text: //===- GlobalMerge.cpp - Internal globals merging -------------------------===//
  //
  // 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
  //
  //===----------------------------------------------------------------------===//
  //
  // This pass merges globals with internal linkage into one. This way all the
  // globals which were merged into a biggest one can be addressed using offsets
  // from the same base pointer (no need for separate base pointer for each of the
  // global). Such a transformation can significantly reduce the register pressure
  // when many globals are involved.
  //
  // For example, consider the code which touches several global variables at
  // once:
  //
  // static int foo[N], bar[N], baz[N];
  //
  // for (i = 0; i < N; ++i) {
  //    foo[i] = bar[i] * baz[i];
  // }
  //
  //  On ARM the addresses of 3 arrays should be kept in the registers, thus
  //  this code has quite large register pressure (loop body):
  //
  //  ldr     r1, [r5], #4
  //  ldr     r2, [r6], #4
  //  mul     r1, r2, r1
  //  str     r1, [r0], #4
  //
  //  Pass converts the code to something like:
  //
  //  static struct {
  //    int foo[N];
  //    int bar[N];
  //    int baz[N];
  //  } merged;
  //
  //  for (i = 0; i < N; ++i) {
  //    merged.foo[i] = merged.bar[i] * merged.baz[i];
  //  }
  //
  //  and in ARM code this becomes:
  //
  //  ldr     r0, [r5, #40]
  //  ldr     r1, [r5, #80]
  //  mul     r0, r1, r0
  //  str     r0, [r5], #4
  //
  //  note that we saved 2 registers here almostly "for free".
  //
  // However, merging globals can have tradeoffs:
  // - it confuses debuggers, tools, and users
  // - it makes linker optimizations less useful (order files, LOHs, ...)
  // - it forces usage of indexed addressing (which isn't necessarily "free")
  // - it can increase register pressure when the uses are disparate enough.
  //
  // We use heuristics to discover the best global grouping we can (cf cl::opts).
  //
  // ===---------------------------------------------------------------------===//
  
  #include "llvm/ADT/BitVector.h"
  #include "llvm/ADT/DenseMap.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/Statistic.h"
  #include "llvm/ADT/StringRef.h"
  #include "llvm/ADT/Triple.h"
  #include "llvm/ADT/Twine.h"
  #include "llvm/CodeGen/Passes.h"
  #include "llvm/IR/BasicBlock.h"
  #include "llvm/IR/Constants.h"
  #include "llvm/IR/DataLayout.h"
  #include "llvm/IR/DerivedTypes.h"
  #include "llvm/IR/Function.h"
  #include "llvm/IR/GlobalAlias.h"
  #include "llvm/IR/GlobalValue.h"
  #include "llvm/IR/GlobalVariable.h"
  #include "llvm/IR/Instruction.h"
  #include "llvm/IR/Module.h"
  #include "llvm/IR/Type.h"
  #include "llvm/IR/Use.h"
  #include "llvm/IR/User.h"
  #include "llvm/Pass.h"
  #include "llvm/Support/Casting.h"
  #include "llvm/Support/CommandLine.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Target/TargetLoweringObjectFile.h"
  #include "llvm/Target/TargetMachine.h"
  #include <algorithm>
  #include <cassert>
  #include <cstddef>
  #include <cstdint>
  #include <string>
  #include <vector>
  
  using namespace llvm;
  
  #define DEBUG_TYPE "global-merge"
  
