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//===- GlobalOpt.cpp - Optimize Global Variables --------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass transforms simple global variables that never have their address
// taken. If obviously true, it marks read/write globals as constant, deletes
// variables only stored to, etc.
#include "llvm/Transforms/IPO/GlobalOpt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/BinaryFormat/Dwarf.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CallingConv.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalAlias.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Support/AtomicOrdering.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/CtorUtils.h"
#include "llvm/Transforms/Utils/Evaluator.h"
#include "llvm/Transforms/Utils/GlobalStatus.h"
#include "llvm/Transforms/Utils/Local.h"
#include <cassert>
#include <cstdint>
#include <optional>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "globalopt"
STATISTIC(NumMarked , "Number of globals marked constant");
STATISTIC(NumUnnamed , "Number of globals marked unnamed_addr");
STATISTIC(NumSRA , "Number of aggregate globals broken into scalars");
STATISTIC(NumSubstitute,"Number of globals with initializers stored into them");
STATISTIC(NumDeleted , "Number of globals deleted");
STATISTIC(NumGlobUses , "Number of global uses devirtualized");
STATISTIC(NumLocalized , "Number of globals localized");
STATISTIC(NumShrunkToBool , "Number of global vars shrunk to booleans");
STATISTIC(NumFastCallFns , "Number of functions converted to fastcc");
STATISTIC(NumCtorsEvaluated, "Number of static ctors evaluated");
STATISTIC(NumNestRemoved , "Number of nest attributes removed");
STATISTIC(NumAliasesResolved, "Number of global aliases resolved");
STATISTIC(NumAliasesRemoved, "Number of global aliases eliminated");
STATISTIC(NumCXXDtorsRemoved, "Number of global C++ destructors removed");
STATISTIC(NumAtExitRemoved, "Number of atexit handlers removed");
STATISTIC(NumInternalFunc, "Number of internal functions");
STATISTIC(NumColdCC, "Number of functions marked coldcc");
STATISTIC(NumIFuncsResolved, "Number of statically resolved IFuncs");
STATISTIC(NumIFuncsDeleted, "Number of IFuncs removed");
static cl::opt<bool>
cl::desc("Enable stress test of coldcc by adding "
"calling conv to all internal functions."),
cl::init(false), cl::Hidden);
static cl::opt<int> ColdCCRelFreq(
"coldcc-rel-freq", cl::Hidden, cl::init(2),
"Maximum block frequency, expressed as a percentage of caller's "
"entry frequency, for a call site to be considered cold for enabling"
/// Is this global variable possibly used by a leak checker as a root? If so,
/// we might not really want to eliminate the stores to it.
static bool isLeakCheckerRoot(GlobalVariable *GV) {
// A global variable is a root if it is a pointer, or could plausibly contain
// a pointer. There are two challenges; one is that we could have a struct
// the has an inner member which is a pointer. We recurse through the type to
// detect these (up to a point). The other is that we may actually be a union
// of a pointer and another type, and so our LLVM type is an integer which
// gets converted into a pointer, or our type is an [i8 x #] with a pointer
// potentially contained here.
if (GV->hasPrivateLinkage())
return false;
SmallVector<Type *, 4> Types;
unsigned Limit = 20;
do {
Type *Ty = Types.pop_back_val();
switch (Ty->getTypeID()) {
default: break;
case Type::PointerTyID:
return true;
case Type::FixedVectorTyID:
case Type::ScalableVectorTyID:
if (cast<VectorType>(Ty)->getElementType()->isPointerTy())
return true;
case Type::ArrayTyID:
case Type::StructTyID: {
StructType *STy = cast<StructType>(Ty);
if (STy->isOpaque()) return true;
for (Type *InnerTy : STy->elements()) {
if (isa<PointerType>(InnerTy)) return true;
if (isa<StructType>(InnerTy) || isa<ArrayType>(InnerTy) ||
if (--Limit == 0) return true;
} while (!Types.empty());
return false;
/// Given a value that is stored to a global but never read, determine whether
/// it's safe to remove the store and the chain of computation that feeds the
/// store.
static bool IsSafeComputationToRemove(
Value *V, function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
do {
if (isa<Constant>(V))
return true;
if (!V->hasOneUse())
return false;
if (isa<LoadInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V) ||
return false;
if (isAllocationFn(V, GetTLI))
return true;
Instruction *I = cast<Instruction>(V);
if (I->mayHaveSideEffects())
return false;
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I)) {
if (!GEP->hasAllConstantIndices())
return false;
} else if (I->getNumOperands() != 1) {
return false;
V = I->getOperand(0);
} while (true);
/// This GV is a pointer root. Loop over all users of the global and clean up
/// any that obviously don't assign the global a value that isn't dynamically
/// allocated.
static bool
CleanupPointerRootUsers(GlobalVariable *GV,
function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
// A brief explanation of leak checkers. The goal is to find bugs where
// pointers are forgotten, causing an accumulating growth in memory
// usage over time. The common strategy for leak checkers is to explicitly
// allow the memory pointed to by globals at exit. This is popular because it
// also solves another problem where the main thread of a C++ program may shut
// down before other threads that are still expecting to use those globals. To
// handle that case, we expect the program may create a singleton and never
// destroy it.
bool Changed = false;
// If Dead[n].first is the only use of a malloc result, we can delete its
// chain of computation and the store to the global in Dead[n].second.
SmallVector<std::pair<Instruction *, Instruction *>, 32> Dead;
SmallVector<User *> Worklist(GV->users());
// Constants can't be pointers to dynamically allocated memory.
while (!Worklist.empty()) {
User *U = Worklist.pop_back_val();
if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
Value *V = SI->getValueOperand();
if (isa<Constant>(V)) {
Changed = true;
} else if (Instruction *I = dyn_cast<Instruction>(V)) {
if (I->hasOneUse())
Dead.push_back(std::make_pair(I, SI));
} else if (MemSetInst *MSI = dyn_cast<MemSetInst>(U)) {
if (isa<Constant>(MSI->getValue())) {
Changed = true;
} else if (Instruction *I = dyn_cast<Instruction>(MSI->getValue())) {
if (I->hasOneUse())
Dead.push_back(std::make_pair(I, MSI));
} else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(U)) {
GlobalVariable *MemSrc = dyn_cast<GlobalVariable>(MTI->getSource());
if (MemSrc && MemSrc->isConstant()) {
Changed = true;
} else if (Instruction *I = dyn_cast<Instruction>(MTI->getSource())) {
if (I->hasOneUse())
Dead.push_back(std::make_pair(I, MTI));
} else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U)) {
if (isa<GEPOperator>(CE))
append_range(Worklist, CE->users());
for (int i = 0, e = Dead.size(); i != e; ++i) {
if (IsSafeComputationToRemove(Dead[i].first, GetTLI)) {
Instruction *I = Dead[i].first;
do {
if (isAllocationFn(I, GetTLI))
Instruction *J = dyn_cast<Instruction>(I->getOperand(0));
if (!J)
I = J;
} while (true);
Changed = true;
return Changed;
/// We just marked GV constant. Loop over all users of the global, cleaning up
/// the obvious ones. This is largely just a quick scan over the use list to
/// clean up the easy and obvious cruft. This returns true if it made a change.
