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//===-- 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 {
static char ID; // Pass identification, replacement for typeid
GlobalDCELegacyPass() : ModulePass(ID) {
// 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;
[&] { return FunctionAnalysisManagerModuleProxy(DummyFAM); });
auto PA =, DummyMAM);
return !PA.areAllPreserved();
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();
} else if (auto *GV = dyn_cast<GlobalValue>(V)) {
} 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) {
/// Mark Global value as Live
void GlobalDCEPass::MarkLive(GlobalValue &GV,
SmallVectorImpl<GlobalValue *> *Updates) {
auto const Ret = AliveGlobals.insert(&GV);
if (!Ret.second)
if (Updates)
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())
// Compute direct dependencies of aliases.
for (GlobalAlias &GA : M.aliases()) {
Changed |= RemoveUnusedGlobalValue(GA);
// Externally visible aliases are needed.
if (!GA.isDiscardableIfUnused())
// Compute direct dependencies of ifuncs.
for (GlobalIFunc &GIF : M.ifuncs()) {
Changed |= RemoveUnusedGlobalValue(GIF);
// Externally visible ifuncs are needed.
if (!GIF.isDiscardableIfUnused())
// Propagate liveness from collected Global Values through the computed
// dependencies.
SmallVector<GlobalValue *, 8> NewLiveGVs{AliveGlobals.begin(),
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();
if (isSafeToDestroyConstant(Init))
// 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())
// The third pass drops targets of aliases which are dead...
std::vector<GlobalAlias*> DeadAliases;
for (GlobalAlias &GA : M.aliases())
if (!AliveGlobals.count(&GA)) {
// The fourth pass drops targets of ifuncs which are dead...
std::vector<GlobalIFunc*> DeadIFuncs;
for (GlobalIFunc &GIF : M.ifuncs())
if (!AliveGlobals.count(&GIF)) {
// Now that all interferences have been dropped, delete the actual objects
// themselves.
auto EraseUnusedGlobalValue = [&](GlobalValue *GV) {
Changed = true;
NumFunctions += DeadFunctions.size();
for (Function *F : DeadFunctions)
NumVariables += DeadGlobalVars.size();
for (GlobalVariable *GV : DeadGlobalVars)
NumAliases += DeadAliases.size();
for (GlobalAlias *GA : DeadAliases)
NumIFuncs += DeadIFuncs.size();
for (GlobalIFunc *GIF : DeadIFuncs)
// Make sure that all memory is released
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;
return GV.use_empty();