgNo);
          if (CI == &RefCI || GTIdArgs.count(ArgOp) ||
              getCallIfRegularCall(*ArgOp,
                                   &RFIs[OMPRTL___kmpc_global_thread_num]))
            continue;
        }
        return false;
      }
      return true;
    };

    // Helper to identify uses of a GTId as GTId arguments.
    auto AddUserArgs = [&](Value &GTId) {
      for (Use &U : GTId.uses())
        if (CallInst *CI = dyn_cast<CallInst>(U.getUser()))
          if (CI->isArgOperand(&U))
            if (Function *Callee = CI->getCalledFunction())
              if (CallArgOpIsGTId(*Callee, U.getOperandNo(), *CI))
                GTIdArgs.insert(Callee->getArg(U.getOperandNo()));
    };

    // The argument users of __kmpc_global_thread_num calls are GTIds.
    RuntimeFunctionInfo &GlobThreadNumRFI =
        RFIs[OMPRTL___kmpc_global_thread_num];
    GlobThreadNumRFI.foreachUse([&](Use &U, Function &F) {
      if (CallInst *CI = getCallIfRegularCall(U, &GlobThreadNumRFI))
        AddUserArgs(*CI);
      return false;
    });

    // Transitively search for more arguments by looking at the users of the
    // ones we know already. During the search the GTIdArgs vector is extended
    // so we cannot cache the size nor can we use a range based for.
    for (unsigned u = 0; u < GTIdArgs.size(); ++u)
      AddUserArgs(*GTIdArgs[u]);
  }

  /// Return the call if \p U is a callee use in a regular call. If \p RFI is
  /// given it has to be the callee or a nullptr is returned.
  CallInst *getCallIfRegularCall(Use &U, RuntimeFunctionInfo *RFI = nullptr) {
    CallInst *CI = dyn_cast<CallInst>(U.getUser());
    if (CI && CI->isCallee(&U) && !CI->hasOperandBundles() &&
        (!RFI || CI->getCalledFunction() == RFI->Declaration))
      return CI;
    return nullptr;
  }

  /// Return the call if \p V is a regular call. If \p RFI is given it has to be
  /// the callee or a nullptr is returned.
  CallInst *getCallIfRegularCall(Value &V, RuntimeFunctionInfo *RFI = nullptr) {
    CallInst *CI = dyn_cast<CallInst>(&V);
    if (CI && !CI->hasOperandBundles() &&
        (!RFI || CI->getCalledFunction() == RFI->Declaration))
      return CI;
    return nullptr;
  }

  /// Returns true if the function declaration \p F matches the runtime
  /// function types, that is, return type \p RTFRetType, and argument types
  /// \p RTFArgTypes.
  static bool declMatchesRTFTypes(Function *F, Type *RTFRetType,
                                  SmallVector<Type *, 8> &RTFArgTypes) {
    // TODO: We should output information to the user (under debug output
    //       and via remarks).

    if (!F)
      return false;
    if (F->getReturnType() != RTFRetType)
      return false;
    if (F->arg_size() != RTFArgTypes.size())
      return false;

    auto RTFTyIt = RTFArgTypes.begin();
    for (Argument &Arg : F->args()) {
      if (Arg.getType() != *RTFTyIt)
        return false;

      ++RTFTyIt;
    }

    return true;
  }

  /// Helper to initialize all runtime function information for those defined in
  /// OpenMPKinds.def.
  void initializeRuntimeFunctions() {
    // Helper to collect all uses of the decleration in the UsesMap.
    auto CollectUses = [&](RuntimeFunctionInfo &RFI) {
      unsigned NumUses = 0;
      if (!RFI.Declaration)
        return NumUses;
      OMPBuilder.addAttributes(RFI.Kind, *RFI.Declaration);

      NumOpenMPRuntimeFunctionsIdentified += 1;
      NumOpenMPRuntimeFunctionUsesIdentified += RFI.Declaration->getNumUses();