static bool CleanupConstantGlobalUsers(GlobalVariable *GV,
const DataLayout &DL) {
Constant *Init = GV->getInitializer();
SmallVector<User *, 8> WorkList(GV->users());
SmallPtrSet<User *, 8> Visited;
bool Changed = false;
SmallVector<WeakTrackingVH> MaybeDeadInsts;
auto EraseFromParent = [&](Instruction *I) {
for (Value *Op : I->operands())
if (auto *OpI = dyn_cast<Instruction>(Op))
Changed = true;
while (!WorkList.empty()) {
User *U = WorkList.pop_back_val();
if (!Visited.insert(U).second)
if (auto *BO = dyn_cast<BitCastOperator>(U))
append_range(WorkList, BO->users());
if (auto *ASC = dyn_cast<AddrSpaceCastOperator>(U))
append_range(WorkList, ASC->users());
else if (auto *GEP = dyn_cast<GEPOperator>(U))
append_range(WorkList, GEP->users());
else if (auto *LI = dyn_cast<LoadInst>(U)) {
// A load from a uniform value is always the same, regardless of any
// applied offset.
Type *Ty = LI->getType();
if (Constant *Res = ConstantFoldLoadFromUniformValue(Init, Ty, DL)) {
Value *PtrOp = LI->getPointerOperand();
APInt Offset(DL.getIndexTypeSizeInBits(PtrOp->getType()), 0);
PtrOp = PtrOp->stripAndAccumulateConstantOffsets(
DL, Offset, /* AllowNonInbounds */ true);
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(PtrOp)) {
if (II->getIntrinsicID() == Intrinsic::threadlocal_address)
PtrOp = II->getArgOperand(0);
if (PtrOp == GV) {
if (auto *Value = ConstantFoldLoadFromConst(Init, Ty, Offset, DL)) {
} else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
// Store must be unreachable or storing Init into the global.
} else if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(U)) { // memset/cpy/mv
if (getUnderlyingObject(MI->getRawDest()) == GV)
} else if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
if (II->getIntrinsicID() == Intrinsic::threadlocal_address)
append_range(WorkList, II->users());
Changed |=
return Changed;
/// Part of the global at a specific offset, which is only accessed through
/// loads and stores with the given type.
struct GlobalPart {
Type *Ty;
Constant *Initializer = nullptr;
bool IsLoaded = false;
bool IsStored = false;
/// Look at all uses of the global and determine which (offset, type) pairs it
/// can be split into.
static bool collectSRATypes(DenseMap<uint64_t, GlobalPart> &Parts,
GlobalVariable *GV, const DataLayout &DL) {
SmallVector<Use *, 16> Worklist;
SmallPtrSet<Use *, 16> Visited;
auto AppendUses = [&](Value *V) {
for (Use &U : V->uses())
if (Visited.insert(&U).second)
while (!Worklist.empty()) {
Use *U = Worklist.pop_back_val();
User *V = U->getUser();
auto *GEP = dyn_cast<GEPOperator>(V);
if (isa<BitCastOperator>(V) || isa<AddrSpaceCastOperator>(V) ||
(GEP && GEP->hasAllConstantIndices())) {
if (Value *Ptr = getLoadStorePointerOperand(V)) {
// This is storing the global address into somewhere, not storing into
// the global.
if (isa<StoreInst>(V) && U->getOperandNo() == 0)
return false;
APInt Offset(DL.getIndexTypeSizeInBits(Ptr->getType()), 0);
Ptr = Ptr->stripAndAccumulateConstantOffsets(DL, Offset,
/* AllowNonInbounds */ true);
if (Ptr != GV || Offset.getActiveBits() >= 64)
return false;
// TODO: We currently require that all accesses at a given offset must
// use the same type. This could be relaxed.
Type *Ty = getLoadStoreType(V);
const auto &[It, Inserted] =
Parts.try_emplace(Offset.getZExtValue(), GlobalPart{Ty});
if (Ty != It->second.Ty)
return false;
if (Inserted) {
It->second.Initializer =
ConstantFoldLoadFromConst(GV->getInitializer(), Ty, Offset, DL);
if (!It->second.Initializer) {
LLVM_DEBUG(dbgs() << "Global SRA: Failed to evaluate initializer of "
<< *GV << " with type " << *Ty << " at offset "
<< Offset.getZExtValue());
return false;
// Scalable types not currently supported.
if (Ty->isScalableTy())
return false;
auto IsStored = [](Value *V, Constant *Initializer) {
auto *SI = dyn_cast<StoreInst>(V);
if (!SI)
return false;
Constant *StoredConst = dyn_cast<Constant>(SI->getOperand(0));
if (!StoredConst)
return true;
// Don't consider stores that only write the initializer value.
return Initializer != StoredConst;
It->second.IsLoaded |= isa<LoadInst>(V);
It->second.IsStored |= IsStored(V, It->second.Initializer);
// Ignore dead constant users.
if (auto *C = dyn_cast<Constant>(V)) {
if (!isSafeToDestroyConstant(C))
return false;
// Unknown user.
return false;
return true;
/// Copy over the debug info for a variable to its SRA replacements.
static void transferSRADebugInfo(GlobalVariable *GV, GlobalVariable *NGV,
uint64_t FragmentOffsetInBits,
uint64_t FragmentSizeInBits,
uint64_t VarSize) {
SmallVector<DIGlobalVariableExpression *, 1> GVs;
for (auto *GVE : GVs) {
DIVariable *Var = GVE->getVariable();
DIExpression *Expr = GVE->getExpression();
int64_t CurVarOffsetInBytes = 0;
uint64_t CurVarOffsetInBits = 0;
uint64_t FragmentEndInBits = FragmentOffsetInBits + FragmentSizeInBits;
// Calculate the offset (Bytes), Continue if unknown.
if (!Expr->extractIfOffset(CurVarOffsetInBytes))
// Ignore negative offset.
if (CurVarOffsetInBytes < 0)
// Convert offset to bits.