  // FIXME: This is only useful as a last-resort way to disable the pass.
  static cl::opt<bool>
  EnableGlobalMerge("enable-global-merge", cl::Hidden,
                    cl::desc("Enable the global merge pass"),
                    cl::init(true));
  
  static cl::opt<unsigned>
  GlobalMergeMaxOffset("global-merge-max-offset", cl::Hidden,
                       cl::desc("Set maximum offset for global merge pass"),
                       cl::init(0));
  
  static cl::opt<bool> GlobalMergeGroupByUse(
      "global-merge-group-by-use", cl::Hidden,
      cl::desc("Improve global merge pass to look at uses"), cl::init(true));
  
  static cl::opt<bool> GlobalMergeIgnoreSingleUse(
      "global-merge-ignore-single-use", cl::Hidden,
      cl::desc("Improve global merge pass to ignore globals only used alone"),
      cl::init(true));
  
  static cl::opt<bool>
  EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
                           cl::desc("Enable global merge pass on constants"),
                           cl::init(false));
  
  // FIXME: this could be a transitional option, and we probably need to remove
  // it if only we are sure this optimization could always benefit all targets.
  static cl::opt<cl::boolOrDefault>
  EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
       cl::desc("Enable global merge pass on external linkage"));
  
  STATISTIC(NumMerged, "Number of globals merged");
  
  namespace {
  
    class GlobalMerge : public FunctionPass {
      const TargetMachine *TM = nullptr;
  
      // FIXME: Infer the maximum possible offset depending on the actual users
      // (these max offsets are different for the users inside Thumb or ARM
      // functions), see the code that passes in the offset in the ARM backend
      // for more information.
      unsigned MaxOffset;
  
      /// Whether we should try to optimize for size only.
      /// Currently, this applies a dead simple heuristic: only consider globals
      /// used in minsize functions for merging.
      /// FIXME: This could learn about optsize, and be used in the cost model.
      bool OnlyOptimizeForSize = false;
  
      /// Whether we should merge global variables that have external linkage.
      bool MergeExternalGlobals = false;
  
      bool IsMachO;
  
      bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                   Module &M, bool isConst, unsigned AddrSpace) const;
  
      /// Merge everything in \p Globals for which the corresponding bit
      /// in \p GlobalSet is set.
      bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
                   const BitVector &GlobalSet, Module &M, bool isConst,
                   unsigned AddrSpace) const;
  
      /// Check if the given variable has been identified as must keep
      /// \pre setMustKeepGlobalVariables must have been called on the Module that
      ///      contains GV
      bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
        return MustKeepGlobalVariables.count(GV);
      }
  
      /// Collect every variables marked as "used" or used in a landing pad
      /// instruction for this Module.
      void setMustKeepGlobalVariables(Module &M);
  
      /// Collect every variables marked as "used"
      void collectUsedGlobalVariables(Module &M, StringRef Name);
  
      /// Keep track of the GlobalVariable that must not be merged away
      SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
  
    public:
      static char ID;             // Pass identification, replacement for typeid.
  
      explicit GlobalMerge()
          : FunctionPass(ID), MaxOffset(GlobalMergeMaxOffset) {
        initializeGlobalMergePass(*PassRegistry::getPassRegistry());
      }
  
      explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset,
                           bool OnlyOptimizeForSize, bool MergeExternalGlobals)
          : FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
            OnlyOptimizeForSize(OnlyOptimizeForSize),
            MergeExternalGlobals(MergeExternalGlobals) {
        initializeGlobalMergePass(*PassRegistry::getPassRegistry());
      }
  
      bool doInitialization(Module &M) override;
      bool runOnFunction(Function &F) override;
      bool doFinalization(Module &M) override;
  
      StringRef getPassName() const override { return "Merge internal globals"; }
  
      void getAnalysisUsage(AnalysisUsage &AU) const override {
        AU.setPreservesCFG();
        FunctionPass::getAnalysisUsage(AU);
      }
    };
  
  } // end anonymous namespace
  
  char GlobalMerge::ID = 0;
  
  INITIALIZE_PASS(GlobalMerge, DEBUG_TYPE, "Merge global variables", false, false)
  
  bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                            Module &M, bool isConst, unsigned AddrSpace) const {
    auto &DL = M.getDataLayout();
    // FIXME: Find better heuristics
    std::stable_sort(Globals.begin(), Globals.end(),
                     [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
                       return DL.getTypeAllocSize(GV1->getValueType()) <
                              DL.getTypeAllocSize(GV2->getValueType());
                     });
  
