GlobalDCE.cpp

//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This transform is designed to eliminate unreachable internal globals from the
// program.  It uses an aggressive algorithm, searching out globals that are
// known to be alive.  After it finds all of the globals which are needed, it
// deletes whatever is left over.  This allows it to delete recursive chunks of
// the program which are unreachable.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/IPO/GlobalDCE.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/CtorUtils.h"
#include "llvm/Transforms/Utils/GlobalStatus.h"

using namespace llvm;

#define DEBUG_TYPE "globaldce"

STATISTIC(NumAliases  , "Number of global aliases removed");
STATISTIC(NumFunctions, "Number of functions removed");
STATISTIC(NumIFuncs,    "Number of indirect functions removed");
STATISTIC(NumVariables, "Number of global variables removed");

namespace {
  class GlobalDCELegacyPass : public ModulePass {
  public:
    static char ID; // Pass identification, replacement for typeid
    GlobalDCELegacyPass() : ModulePass(ID) {
      initializeGlobalDCELegacyPassPass(*PassRegistry::getPassRegistry());
    }

    // run - Do the GlobalDCE pass on the specified module, optionally updating
    // the specified callgraph to reflect the changes.
    //
    bool runOnModule(Module &M) override {
      if (skipModule(M))
        return false;

      // We need a minimally functional dummy module analysis manager. It needs
      // to at least know about the possibility of proxying a function analysis
      // manager.
      FunctionAnalysisManager DummyFAM;
      ModuleAnalysisManager DummyMAM;
      DummyMAM.registerPass(
          [&] { return FunctionAnalysisManagerModuleProxy(DummyFAM); });

      auto PA = Impl.run(M, DummyMAM);
      return !PA.areAllPreserved();
    }

  private:
    GlobalDCEPass Impl;
  };
}

char GlobalDCELegacyPass::ID = 0;
INITIALIZE_PASS(GlobalDCELegacyPass, "globaldce",
                "Dead Global Elimination", false, false)

// Public interface to the GlobalDCEPass.
ModulePass *llvm::createGlobalDCEPass() {
  return new GlobalDCELegacyPass();
}

/// Returns true if F contains only a single "ret" instruction.
static bool isEmptyFunction(Function *F) {
  BasicBlock &Entry = F->getEntryBlock();
  if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
    return false;
  ReturnInst &RI = cast<ReturnInst>(Entry.front());
  return RI.getReturnValue() == nullptr;
}

/// Compute the set of GlobalValue that depends from V.
/// The recursion stops as soon as a GlobalValue is met.
void GlobalDCEPass::ComputeDependencies(Value *V,
                                        SmallPtrSetImpl<GlobalValue *> &Deps) {
  if (auto *I = dyn_cast<Instruction>(V)) {
    Function *Parent = I->getParent()->getParent();
    Deps.insert(Parent);
  } else if (auto *GV = dyn_cast<GlobalValue>(V)) {
    Deps.insert(GV);
  } else if (auto *CE = dyn_cast<Constant>(V)) {
    // Avoid walking the whole tree of a big ConstantExprs multiple times.
    auto Where = ConstantDependenciesCache.find(CE);
    if (Where != ConstantDependenciesCache.end()) {
      auto const &K = Where->second;
      Deps.insert(K.begin(), K.end());
    } else {
      SmallPtrSetImpl<GlobalValue *> &LocalDeps = ConstantDependenciesCache[CE];
      for (User *CEUser : CE->users())
        ComputeDependencies(CEUser, LocalDeps);
      Deps.insert(LocalDeps.begin(), LocalDeps.end());
    }
  }
}

void GlobalDCEPass::UpdateGVDependencies(GlobalValue &GV) {
  SmallPtrSet<GlobalValue *, 8> Deps;
  for (User *User : GV.users())
    ComputeDependencies(User, Deps);
  Deps.erase(&GV); // Remove self-reference.
  for (GlobalValue *GVU : Deps) {
    GVDependencies[GVU].insert(&GV);
  }
}

/// Mark Global value as Live
void GlobalDCEPass::MarkLive(GlobalValue &GV,
                             SmallVectorImpl<GlobalValue *> *Updates) {
  auto const Ret = AliveGlobals.insert(&GV);
  if (!Ret.second)
    return;

  if (Updates)
    Updates->push_back(&GV);
  if (Comdat *C = GV.getComdat()) {
    for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
      MarkLive(*CM.second, Updates); // Recursion depth is only two because only
                                     // globals in the same comdat are visited.
  }
}

PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
  bool Changed = false;

  // The algorithm first computes the set L of global variables that are
  // trivially live.  Then it walks the initialization of these variables to
  // compute the globals used to initialize them, which effectively builds a
  // directed graph where nodes are global variables, and an edge from A to B
  // means B is used to initialize A.  Finally, it propagates the liveness
  // information through the graph starting from the nodes in L. Nodes note
  // marked as alive are discarded.