CurVarOffsetInBits = CHAR_BIT * (uint64_t)CurVarOffsetInBytes;
// Current var starts after the fragment, ignore.
if (CurVarOffsetInBits >= FragmentEndInBits)
uint64_t CurVarSize = Var->getType()->getSizeInBits();
uint64_t CurVarEndInBits = CurVarOffsetInBits + CurVarSize;
// Current variable ends before start of fragment, ignore.
if (CurVarSize != 0 && /* CurVarSize is known */
CurVarEndInBits <= FragmentOffsetInBits)
// Current variable fits in (not greater than) the fragment,
// does not need fragment expression.
if (CurVarSize != 0 && /* CurVarSize is known */
CurVarOffsetInBits >= FragmentOffsetInBits &&
CurVarEndInBits <= FragmentEndInBits) {
uint64_t CurVarOffsetInFragment =
(CurVarOffsetInBits - FragmentOffsetInBits) / 8;
if (CurVarOffsetInFragment != 0)
Expr = DIExpression::get(Expr->getContext(), {dwarf::DW_OP_plus_uconst,
Expr = DIExpression::get(Expr->getContext(), {});
auto *NGVE =
DIGlobalVariableExpression::get(GVE->getContext(), Var, Expr);
// Current variable does not fit in single fragment,
// emit a fragment expression.
if (FragmentSizeInBits < VarSize) {
if (CurVarOffsetInBits > FragmentOffsetInBits)
uint64_t CurVarFragmentOffsetInBits =
FragmentOffsetInBits - CurVarOffsetInBits;
uint64_t CurVarFragmentSizeInBits = FragmentSizeInBits;
if (CurVarSize != 0 && CurVarEndInBits < FragmentEndInBits)
CurVarFragmentSizeInBits -= (FragmentEndInBits - CurVarEndInBits);
if (CurVarOffsetInBits)
Expr = DIExpression::get(Expr->getContext(), {});
if (auto E = DIExpression::createFragmentExpression(
Expr, CurVarFragmentOffsetInBits, CurVarFragmentSizeInBits))
Expr = *E;
auto *NGVE = DIGlobalVariableExpression::get(GVE->getContext(), Var, Expr);
/// Perform scalar replacement of aggregates on the specified global variable.
/// This opens the door for other optimizations by exposing the behavior of the
/// program in a more fine-grained way. We have determined that this
/// transformation is safe already. We return the first global variable we
/// insert so that the caller can reprocess it.
static GlobalVariable *SRAGlobal(GlobalVariable *GV, const DataLayout &DL) {
// Collect types to split into.
DenseMap<uint64_t, GlobalPart> Parts;
if (!collectSRATypes(Parts, GV, DL) || Parts.empty())
return nullptr;
// Make sure we don't SRA back to the same type.
if (Parts.size() == 1 && Parts.begin()->second.Ty == GV->getValueType())
return nullptr;
// Don't perform SRA if we would have to split into many globals. Ignore
// parts that are either only loaded or only stored, because we expect them
// to be optimized away.
unsigned NumParts = count_if(Parts, [](const auto &Pair) {
return Pair.second.IsLoaded && Pair.second.IsStored;
if (NumParts > 16)
return nullptr;
// Sort by offset.
SmallVector<std::tuple<uint64_t, Type *, Constant *>, 16> TypesVector;
for (const auto &Pair : Parts) {
{Pair.first, Pair.second.Ty, Pair.second.Initializer});
sort(TypesVector, llvm::less_first());
// Check that the types are non-overlapping.
uint64_t Offset = 0;
for (const auto &[OffsetForTy, Ty, _] : TypesVector) {
// Overlaps with previous type.
if (OffsetForTy < Offset)
return nullptr;
Offset = OffsetForTy + DL.getTypeAllocSize(Ty);
// Some accesses go beyond the end of the global, don't bother.
if (Offset > DL.getTypeAllocSize(GV->getValueType()))
return nullptr;
LLVM_DEBUG(dbgs() << "PERFORMING GLOBAL SRA ON: " << *GV << "\n");
// Get the alignment of the global, either explicit or target-specific.
Align StartAlignment =
DL.getValueOrABITypeAlignment(GV->getAlign(), GV->getValueType());
uint64_t VarSize = DL.getTypeSizeInBits(GV->getValueType());
// Create replacement globals.
DenseMap<uint64_t, GlobalVariable *> NewGlobals;
unsigned NameSuffix = 0;
for (auto &[OffsetForTy, Ty, Initializer] : TypesVector) {
GlobalVariable *NGV = new GlobalVariable(
*GV->getParent(), Ty, false, GlobalVariable::InternalLinkage,
Initializer, GV->getName() + "." + Twine(NameSuffix++), GV,
GV->getThreadLocalMode(), GV->getAddressSpace());
NewGlobals.insert({OffsetForTy, NGV});
// Calculate the known alignment of the field. If the original aggregate
// had 256 byte alignment for example, something might depend on that:
// propagate info to each field.
Align NewAlign = commonAlignment(StartAlignment, OffsetForTy);
if (NewAlign > DL.getABITypeAlign(Ty))
// Copy over the debug info for the variable.
transferSRADebugInfo(GV, NGV, OffsetForTy * 8,
DL.getTypeAllocSizeInBits(Ty), VarSize);
// Replace uses of the original global with uses of the new global.
SmallVector<Value *, 16> Worklist;
SmallPtrSet<Value *, 16> Visited;
SmallVector<WeakTrackingVH, 16> DeadInsts;
auto AppendUsers = [&](Value *V) {
for (User *U : V->users())
if (Visited.insert(U).second)
while (!Worklist.empty()) {
Value *V = Worklist.pop_back_val();
if (isa<BitCastOperator>(V) || isa<AddrSpaceCastOperator>(V) ||
isa<GEPOperator>(V)) {
if (isa<Instruction>(V))
if (Value *Ptr = getLoadStorePointerOperand(V)) {
APInt Offset(DL.getIndexTypeSizeInBits(Ptr->getType()), 0);
Ptr = Ptr->stripAndAccumulateConstantOffsets(DL, Offset,
/* AllowNonInbounds */ true);
assert(Ptr == GV && "Load/store must be from/to global");
GlobalVariable *NGV = NewGlobals[Offset.getZExtValue()];
assert(NGV && "Must have replacement global for this offset");
// Update the pointer operand and recalculate alignment.