    // If we want to just blindly group all globals together, do so.
    if (!GlobalMergeGroupByUse) {
      BitVector AllGlobals(Globals.size());
      AllGlobals.set();
      return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
    }
  
    // If we want to be smarter, look at all uses of each global, to try to
    // discover all sets of globals used together, and how many times each of
    // these sets occurred.
    //
    // Keep this reasonably efficient, by having an append-only list of all sets
    // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
    // code (currently, a Function) to the set of globals seen so far that are
    // used together in that unit (GlobalUsesByFunction).
    //
    // When we look at the Nth global, we know that any new set is either:
    // - the singleton set {N}, containing this global only, or
    // - the union of {N} and a previously-discovered set, containing some
    //   combination of the previous N-1 globals.
    // Using that knowledge, when looking at the Nth global, we can keep:
    // - a reference to the singleton set {N} (CurGVOnlySetIdx)
    // - a list mapping each previous set to its union with {N} (EncounteredUGS),
    //   if it actually occurs.
  
    // We keep track of the sets of globals used together "close enough".
    struct UsedGlobalSet {
      BitVector Globals;
      unsigned UsageCount = 1;
  
      UsedGlobalSet(size_t Size) : Globals(Size) {}
    };
  
    // Each set is unique in UsedGlobalSets.
    std::vector<UsedGlobalSet> UsedGlobalSets;
  
    // Avoid repeating the create-global-set pattern.
    auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
      UsedGlobalSets.emplace_back(Globals.size());
      return UsedGlobalSets.back();
    };
  
    // The first set is the empty set.
    CreateGlobalSet().UsageCount = 0;
  
    // We define "close enough" to be "in the same function".
    // FIXME: Grouping uses by function is way too aggressive, so we should have
    // a better metric for distance between uses.
    // The obvious alternative would be to group by BasicBlock, but that's in
    // turn too conservative..
    // Anything in between wouldn't be trivial to compute, so just stick with
    // per-function grouping.
  
    // The value type is an index into UsedGlobalSets.
    // The default (0) conveniently points to the empty set.
    DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
  
    // Now, look at each merge-eligible global in turn.
  
    // Keep track of the sets we already encountered to which we added the
    // current global.
    // Each element matches the same-index element in UsedGlobalSets.
    // This lets us efficiently tell whether a set has already been expanded to
    // include the current global.
    std::vector<size_t> EncounteredUGS;
  
    for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
      GlobalVariable *GV = Globals[GI];
  
      // Reset the encountered sets for this global...
      std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
      // ...and grow it in case we created new sets for the previous global.
      EncounteredUGS.resize(UsedGlobalSets.size());
  
      // We might need to create a set that only consists of the current global.
      // Keep track of its index into UsedGlobalSets.
      size_t CurGVOnlySetIdx = 0;
  
      // For each global, look at all its Uses.
      for (auto &U : GV->uses()) {
        // This Use might be a ConstantExpr.  We're interested in Instruction
        // users, so look through ConstantExpr...
        Use *UI, *UE;
        if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
          if (CE->use_empty())
            continue;
          UI = &*CE->use_begin();
          UE = nullptr;
        } else if (isa<Instruction>(U.getUser())) {
          UI = &U;
          UE = UI->getNext();
        } else {
          continue;
        }
  
        // ...to iterate on all the instruction users of the global.
        // Note that we iterate on Uses and not on Users to be able to getNext().
        for (; UI != UE; UI = UI->getNext()) {
          Instruction *I = dyn_cast<Instruction>(UI->getUser());
          if (!I)
            continue;
  
          Function *ParentFn = I->getParent()->getParent();
  
          // If we're only optimizing for size, ignore non-minsize functions.
          if (OnlyOptimizeForSize && !ParentFn->hasMinSize())
            continue;
  
          size_t UGSIdx = GlobalUsesByFunction[ParentFn];
  