      // TODO: We directly convert uses into proper calls and unknown uses.
      for (Use &U : RFI.Declaration->uses()) {
        if (Instruction *UserI = dyn_cast<Instruction>(U.getUser())) {
          if (ModuleSlice.count(UserI->getFunction())) {
            RFI.getOrCreateUseVector(UserI->getFunction()).push_back(&U);
            ++NumUses;
          }
        } else {
          RFI.getOrCreateUseVector(nullptr).push_back(&U);
          ++NumUses;
        }
      }
      return NumUses;
    };

#define OMP_RTL(_Enum, _Name, _IsVarArg, _ReturnType, ...)                     \
  {                                                                            \
    SmallVector<Type *, 8> ArgsTypes({__VA_ARGS__});                           \
    Function *F = M.getFunction(_Name);                                        \
    if (declMatchesRTFTypes(F, _ReturnType, ArgsTypes)) {                      \
      auto &RFI = RFIs[_Enum];                                                 \
      RFI.Kind = _Enum;                                                        \
      RFI.Name = _Name;                                                        \
      RFI.IsVarArg = _IsVarArg;                                                \
      RFI.ReturnType = _ReturnType;                                            \
      RFI.ArgumentTypes = std::move(ArgsTypes);                                \
      RFI.Declaration = F;                                                     \
      unsigned NumUses = CollectUses(RFI);                                     \
      (void)NumUses;                                                           \
      LLVM_DEBUG({                                                             \
        dbgs() << TAG << RFI.Name << (RFI.Declaration ? "" : " not")           \
               << " found\n";                                                  \
        if (RFI.Declaration)                                                   \
          dbgs() << TAG << "-> got " << NumUses << " uses in "                 \
                 << RFI.getNumFunctionsWithUses()                              \
                 << " different functions.\n";                                 \
      });                                                                      \
    }                                                                          \
  }
#include "llvm/Frontend/OpenMP/OMPKinds.def"

    // TODO: We should attach the attributes defined in OMPKinds.def.
  }

  /// Emit a remark generically
  ///
  /// This template function can be used to generically emit a remark. The
  /// RemarkKind should be one of the following:
  ///   - OptimizationRemark to indicate a successful optimization attempt
  ///   - OptimizationRemarkMissed to report a failed optimization attempt
  ///   - OptimizationRemarkAnalysis to provide additional information about an
  ///     optimization attempt
  ///
  /// The remark is built using a callback function provided by the caller that
  /// takes a RemarkKind as input and returns a RemarkKind.
  template <typename RemarkKind,
            typename RemarkCallBack = function_ref<RemarkKind(RemarkKind &&)>>
  void emitRemark(Instruction *Inst, StringRef RemarkName,
                  RemarkCallBack &&RemarkCB) {
    Function *F = Inst->getParent()->getParent();
    auto &ORE = OREGetter(F);

    ORE.emit([&]() {
      return RemarkCB(RemarkKind(DEBUG_TYPE, RemarkName, Inst));
    });
  }

  /// The underyling module.
  Module &M;

  /// The SCC we are operating on.
  SmallVectorImpl<Function *> &SCC;

  /// The slice of the module we are allowed to look at.
  SmallPtrSetImpl<Function *> &ModuleSlice;

  /// An OpenMP-IR-Builder instance
  OpenMPIRBuilder OMPBuilder;

  /// Callback to update the call graph, the first argument is a removed call,
  /// the second an optional replacement call.
  CallGraphUpdater &CGUpdater;

  /// Callback to get an OptimizationRemarkEmitter from a Function *
  OptimizationRemarkGetter OREGetter;

  /// Map from runtime function kind to the runtime function description.
  EnumeratedArray<RuntimeFunctionInfo, RuntimeFunction,
                  RuntimeFunction::OMPRTL___last>
      RFIs;
};
} // namespace

PreservedAnalyses OpenMPOptPass::run(LazyCallGraph::SCC &C,
                                     CGSCCAnalysisManager &AM,
                                     LazyCallGraph &CG, CGSCCUpdateResult &UR) {
  if (!containsOpenMP(*C.begin()->getFunction().getParent(), OMPInModule))
    return PreservedAnalyses::all();

  if (DisableOpenMPOptimizations)
    return PreservedAnalyses::all();