Align PrefAlign = DL.getPrefTypeAlign(getLoadStoreType(V));
Align NewAlign =
getOrEnforceKnownAlignment(NGV, PrefAlign, DL, cast<Instruction>(V));
if (auto *LI = dyn_cast<LoadInst>(V)) {
LI->setOperand(0, NGV);
} else {
auto *SI = cast<StoreInst>(V);
SI->setOperand(1, NGV);
assert(isa<Constant>(V) && isSafeToDestroyConstant(cast<Constant>(V)) &&
"Other users can only be dead constants");
// Delete old instructions and global.
assert(NewGlobals.size() > 0);
return NewGlobals.begin()->second;
/// Return true if all users of the specified value will trap if the value is
/// dynamically null. PHIs keeps track of any phi nodes we've seen to avoid
/// reprocessing them.
static bool AllUsesOfValueWillTrapIfNull(const Value *V,
SmallPtrSetImpl<const PHINode*> &PHIs) {
for (const User *U : V->users()) {
if (const Instruction *I = dyn_cast<Instruction>(U)) {
// If null pointer is considered valid, then all uses are non-trapping.
// Non address-space 0 globals have already been pruned by the caller.
if (NullPointerIsDefined(I->getFunction()))
return false;
if (isa<LoadInst>(U)) {
// Will trap.
} else if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
if (SI->getOperand(0) == V) {
return false; // Storing the value.
} else if (const CallInst *CI = dyn_cast<CallInst>(U)) {
if (CI->getCalledOperand() != V) {
return false; // Not calling the ptr
} else if (const InvokeInst *II = dyn_cast<InvokeInst>(U)) {
if (II->getCalledOperand() != V) {
return false; // Not calling the ptr
} else if (const AddrSpaceCastInst *CI = dyn_cast<AddrSpaceCastInst>(U)) {
if (!AllUsesOfValueWillTrapIfNull(CI, PHIs))
return false;
} else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(U)) {
if (!AllUsesOfValueWillTrapIfNull(GEPI, PHIs)) return false;
} else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
// If we've already seen this phi node, ignore it, it has already been
// checked.
if (PHIs.insert(PN).second && !AllUsesOfValueWillTrapIfNull(PN, PHIs))
return false;
} else if (isa<ICmpInst>(U) &&
!ICmpInst::isSigned(cast<ICmpInst>(U)->getPredicate()) &&
isa<LoadInst>(U->getOperand(0)) &&
isa<ConstantPointerNull>(U->getOperand(1))) {
->stripPointerCasts()) &&
"Should be GlobalVariable");
// This and only this kind of non-signed ICmpInst is to be replaced with
// the comparing of the value of the created global init bool later in
// optimizeGlobalAddressOfAllocation for the global variable.
} else {
return false;
return true;
/// Return true if all uses of any loads from GV will trap if the loaded value
/// is null. Note that this also permits comparisons of the loaded value
/// against null, as a special case.
static bool allUsesOfLoadedValueWillTrapIfNull(const GlobalVariable *GV) {
SmallVector<const Value *, 4> Worklist;
while (!Worklist.empty()) {
const Value *P = Worklist.pop_back_val();
for (const auto *U : P->users()) {
if (auto *LI = dyn_cast<LoadInst>(U)) {
SmallPtrSet<const PHINode *, 8> PHIs;
if (!AllUsesOfValueWillTrapIfNull(LI, PHIs))
return false;
} else if (auto *SI = dyn_cast<StoreInst>(U)) {
// Ignore stores to the global.
if (SI->getPointerOperand() != P)
return false;
} else if (auto *CE = dyn_cast<ConstantExpr>(U)) {
if (CE->stripPointerCasts() != GV)
return false;
// Check further the ConstantExpr.
} else {
// We don't know or understand this user, bail out.
return false;
return true;
/// Get all the loads/store uses for global variable \p GV.
static void allUsesOfLoadAndStores(GlobalVariable *GV,
SmallVector<Value *, 4> &Uses) {
SmallVector<Value *, 4> Worklist;
while (!Worklist.empty()) {
auto *P = Worklist.pop_back_val();
for (auto *U : P->users()) {
if (auto *CE = dyn_cast<ConstantExpr>(U)) {
assert((isa<LoadInst>(U) || isa<StoreInst>(U)) &&
"Expect only load or store instructions");
static bool OptimizeAwayTrappingUsesOfValue(Value *V, Constant *NewV) {
bool Changed = false;
for (auto UI = V->user_begin(), E = V->user_end(); UI != E; ) {
Instruction *I = cast<Instruction>(*UI++);
// Uses are non-trapping if null pointer is considered valid.
// Non address-space 0 globals are already pruned by the caller.
if (NullPointerIsDefined(I->getFunction()))
return false;
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
LI->setOperand(0, NewV);
Changed = true;
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
if (SI->getOperand(1) == V) {
SI->setOperand(1, NewV);
Changed = true;
} else if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
CallBase *CB = cast<CallBase>(I);
if (CB->getCalledOperand() == V) {
// Calling through the pointer! Turn into a direct call, but be careful
// that the pointer is not also being passed as an argument.
Changed = true;
bool PassedAsArg = false;
for (unsigned i = 0, e = CB->arg_size(); i != e; ++i)
if (CB->getArgOperand(i) == V) {
PassedAsArg = true;
CB->setArgOperand(i, NewV);
if (PassedAsArg) {
// Being passed as an argument also. Be careful to not invalidate UI!
UI = V->user_begin();
} else if (AddrSpaceCastInst *CI = dyn_cast<AddrSpaceCastInst>(I)) {
Changed |= OptimizeAwayTrappingUsesOfValue(
CI, ConstantExpr::getAddrSpaceCast(NewV, CI->getType()));
if (CI->use_empty()) {
Changed = true;
} else if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(I)) {
// Should handle GEP here.