          // If this is the first global the basic block uses, map it to the set
          // consisting of this global only.
          if (!UGSIdx) {
            // If that set doesn't exist yet, create it.
            if (!CurGVOnlySetIdx) {
              CurGVOnlySetIdx = UsedGlobalSets.size();
              CreateGlobalSet().Globals.set(GI);
            } else {
              ++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
            }
  
            GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
            continue;
          }
  
          // If we already encountered this BB, just increment the counter.
          if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
            ++UsedGlobalSets[UGSIdx].UsageCount;
            continue;
          }
  
          // If not, the previous set wasn't actually used in this function.
          --UsedGlobalSets[UGSIdx].UsageCount;
  
          // If we already expanded the previous set to include this global, just
          // reuse that expanded set.
          if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
            ++UsedGlobalSets[ExpandedIdx].UsageCount;
            GlobalUsesByFunction[ParentFn] = ExpandedIdx;
            continue;
          }
  
          // If not, create a new set consisting of the union of the previous set
          // and this global.  Mark it as encountered, so we can reuse it later.
          GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
              UsedGlobalSets.size();
  
          UsedGlobalSet &NewUGS = CreateGlobalSet();
          NewUGS.Globals.set(GI);
          NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
        }
      }
    }
  
    // Now we found a bunch of sets of globals used together.  We accumulated
    // the number of times we encountered the sets (i.e., the number of blocks
    // that use that exact set of globals).
    //
    // Multiply that by the size of the set to give us a crude profitability
    // metric.
    std::stable_sort(UsedGlobalSets.begin(), UsedGlobalSets.end(),
              [](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
                return UGS1.Globals.count() * UGS1.UsageCount <
                       UGS2.Globals.count() * UGS2.UsageCount;
              });
  
    // We can choose to merge all globals together, but ignore globals never used
    // with another global.  This catches the obviously non-profitable cases of
    // having a single global, but is aggressive enough for any other case.
    if (GlobalMergeIgnoreSingleUse) {
      BitVector AllGlobals(Globals.size());
      for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
        const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
        if (UGS.UsageCount == 0)
          continue;
        if (UGS.Globals.count() > 1)
          AllGlobals |= UGS.Globals;
      }
      return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
    }
  
    // Starting from the sets with the best (=biggest) profitability, find a
    // good combination.
    // The ideal (and expensive) solution can only be found by trying all
    // combinations, looking for the one with the best profitability.
    // Don't be smart about it, and just pick the first compatible combination,
    // starting with the sets with the best profitability.
    BitVector PickedGlobals(Globals.size());
    bool Changed = false;
  
    for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
      const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
      if (UGS.UsageCount == 0)
        continue;
      if (PickedGlobals.anyCommon(UGS.Globals))
        continue;
      PickedGlobals |= UGS.Globals;
      // If the set only contains one global, there's no point in merging.
      // Ignore the global for inclusion in other sets though, so keep it in
      // PickedGlobals.
      if (UGS.Globals.count() < 2)
        continue;
      Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace);
    }
  
    return Changed;
  }
  
  bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
                            const BitVector &GlobalSet, Module &M, bool isConst,
                            unsigned AddrSpace) const {
    assert(Globals.size() > 1);
  
    Type *Int32Ty = Type::getInt32Ty(M.getContext());
    Type *Int8Ty = Type::getInt8Ty(M.getContext());
    auto &DL = M.getDataLayout();
  
    LLVM_DEBUG(dbgs() << " Trying to merge set, starts with #"
                      << GlobalSet.find_first() << "\n");
  
    bool Changed = false;
    ssize_t i = GlobalSet.find_first();
    while (i != -1) {
      ssize_t j = 0;
      uint64_t MergedSize = 0;
      std::vector<Type*> Tys;
      std::vector<Constant*> Inits;
      std::vector<unsigned> StructIdxs;
  
      bool HasExternal = false;
      StringRef FirstExternalName;
      unsigned MaxAlign = 1;
      unsigned CurIdx = 0;
      for (j = i; j != -1; j = GlobalSet.find_next(j)) {
        Type *Ty = Globals[j]->getValueType();
  