  // Remove empty functions from the global ctors list.
  Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);

  // Collect the set of members for each comdat.
  for (Function &F : M)
    if (Comdat *C = F.getComdat())
      ComdatMembers.insert(std::make_pair(C, &F));
  for (GlobalVariable &GV : M.globals())
    if (Comdat *C = GV.getComdat())
      ComdatMembers.insert(std::make_pair(C, &GV));
  for (GlobalAlias &GA : M.aliases())
    if (Comdat *C = GA.getComdat())
      ComdatMembers.insert(std::make_pair(C, &GA));

  // Loop over the module, adding globals which are obviously necessary.
  for (GlobalObject &GO : M.global_objects()) {
    Changed |= RemoveUnusedGlobalValue(GO);
    // Functions with external linkage are needed if they have a body.
    // Externally visible & appending globals are needed, if they have an
    // initializer.
    if (!GO.isDeclaration() && !GO.hasAvailableExternallyLinkage())
      if (!GO.isDiscardableIfUnused())
        MarkLive(GO);

    UpdateGVDependencies(GO);
  }

  // Compute direct dependencies of aliases.
  for (GlobalAlias &GA : M.aliases()) {
    Changed |= RemoveUnusedGlobalValue(GA);
    // Externally visible aliases are needed.
    if (!GA.isDiscardableIfUnused())
      MarkLive(GA);

    UpdateGVDependencies(GA);
  }

  // Compute direct dependencies of ifuncs.
  for (GlobalIFunc &GIF : M.ifuncs()) {
    Changed |= RemoveUnusedGlobalValue(GIF);
    // Externally visible ifuncs are needed.
    if (!GIF.isDiscardableIfUnused())
      MarkLive(GIF);

    UpdateGVDependencies(GIF);
  }

  // Propagate liveness from collected Global Values through the computed
  // dependencies.
  SmallVector<GlobalValue *, 8> NewLiveGVs{AliveGlobals.begin(),
                                           AliveGlobals.end()};
  while (!NewLiveGVs.empty()) {
    GlobalValue *LGV = NewLiveGVs.pop_back_val();
    for (auto *GVD : GVDependencies[LGV])
      MarkLive(*GVD, &NewLiveGVs);
  }

  // Now that all globals which are needed are in the AliveGlobals set, we loop
  // through the program, deleting those which are not alive.
  //

  // The first pass is to drop initializers of global variables which are dead.
  std::vector<GlobalVariable *> DeadGlobalVars; // Keep track of dead globals
  for (GlobalVariable &GV : M.globals())
    if (!AliveGlobals.count(&GV)) {
      DeadGlobalVars.push_back(&GV);         // Keep track of dead globals
      if (GV.hasInitializer()) {
        Constant *Init = GV.getInitializer();
        GV.setInitializer(nullptr);
        if (isSafeToDestroyConstant(Init))
          Init->destroyConstant();
      }
    }

  // The second pass drops the bodies of functions which are dead...
  std::vector<Function *> DeadFunctions;
  for (Function &F : M)
    if (!AliveGlobals.count(&F)) {
      DeadFunctions.push_back(&F);         // Keep track of dead globals
      if (!F.isDeclaration())
        F.deleteBody();
    }

  // The third pass drops targets of aliases which are dead...
  std::vector<GlobalAlias*> DeadAliases;
  for (GlobalAlias &GA : M.aliases())
    if (!AliveGlobals.count(&GA)) {
      DeadAliases.push_back(&GA);
      GA.setAliasee(nullptr);
    }

  // The fourth pass drops targets of ifuncs which are dead...
  std::vector<GlobalIFunc*> DeadIFuncs;
  for (GlobalIFunc &GIF : M.ifuncs())
    if (!AliveGlobals.count(&GIF)) {
      DeadIFuncs.push_back(&GIF);
      GIF.setResolver(nullptr);
    }

  // Now that all interferences have been dropped, delete the actual objects
  // themselves.
  auto EraseUnusedGlobalValue = [&](GlobalValue *GV) {
    RemoveUnusedGlobalValue(*GV);
    GV->eraseFromParent();
    Changed = true;
  };

  NumFunctions += DeadFunctions.size();
  for (Function *F : DeadFunctions)
    EraseUnusedGlobalValue(F);

  NumVariables += DeadGlobalVars.size();
  for (GlobalVariable *GV : DeadGlobalVars)
    EraseUnusedGlobalValue(GV);

  NumAliases += DeadAliases.size();
  for (GlobalAlias *GA : DeadAliases)
    EraseUnusedGlobalValue(GA);

  NumIFuncs += DeadIFuncs.size();
  for (GlobalIFunc *GIF : DeadIFuncs)
    EraseUnusedGlobalValue(GIF);

  // Make sure that all memory is released
  AliveGlobals.clear();
  ConstantDependenciesCache.clear();
  GVDependencies.clear();
  ComdatMembers.clear();

  if (Changed)
    return PreservedAnalyses::none();
  return PreservedAnalyses::all();
}

// RemoveUnusedGlobalValue - Loop over all of the uses of the specified
// GlobalValue, looking for the constant pointer ref that may be pointing to it.
// If found, check to see if the constant pointer ref is safe to destroy, and if
// so, nuke it.  This will reduce the reference count on the global value, which
// might make it deader.
//
bool GlobalDCEPass::RemoveUnusedGlobalValue(GlobalValue &GV) {
  if (GV.use_empty())
    return false;
  GV.removeDeadConstantUsers();
  return GV.use_empty();
}