SmallVector<Constant*, 8> Idxs;
for (User::op_iterator i = GEPI->op_begin() + 1, e = GEPI->op_end();
i != e; ++i)
if (Constant *C = dyn_cast<Constant>(*i))
if (Idxs.size() == GEPI->getNumOperands()-1)
Changed |= OptimizeAwayTrappingUsesOfValue(
GEPI, ConstantExpr::getGetElementPtr(GEPI->getSourceElementType(),
NewV, Idxs));
if (GEPI->use_empty()) {
Changed = true;
return Changed;
/// The specified global has only one non-null value stored into it. If there
/// are uses of the loaded value that would trap if the loaded value is
/// dynamically null, then we know that they cannot be reachable with a null
/// optimize away the load.
static bool OptimizeAwayTrappingUsesOfLoads(
GlobalVariable *GV, Constant *LV, const DataLayout &DL,
function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
bool Changed = false;
// Keep track of whether we are able to remove all the uses of the global
// other than the store that defines it.
bool AllNonStoreUsesGone = true;
// Replace all uses of loads with uses of uses of the stored value.
for (User *GlobalUser : llvm::make_early_inc_range(GV->users())) {
if (LoadInst *LI = dyn_cast<LoadInst>(GlobalUser)) {
Changed |= OptimizeAwayTrappingUsesOfValue(LI, LV);
// If we were able to delete all uses of the loads
if (LI->use_empty()) {
Changed = true;
} else {
AllNonStoreUsesGone = false;
} else if (isa<StoreInst>(GlobalUser)) {
// Ignore the store that stores "LV" to the global.
assert(GlobalUser->getOperand(1) == GV &&
"Must be storing *to* the global");
} else {
AllNonStoreUsesGone = false;
// If we get here we could have other crazy uses that are transitively
// loaded.
assert((isa<PHINode>(GlobalUser) || isa<SelectInst>(GlobalUser) ||
isa<ConstantExpr>(GlobalUser) || isa<CmpInst>(GlobalUser) ||
isa<BitCastInst>(GlobalUser) ||
isa<GetElementPtrInst>(GlobalUser) ||
isa<AddrSpaceCastInst>(GlobalUser)) &&
"Only expect load and stores!");
if (Changed) {
<< "\n");
// If we nuked all of the loads, then none of the stores are needed either,
// nor is the global.
if (AllNonStoreUsesGone) {
if (isLeakCheckerRoot(GV)) {
Changed |= CleanupPointerRootUsers(GV, GetTLI);
} else {
Changed = true;
CleanupConstantGlobalUsers(GV, DL);
if (GV->use_empty()) {
LLVM_DEBUG(dbgs() << " *** GLOBAL NOW DEAD!\n");
Changed = true;
return Changed;
/// Walk the use list of V, constant folding all of the instructions that are
/// foldable.
static void ConstantPropUsersOf(Value *V, const DataLayout &DL,
TargetLibraryInfo *TLI) {
for (Value::user_iterator UI = V->user_begin(), E = V->user_end(); UI != E; )
if (Instruction *I = dyn_cast<Instruction>(*UI++))
if (Constant *NewC = ConstantFoldInstruction(I, DL, TLI)) {
// Advance UI to the next non-I use to avoid invalidating it!
// Instructions could multiply use V.
while (UI != E && *UI == I)
if (isInstructionTriviallyDead(I, TLI))
/// This function takes the specified global variable, and transforms the
/// program as if it always contained the result of the specified malloc.
/// Because it is always the result of the specified malloc, there is no reason
/// to actually DO the malloc. Instead, turn the malloc into a global, and any
/// loads of GV as uses of the new global.
static GlobalVariable *
OptimizeGlobalAddressOfAllocation(GlobalVariable *GV, CallInst *CI,
uint64_t AllocSize, Constant *InitVal,
const DataLayout &DL,
TargetLibraryInfo *TLI) {
LLVM_DEBUG(errs() << "PROMOTING GLOBAL: " << *GV << " CALL = " << *CI
<< '\n');
// Create global of type [AllocSize x i8].
Type *GlobalType = ArrayType::get(Type::getInt8Ty(GV->getContext()),
// Create the new global variable. The contents of the allocated memory is
// undefined initially, so initialize with an undef value.
GlobalVariable *NewGV = new GlobalVariable(
*GV->getParent(), GlobalType, false, GlobalValue::InternalLinkage,
UndefValue::get(GlobalType), GV->getName() + ".body", nullptr,
// Initialize the global at the point of the original call. Note that this
// is a different point from the initialization referred to below for the
// nullability handling. Sublety: We have not proven the original global was
// only initialized once. As such, we can not fold this into the initializer
// of the new global as may need to re-init the storage multiple times.
if (!isa<UndefValue>(InitVal)) {
IRBuilder<> Builder(CI->getNextNode());
// TODO: Use alignment above if align!=1
Builder.CreateMemSet(NewGV, InitVal, AllocSize, std::nullopt);
// Update users of the allocation to use the new global instead.
// If there is a comparison against null, we will insert a global bool to
// keep track of whether the global was initialized yet or not.
GlobalVariable *InitBool =
new GlobalVariable(Type::getInt1Ty(GV->getContext()), false,
GV->getName()+".init", GV->getThreadLocalMode());
bool InitBoolUsed = false;
// Loop over all instruction uses of GV, processing them in turn.
SmallVector<Value *, 4> Guses;
allUsesOfLoadAndStores(GV, Guses);
for (auto *U : Guses) {
if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
// The global is initialized when the store to it occurs. If the stored
// value is null value, the global bool is set to false, otherwise true.
new StoreInst(ConstantInt::getBool(
InitBool, false, Align(1), SI->getOrdering(),
SI->getSyncScopeID(), SI->getIterator());
LoadInst *LI = cast<LoadInst>(U);
while (!LI->use_empty()) {
Use &LoadUse = *LI->use_begin();
ICmpInst *ICI = dyn_cast<ICmpInst>(LoadUse.getUser());
if (!ICI) {
// Replace the cmp X, 0 with a use of the bool value.
Value *LV = new LoadInst(InitBool->getValueType(), InitBool,
InitBool->getName() + ".val", false, Align(1),
LI->getOrdering(), LI->getSyncScopeID(),
InitBoolUsed = true;
switch (ICI->getPredicate()) {
default: llvm_unreachable("Unknown ICmp Predicate!");
case ICmpInst::ICMP_ULT: // X < null -> always false
LV = ConstantInt::getFalse(GV->getContext());
case ICmpInst::ICMP_UGE: // X >= null -> always true
LV = ConstantInt::getTrue(GV->getContext());
case ICmpInst::ICMP_ULE:
case ICmpInst::ICMP_EQ:
LV = BinaryOperator::CreateNot(LV, "notinit", ICI->getIterator());
case ICmpInst::ICMP_NE:
case ICmpInst::ICMP_UGT:
break; // no change.