        // Make sure we use the same alignment AsmPrinter would use.
        unsigned Align = DL.getPreferredAlignment(Globals[j]);
        unsigned Padding = alignTo(MergedSize, Align) - MergedSize;
        MergedSize += Padding;
        MergedSize += DL.getTypeAllocSize(Ty);
        if (MergedSize > MaxOffset) {
          break;
        }
        if (Padding) {
          Tys.push_back(ArrayType::get(Int8Ty, Padding));
          Inits.push_back(ConstantAggregateZero::get(Tys.back()));
          ++CurIdx;
        }
        Tys.push_back(Ty);
        Inits.push_back(Globals[j]->getInitializer());
        StructIdxs.push_back(CurIdx++);
  
        MaxAlign = std::max(MaxAlign, Align);
  
        if (Globals[j]->hasExternalLinkage() && !HasExternal) {
          HasExternal = true;
          FirstExternalName = Globals[j]->getName();
        }
      }
  
      // Exit early if there is only one global to merge.
      if (Tys.size() < 2) {
        i = j;
        continue;
      }
  
      // If merged variables doesn't have external linkage, we needn't to expose
      // the symbol after merging.
      GlobalValue::LinkageTypes Linkage = HasExternal
                                              ? GlobalValue::ExternalLinkage
                                              : GlobalValue::InternalLinkage;
      // Use a packed struct so we can control alignment.
      StructType *MergedTy = StructType::get(M.getContext(), Tys, true);
      Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
  
      // On Darwin external linkage needs to be preserved, otherwise
      // dsymutil cannot preserve the debug info for the merged
      // variables.  If they have external linkage, use the symbol name
      // of the first variable merged as the suffix of global symbol
      // name.  This avoids a link-time naming conflict for the
      // _MergedGlobals symbols.
      Twine MergedName =
          (IsMachO && HasExternal)
              ? "_MergedGlobals_" + FirstExternalName
              : "_MergedGlobals";
      auto MergedLinkage = IsMachO ? Linkage : GlobalValue::PrivateLinkage;
      auto *MergedGV = new GlobalVariable(
          M, MergedTy, isConst, MergedLinkage, MergedInit, MergedName, nullptr,
          GlobalVariable::NotThreadLocal, AddrSpace);
  
      MergedGV->setAlignment(MaxAlign);
      MergedGV->setSection(Globals[i]->getSection());
  
      const StructLayout *MergedLayout = DL.getStructLayout(MergedTy);
      for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
        GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
        std::string Name = Globals[k]->getName();
        GlobalValue::DLLStorageClassTypes DLLStorage =
            Globals[k]->getDLLStorageClass();
  
        // Copy metadata while adjusting any debug info metadata by the original
        // global's offset within the merged global.
        MergedGV->copyMetadata(Globals[k],
                               MergedLayout->getElementOffset(StructIdxs[idx]));
  
        Constant *Idx[2] = {
            ConstantInt::get(Int32Ty, 0),
            ConstantInt::get(Int32Ty, StructIdxs[idx]),
        };
        Constant *GEP =
            ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
        Globals[k]->replaceAllUsesWith(GEP);
        Globals[k]->eraseFromParent();
  
        // When the linkage is not internal we must emit an alias for the original
        // variable name as it may be accessed from another object. On non-Mach-O
        // we can also emit an alias for internal linkage as it's safe to do so.
        // It's not safe on Mach-O as the alias (and thus the portion of the
        // MergedGlobals variable) may be dead stripped at link time.
        if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
          GlobalAlias *GA = GlobalAlias::create(Tys[StructIdxs[idx]], AddrSpace,
                                                Linkage, Name, GEP, &M);
          GA->setDLLStorageClass(DLLStorage);
        }
  