  SmallPtrSet<Function *, 16> ModuleSlice;
  SmallVector<Function *, 16> SCC;
  for (LazyCallGraph::Node &N : C) {
    SCC.push_back(&N.getFunction());
    ModuleSlice.insert(SCC.back());
  }

  if (SCC.empty())
    return PreservedAnalyses::all();

  auto OREGetter = [&C, &CG, &AM](Function *F) -> OptimizationRemarkEmitter & {
    FunctionAnalysisManager &FAM =
        AM.getResult<FunctionAnalysisManagerCGSCCProxy>(C, CG).getManager();
    return FAM.getResult<OptimizationRemarkEmitterAnalysis>(*F);
  };

  CallGraphUpdater CGUpdater;
  CGUpdater.initialize(CG, C, AM, UR);
  // TODO: Compute the module slice we are allowed to look at.
  OpenMPOpt OMPOpt(SCC, ModuleSlice, CGUpdater, OREGetter);
  bool Changed = OMPOpt.run();
  (void)Changed;
  return PreservedAnalyses::all();
}

namespace {

struct OpenMPOptLegacyPass : public CallGraphSCCPass {
  CallGraphUpdater CGUpdater;
  OpenMPInModule OMPInModule;
  static char ID;

  OpenMPOptLegacyPass() : CallGraphSCCPass(ID) {
    initializeOpenMPOptLegacyPassPass(*PassRegistry::getPassRegistry());
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    CallGraphSCCPass::getAnalysisUsage(AU);
  }

  bool doInitialization(CallGraph &CG) override {
    // Disable the pass if there is no OpenMP (runtime call) in the module.
    containsOpenMP(CG.getModule(), OMPInModule);
    return false;
  }

  bool runOnSCC(CallGraphSCC &CGSCC) override {
    if (!containsOpenMP(CGSCC.getCallGraph().getModule(), OMPInModule))
      return false;
    if (DisableOpenMPOptimizations || skipSCC(CGSCC))
      return false;

    SmallPtrSet<Function *, 16> ModuleSlice;
    SmallVector<Function *, 16> SCC;
    for (CallGraphNode *CGN : CGSCC)
      if (Function *Fn = CGN->getFunction())
        if (!Fn->isDeclaration()) {
          SCC.push_back(Fn);
          ModuleSlice.insert(Fn);
        }

    if (SCC.empty())
      return false;

    CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
    CGUpdater.initialize(CG, CGSCC);

    // Maintain a map of functions to avoid rebuilding the ORE
    DenseMap<Function *, std::unique_ptr<OptimizationRemarkEmitter>> OREMap;
    auto OREGetter = [&OREMap](Function *F) -> OptimizationRemarkEmitter & {
      std::unique_ptr<OptimizationRemarkEmitter> &ORE = OREMap[F];
      if (!ORE)
        ORE = std::make_unique<OptimizationRemarkEmitter>(F);
      return *ORE;
    };

    // TODO: Compute the module slice we are allowed to look at.
    OpenMPOpt OMPOpt(SCC, ModuleSlice, CGUpdater, OREGetter);
    return OMPOpt.run();
  }

  bool doFinalization(CallGraph &CG) override { return CGUpdater.finalize(); }
};

} // end anonymous namespace

bool llvm::omp::containsOpenMP(Module &M, OpenMPInModule &OMPInModule) {
  if (OMPInModule.isKnown())
    return OMPInModule;

#define OMP_RTL(_Enum, _Name, ...)                                             \
  if (M.getFunction(_Name))                                                    \
    return OMPInModule = true;
#include "llvm/Frontend/OpenMP/OMPKinds.def"
  return OMPInModule = false;
}

char OpenMPOptLegacyPass::ID = 0;

INITIALIZE_PASS_BEGIN(OpenMPOptLegacyPass, "openmpopt",
                      "OpenMP specific optimizations", false, false)
INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
INITIALIZE_PASS_END(OpenMPOptLegacyPass, "openmpopt",
                    "OpenMP specific optimizations", false, false)

Pass *llvm::createOpenMPOptLegacyPass() { return new OpenMPOptLegacyPass(); }