// If the initialization boolean was used, insert it, otherwise delete it.
if (!InitBoolUsed) {
while (!InitBool->use_empty()) // Delete initializations
delete InitBool;
} else
GV->getParent()->insertGlobalVariable(GV->getIterator(), InitBool);
// Now the GV is dead, nuke it and the allocation..
// To further other optimizations, loop over all users of NewGV and try to
// constant prop them. This will promote GEP instructions with constant
// indices into GEP constant-exprs, which will allow global-opt to hack on it.
ConstantPropUsersOf(NewGV, DL, TLI);
return NewGV;
/// Scan the use-list of GV checking to make sure that there are no complex uses
/// of GV. We permit simple things like dereferencing the pointer, but not
/// storing through the address, unless it is to the specified global.
static bool
valueIsOnlyUsedLocallyOrStoredToOneGlobal(const CallInst *CI,
const GlobalVariable *GV) {
SmallPtrSet<const Value *, 4> Visited;
SmallVector<const Value *, 4> Worklist;
while (!Worklist.empty()) {
const Value *V = Worklist.pop_back_val();
if (!Visited.insert(V).second)
for (const Use &VUse : V->uses()) {
const User *U = VUse.getUser();
if (isa<LoadInst>(U) || isa<CmpInst>(U))
continue; // Fine, ignore.
if (auto *SI = dyn_cast<StoreInst>(U)) {
if (SI->getValueOperand() == V &&
SI->getPointerOperand()->stripPointerCasts() != GV)
return false; // Storing the pointer not into GV... bad.
continue; // Otherwise, storing through it, or storing into GV... fine.
if (auto *BCI = dyn_cast<BitCastInst>(U)) {
if (auto *GEPI = dyn_cast<GetElementPtrInst>(U)) {
return false;
return true;
/// If we have a global that is only initialized with a fixed size allocation
/// try to transform the program to use global memory instead of heap
/// allocated memory. This eliminates dynamic allocation, avoids an indirection
/// accessing the data, and exposes the resultant global to further GlobalOpt.
static bool tryToOptimizeStoreOfAllocationToGlobal(GlobalVariable *GV,
CallInst *CI,
const DataLayout &DL,
TargetLibraryInfo *TLI) {
if (!isRemovableAlloc(CI, TLI))
// Must be able to remove the call when we get done..
return false;
Type *Int8Ty = Type::getInt8Ty(CI->getFunction()->getContext());
Constant *InitVal = getInitialValueOfAllocation(CI, TLI, Int8Ty);
if (!InitVal)
// Must be able to emit a memset for initialization
return false;
uint64_t AllocSize;
if (!getObjectSize(CI, AllocSize, DL, TLI, ObjectSizeOpts()))
return false;
// Restrict this transformation to only working on small allocations
// (2048 bytes currently), as we don't want to introduce a 16M global or
// something.
if (AllocSize >= 2048)
return false;
// We can't optimize this global unless all uses of it are *known* to be
// of the malloc value, not of the null initializer value (consider a use
// that compares the global's value against zero to see if the malloc has
// been reached). To do this, we check to see if all uses of the global
// would trap if the global were null: this proves that they must all
// happen after the malloc.
if (!allUsesOfLoadedValueWillTrapIfNull(GV))
return false;
// We can't optimize this if the malloc itself is used in a complex way,
// for example, being stored into multiple globals. This allows the
// malloc to be stored into the specified global, loaded, gep, icmp'd.
// These are all things we could transform to using the global for.
if (!valueIsOnlyUsedLocallyOrStoredToOneGlobal(CI, GV))
return false;
OptimizeGlobalAddressOfAllocation(GV, CI, AllocSize, InitVal, DL, TLI);
return true;
// Try to optimize globals based on the knowledge that only one value (besides
// its initializer) is ever stored to the global.
static bool
optimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal,
const DataLayout &DL,
function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
// Ignore no-op GEPs and bitcasts.
StoredOnceVal = StoredOnceVal->stripPointerCasts();
// If we are dealing with a pointer global that is initialized to null and
// only has one (non-null) value stored into it, then we can optimize any
// users of the loaded value (often calls and loads) that would trap if the
// value was null.
if (GV->getInitializer()->getType()->isPointerTy() &&
GV->getInitializer()->isNullValue() &&
StoredOnceVal->getType()->isPointerTy() &&
nullptr /* F */,
GV->getInitializer()->getType()->getPointerAddressSpace())) {
if (Constant *SOVC = dyn_cast<Constant>(StoredOnceVal)) {
// Optimize away any trapping uses of the loaded value.
if (OptimizeAwayTrappingUsesOfLoads(GV, SOVC, DL, GetTLI))
return true;
} else if (isAllocationFn(StoredOnceVal, GetTLI)) {
if (auto *CI = dyn_cast<CallInst>(StoredOnceVal)) {
auto *TLI = &GetTLI(*CI->getFunction());
if (tryToOptimizeStoreOfAllocationToGlobal(GV, CI, DL, TLI))
return true;
return false;
/// At this point, we have learned that the only two values ever stored into GV
/// are its initializer and OtherVal. See if we can shrink the global into a
/// boolean and select between the two values whenever it is used. This exposes
/// the values to other scalar optimizations.
static bool TryToShrinkGlobalToBoolean(GlobalVariable *GV, Constant *OtherVal) {
Type *GVElType = GV->getValueType();
// If GVElType is already i1, it is already shrunk. If the type of the GV is
// an FP value, pointer or vector, don't do this optimization because a select
// between them is very expensive and unlikely to lead to later
// simplification. In these cases, we typically end up with "cond ? v1 : v2"
// where v1 and v2 both require constant pool loads, a big loss.
if (GVElType == Type::getInt1Ty(GV->getContext()) ||
GVElType->isFloatingPointTy() ||
GVElType->isPointerTy() || GVElType->isVectorTy())
return false;
// Walk the use list of the global seeing if all the uses are load or store.
// If there is anything else, bail out.
for (User *U : GV->users()) {
if (!isa<LoadInst>(U) && !isa<StoreInst>(U))
return false;
if (getLoadStoreType(U) != GVElType)
return false;
LLVM_DEBUG(dbgs() << " *** SHRINKING TO BOOL: " << *GV << "\n");
// Create the new global, initializing it to false.