        NumMerged++;
      }
      Changed = true;
      i = j;
    }
  
    return Changed;
  }
  
  void GlobalMerge::collectUsedGlobalVariables(Module &M, StringRef Name) {
    // Extract global variables from llvm.used array
    const GlobalVariable *GV = M.getGlobalVariable(Name);
    if (!GV || !GV->hasInitializer()) return;
  
    // Should be an array of 'i8*'.
    const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
  
    for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
      if (const GlobalVariable *G =
          dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
        MustKeepGlobalVariables.insert(G);
  }
  
  void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
    collectUsedGlobalVariables(M, "llvm.used");
    collectUsedGlobalVariables(M, "llvm.compiler.used");
  
    for (Function &F : M) {
      for (BasicBlock &BB : F) {
        Instruction *Pad = BB.getFirstNonPHI();
        if (!Pad->isEHPad())
          continue;
  
        // Keep globals used by landingpads and catchpads.
        for (const Use &U : Pad->operands()) {
          if (const GlobalVariable *GV =
                  dyn_cast<GlobalVariable>(U->stripPointerCasts()))
            MustKeepGlobalVariables.insert(GV);
        }
      }
    }
  }
  
  bool GlobalMerge::doInitialization(Module &M) {
    if (!EnableGlobalMerge)
      return false;
  
    IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO();
  
    auto &DL = M.getDataLayout();
    DenseMap<std::pair<unsigned, StringRef>, SmallVector<GlobalVariable *, 16>>
        Globals, ConstGlobals, BSSGlobals;
    bool Changed = false;
    setMustKeepGlobalVariables(M);
  
    // Grab all non-const globals.
    for (auto &GV : M.globals()) {
      // Merge is safe for "normal" internal or external globals only
      if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasImplicitSection())
        continue;
  
      // It's not safe to merge globals that may be preempted
      if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
        continue;
  
      if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
          !GV.hasInternalLinkage())
        continue;
  
      PointerType *PT = dyn_cast<PointerType>(GV.getType());
      assert(PT && "Global variable is not a pointer!");
  
      unsigned AddressSpace = PT->getAddressSpace();
      StringRef Section = GV.getSection();
  
      // Ignore all 'special' globals.
      if (GV.getName().startswith("llvm.") ||
          GV.getName().startswith(".llvm."))
        continue;
  
      // Ignore all "required" globals:
      if (isMustKeepGlobalVariable(&GV))
        continue;
  
      Type *Ty = GV.getValueType();
      if (DL.getTypeAllocSize(Ty) < MaxOffset) {
        if (TM &&
            TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSS())
          BSSGlobals[{AddressSpace, Section}].push_back(&GV);
        else if (GV.isConstant())
          ConstGlobals[{AddressSpace, Section}].push_back(&GV);
        else
          Globals[{AddressSpace, Section}].push_back(&GV);
      }
    }
  
    for (auto &P : Globals)
      if (P.second.size() > 1)
        Changed |= doMerge(P.second, M, false, P.first.first);
  
    for (auto &P : BSSGlobals)
      if (P.second.size() > 1)
        Changed |= doMerge(P.second, M, false, P.first.first);
  
    if (EnableGlobalMergeOnConst)
      for (auto &P : ConstGlobals)
        if (P.second.size() > 1)
          Changed |= doMerge(P.second, M, true, P.first.first);
  
    return Changed;
  }
  
  bool GlobalMerge::runOnFunction(Function &F) {
    return false;
  }
  
  bool GlobalMerge::doFinalization(Module &M) {
    MustKeepGlobalVariables.clear();
    return false;
  }
  
  Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
                                    bool OnlyOptimizeForSize,
                                    bool MergeExternalByDefault) {
    bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
      MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
    return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
  }
  