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//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
//
// 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 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/Analysis/TypeMetadataUtils.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.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"
static cl::opt<bool>
ClEnableVFE("enable-vfe", cl::Hidden, cl::init(true), cl::ZeroOrMore,
cl::desc("Enable virtual function elimination"));
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");
STATISTIC(NumVFuncs, "Number of virtual functions 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 is effectively empty.
static bool isEmptyFunction(Function *F) {
BasicBlock &Entry = F->getEntryBlock();
for (auto &I : Entry) {
if (isa<DbgInfoIntrinsic>(I))
continue;
if (auto *RI = dyn_cast<ReturnInst>(&I))
return !RI->getReturnValue();
break;
}
return false;
}
/// 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) {
// If this is a dep from a vtable to a virtual function, and we have
// complete information about all virtual call sites which could call
// though this vtable, then skip it, because the call site information will
// be more precise.
if (VFESafeVTables.count(GVU) && isa<Function>(&GV)) {
LLVM_DEBUG(dbgs() << "Ignoring dep " << GVU->getName() << " -> "
<< GV.getName() << "\n");
continue;
}
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.
}
}
}
void GlobalDCEPass::ScanVTables(Module &M) {
SmallVector<MDNode *, 2> Types;
LLVM_DEBUG(dbgs() << "Building type info -> vtable map\n");
auto *LTOPostLinkMD =
cast_or_null<ConstantAsMetadata>(M.getModuleFlag("LTOPostLink"));
bool LTOPostLink =
LTOPostLinkMD &&
(cast<ConstantInt>(LTOPostLinkMD->getValue())->getZExtValue() != 0);
for (GlobalVariable &GV : M.globals()) {
Types.clear();
GV.getMetadata(LLVMContext::MD_type, Types);
if (GV.isDeclaration() || Types.empty())
continue;
// Use the typeid metadata on the vtable to build a mapping from typeids to
// the list of (GV, offset) pairs which are the possible vtables for that
// typeid.
for (MDNode *Type : Types) {
Metadata *TypeID = Type->getOperand(1).get();
uint64_t Offset =
cast<ConstantInt>(
cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
->getZExtValue();
TypeIdMap[TypeID].insert(std::make_pair(&GV, Offset));
}
// If the type corresponding to the vtable is private to this translation
// unit, we know that we can see all virtual functions which might use it,
// so VFE is safe.
if (auto GO = dyn_cast<GlobalObject>(&GV)) {
GlobalObject::VCallVisibility TypeVis = GO->getVCallVisibility();
if (TypeVis == GlobalObject::VCallVisibilityTranslationUnit ||
(LTOPostLink &&
TypeVis == GlobalObject::VCallVisibilityLinkageUnit)) {
LLVM_DEBUG(dbgs() << GV.getName() << " is safe for VFE\n");
VFESafeVTables.insert(&GV);
}
}
}
}
void GlobalDCEPass::ScanVTableLoad(Function *Caller, Metadata *TypeId,
uint64_t CallOffset) {
for (auto &VTableInfo : TypeIdMap[TypeId]) {
GlobalVariable *VTable = VTableInfo.first;
uint64_t VTableOffset = VTableInfo.second;
Constant *Ptr =
getPointerAtOffset(VTable->getInitializer(), VTableOffset + CallOffset,
*Caller->getParent());
if (!Ptr) {
LLVM_DEBUG(dbgs() << "can't find pointer in vtable!\n");
VFESafeVTables.erase(VTable);
return;
}
auto Callee = dyn_cast<Function>(Ptr->stripPointerCasts());
if (!Callee) {
LLVM_DEBUG(dbgs() << "vtable entry is not function pointer!\n");
VFESafeVTables.erase(VTable);
return;
}
LLVM_DEBUG(dbgs() << "vfunc dep " << Caller->getName() << " -> "
<< Callee->getName() << "\n");
GVDependencies[Caller].insert(Callee);
}
}
void GlobalDCEPass::ScanTypeCheckedLoadIntrinsics(Module &M) {
LLVM_DEBUG(dbgs() << "Scanning type.checked.load intrinsics\n");
Function *TypeCheckedLoadFunc =
M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
if (!TypeCheckedLoadFunc)
return;
for (auto U : TypeCheckedLoadFunc->users()) {
auto CI = dyn_cast<CallInst>(U);
if (!CI)
continue;
auto *Offset = dyn_cast<ConstantInt>(CI->getArgOperand(1));
Value *TypeIdValue = CI->getArgOperand(2);
auto *TypeId = cast<MetadataAsValue>(TypeIdValue)->getMetadata();
if (Offset) {
ScanVTableLoad(CI->getFunction(), TypeId, Offset->getZExtValue());
} else {
// type.checked.load with a non-constant offset, so assume every entry in
// every matching vtable is used.
for (auto &VTableInfo : TypeIdMap[TypeId]) {
VFESafeVTables.erase(VTableInfo.first);
}
}
}
}
void GlobalDCEPass::AddVirtualFunctionDependencies(Module &M) {
if (!ClEnableVFE)
return;
ScanVTables(M);
if (VFESafeVTables.empty())
return;
ScanTypeCheckedLoadIntrinsics(M);
LLVM_DEBUG(
dbgs() << "VFE safe vtables:\n";
for (auto *VTable : VFESafeVTables)
dbgs() << " " << VTable->getName() << "\n";
);
}
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));
// Add dependencies between virtual call sites and the virtual functions they
// might call, if we have that information.
AddVirtualFunctionDependencies(M);
// 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())
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) {
if (!F->use_empty()) {
// Virtual functions might still be referenced by one or more vtables,
// but if we've proven them to be unused then it's safe to replace the
// virtual function pointers with null, allowing us to remove the
// function itself.
++NumVFuncs;
F->replaceNonMetadataUsesWith(ConstantPointerNull::get(F->getType()));
}
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();
TypeIdMap.clear();
VFESafeVTables.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();
}