GlobalVariable *NewGV = new GlobalVariable(Type::getInt1Ty(GV->getContext()),
GV->getParent()->insertGlobalVariable(GV->getIterator(), NewGV);
Constant *InitVal = GV->getInitializer();
assert(InitVal->getType() != Type::getInt1Ty(GV->getContext()) &&
"No reason to shrink to bool!");
SmallVector<DIGlobalVariableExpression *, 1> GVs;
// If initialized to zero and storing one into the global, we can use a cast
// instead of a select to synthesize the desired value.
bool IsOneZero = false;
bool EmitOneOrZero = true;
auto *CI = dyn_cast<ConstantInt>(OtherVal);
if (CI && CI->getValue().getActiveBits() <= 64) {
IsOneZero = InitVal->isNullValue() && CI->isOne();
auto *CIInit = dyn_cast<ConstantInt>(GV->getInitializer());
if (CIInit && CIInit->getValue().getActiveBits() <= 64) {
uint64_t ValInit = CIInit->getZExtValue();
uint64_t ValOther = CI->getZExtValue();
uint64_t ValMinus = ValOther - ValInit;
for(auto *GVe : GVs){
DIGlobalVariable *DGV = GVe->getVariable();
DIExpression *E = GVe->getExpression();
const DataLayout &DL = GV->getParent()->getDataLayout();
unsigned SizeInOctets =
DL.getTypeAllocSizeInBits(NewGV->getValueType()) / 8;
// It is expected that the address of global optimized variable is on
// top of the stack. After optimization, value of that variable will
// be ether 0 for initial value or 1 for other value. The following
// expression should return constant integer value depending on the
// value at global object address:
// val * (ValOther - ValInit) + ValInit:
// DW_OP_deref DW_OP_constu <ValMinus>
// DW_OP_mul DW_OP_constu <ValInit> DW_OP_plus DW_OP_stack_value
SmallVector<uint64_t, 12> Ops = {
dwarf::DW_OP_deref_size, SizeInOctets,
dwarf::DW_OP_constu, ValMinus,
dwarf::DW_OP_mul, dwarf::DW_OP_constu, ValInit,
bool WithStackValue = true;
E = DIExpression::prependOpcodes(E, Ops, WithStackValue);
DIGlobalVariableExpression *DGVE =
DIGlobalVariableExpression::get(NewGV->getContext(), DGV, E);
EmitOneOrZero = false;
if (EmitOneOrZero) {
// FIXME: This will only emit address for debugger on which will
// be written only 0 or 1.
for(auto *GV : GVs)
while (!GV->use_empty()) {
Instruction *UI = cast<Instruction>(GV->user_back());
if (StoreInst *SI = dyn_cast<StoreInst>(UI)) {
// Change the store into a boolean store.
bool StoringOther = SI->getOperand(0) == OtherVal;
// Only do this if we weren't storing a loaded value.
Value *StoreVal;
if (StoringOther || SI->getOperand(0) == InitVal) {
StoreVal = ConstantInt::get(Type::getInt1Ty(GV->getContext()),
} else {
// Otherwise, we are storing a previously loaded copy. To do this,
// change the copy from copying the original value to just copying the
// bool.
Instruction *StoredVal = cast<Instruction>(SI->getOperand(0));
// If we've already replaced the input, StoredVal will be a cast or
// select instruction. If not, it will be a load of the original
// global.
if (LoadInst *LI = dyn_cast<LoadInst>(StoredVal)) {
assert(LI->getOperand(0) == GV && "Not a copy!");
// Insert a new load, to preserve the saved value.
StoreVal =
new LoadInst(NewGV->getValueType(), NewGV, LI->getName() + ".b",
false, Align(1), LI->getOrdering(),
LI->getSyncScopeID(), LI->getIterator());
} else {
assert((isa<CastInst>(StoredVal) || isa<SelectInst>(StoredVal)) &&
"This is not a form that we understand!");
StoreVal = StoredVal->getOperand(0);
assert(isa<LoadInst>(StoreVal) && "Not a load of NewGV!");
StoreInst *NSI =
new StoreInst(StoreVal, NewGV, false, Align(1), SI->getOrdering(),
SI->getSyncScopeID(), SI->getIterator());
} else {
// Change the load into a load of bool then a select.
LoadInst *LI = cast<LoadInst>(UI);
LoadInst *NLI = new LoadInst(
NewGV->getValueType(), NewGV, LI->getName() + ".b", false, Align(1),
LI->getOrdering(), LI->getSyncScopeID(), LI->getIterator());
Instruction *NSI;
if (IsOneZero)
NSI = new ZExtInst(NLI, LI->getType(), "", LI->getIterator());
NSI = SelectInst::Create(NLI, OtherVal, InitVal, "", LI->getIterator());
// Since LI is split into two instructions, NLI and NSI both inherit the
// same DebugLoc
// Retain the name of the old global variable. People who are debugging their
// programs may expect these variables to be named the same.
return true;
static bool
deleteIfDead(GlobalValue &GV,
SmallPtrSetImpl<const Comdat *> &NotDiscardableComdats,
function_ref<void(Function &)> DeleteFnCallback = nullptr) {
if (!GV.isDiscardableIfUnused() && !GV.isDeclaration())
return false;
if (const Comdat *C = GV.getComdat())
if (!GV.hasLocalLinkage() && NotDiscardableComdats.count(C))
return false;
bool Dead;
if (auto *F = dyn_cast<Function>(&GV))
Dead = (F->isDeclaration() && F->use_empty()) || F->isDefTriviallyDead();
Dead = GV.use_empty();
if (!Dead)
return false;
LLVM_DEBUG(dbgs() << "GLOBAL DEAD: " << GV << "\n");
if (auto *F = dyn_cast<Function>(&GV)) {
if (DeleteFnCallback)
return true;
static bool isPointerValueDeadOnEntryToFunction(
const Function *F, GlobalValue *GV,
function_ref<DominatorTree &(Function &)> LookupDomTree) {
// Find all uses of GV. We expect them all to be in F, and if we can't
// identify any of the uses we bail out.
// On each of these uses, identify if the memory that GV points to is
// used/required/live at the start of the function. If it is not, for example
// if the first thing the function does is store to the GV, the GV can
// possibly be demoted.
// We don't do an exhaustive search for memory operations - simply look
// through bitcasts as they're quite common and benign.
const DataLayout &DL = GV->getParent()->getDataLayout();
SmallVector<LoadInst *, 4> Loads;
SmallVector<StoreInst *, 4> Stores;
for (auto *U : GV->users()) {
Instruction *I = dyn_cast<Instruction>(U);
if (!I)
return false;
assert(I->getParent()->getParent() == F);
if (auto *LI = dyn_cast<LoadInst>(I))
else if (auto *SI = dyn_cast<StoreInst>(I))
return false;
// We have identified all uses of GV into loads and stores. Now check if all
// of them are known not to depend on the value of the global at the function
// entry point. We do this by ensuring that every load is dominated by at
// least one store.
auto &DT = LookupDomTree(*const_cast<Function *>(F));
// The below check is quadratic. Check we're not going to do too many tests.
// FIXME: Even though this will always have worst-case quadratic time, we
// could put effort into minimizing the average time by putting stores that
// have been shown to dominate at least one load at the beginning of the
// Stores array, making subsequent dominance checks more likely to succeed
// early.
// The threshold here is fairly large because global->local demotion is a
// very powerful optimization should it fire.
const unsigned Threshold = 100;
if (Loads.size() * Stores.size() > Threshold)
return false;
for (auto *L : Loads) {
auto *LTy = L->getType();
if (none_of(Stores, [&](const StoreInst *S) {
auto *STy = S->getValueOperand()->getType();
// The load is only dominated by the store if DomTree says so
// and the number of bits loaded in L is less than or equal to
// the number of bits stored in S.
return DT.dominates(S, L) &&
DL.getTypeStoreSize(LTy).getFixedValue() <=
return false;
// All loads have known dependences inside F, so the global can be localized.
return true;
// For a global variable with one store, if the store dominates any loads,
// those loads will always load the stored value (as opposed to the
// initializer), even in the presence of recursion.
static bool forwardStoredOnceStore(
GlobalVariable *GV, const StoreInst *StoredOnceStore,
function_ref<DominatorTree &(Function &)> LookupDomTree) {
const Value *StoredOnceValue = StoredOnceStore->getValueOperand();
// We can do this optimization for non-constants in nosync + norecurse
// functions, but globals used in exactly one norecurse functions are already
// promoted to an alloca.
if (!isa<Constant>(StoredOnceValue))
return false;
const Function *F = StoredOnceStore->getFunction();
SmallVector<LoadInst *> Loads;
for (User *U : GV->users()) {
if (auto *LI = dyn_cast<LoadInst>(U)) {
if (LI->getFunction() == F &&
LI->getType() == StoredOnceValue->getType() && LI->isSimple())
// Only compute DT if we have any loads to examine.
bool MadeChange = false;
if (!Loads.empty()) {
auto &DT = LookupDomTree(*const_cast<Function *>(F));
for (auto *LI : Loads) {
if (DT.dominates(StoredOnceStore, LI)) {
LI->replaceAllUsesWith(const_cast<Value *>(StoredOnceValue));
MadeChange = true;
return MadeChange;
/// Analyze the specified global variable and optimize
/// it if possible. If we make a change, return true.
static bool
processInternalGlobal(GlobalVariable *GV, const GlobalStatus &GS,
function_ref<TargetTransformInfo &(Function &)> GetTTI,
function_ref<TargetLibraryInfo &(Function &)> GetTLI,
function_ref<DominatorTree &(Function &)> LookupDomTree) {
auto &DL = GV->getParent()->getDataLayout();
// If this is a first class global and has only one accessing function and
// this function is non-recursive, we replace the global with a local alloca
// in this function.
// NOTE: It doesn't make sense to promote non-single-value types since we
// are just replacing static memory to stack memory.
// If the global is in different address space, don't bring it to stack.
if (!GS.HasMultipleAccessingFunctions &&
GS.AccessingFunction &&
GV->getValueType()->isSingleValueType() &&
GV->getType()->getAddressSpace() == DL.getAllocaAddrSpace() &&
!GV->isExternallyInitialized() &&
GS.AccessingFunction->doesNotRecurse() &&
isPointerValueDeadOnEntryToFunction(GS.AccessingFunction, GV,
LookupDomTree)) {
const DataLayout &DL = GV->getParent()->getDataLayout();
LLVM_DEBUG(dbgs() << "LOCALIZING GLOBAL: " << *GV << "\n");
BasicBlock::iterator FirstI =
Type *ElemTy = GV->getValueType();
// FIXME: Pass Global's alignment when globals have alignment
AllocaInst *Alloca = new AllocaInst(ElemTy, DL.getAllocaAddrSpace(),
nullptr, GV->getName(), FirstI);
if (!isa<UndefValue>(GV->getInitializer()))
new StoreInst(GV->getInitializer(), Alloca, FirstI);
return true;
bool Changed = false;
// If the global is never loaded (but may be stored to), it is dead.
// Delete it now.
if (!GS.IsLoaded) {
LLVM_DEBUG(dbgs() << "GLOBAL NEVER LOADED: " << *GV << "\n");
if (isLeakCheckerRoot(GV)) {
// Delete any constant stores to the global.
Changed = CleanupPointerRootUsers(GV, GetTLI);
} else {
// Delete any stores we can find to the global. We may not be able to
// make it completely dead though.
Changed = CleanupConstantGlobalUsers(GV, DL);
// If the global is dead now, delete it.
if (GV->use_empty()) {
Changed = true;
return Changed;
if (GS.StoredType <= GlobalStatus::InitializerStored) {
LLVM_DEBUG(dbgs() << "MARKING CONSTANT: " << *GV << "\n");
// Don't actually mark a global constant if it's atomic because atomic loads
// are implemented by a trivial cmpxchg in some edge-cases and that usually
// requires write access to the variable even if it's not actually changed.
if (GS.Ordering == AtomicOrdering::NotAtomic) {
assert(!GV->isConstant() && "Expected a non-constant global");
Changed = true;
// Clean up any obviously simplifiable users now.
Changed |= CleanupConstantGlobalUsers(GV, DL);
// If the global is dead now, just nuke it.
if (GV->use_empty()) {
LLVM_DEBUG(dbgs() << " *** Marking constant allowed us to simplify "
<< "all users and delete global!\n");
return true;
// Fall through to the next check; see if we can optimize further.
if (!GV->getInitializer()->getType()->isSingleValueType()) {
const DataLayout &DL = GV->getParent()->getDataLayout();
if (SRAGlobal(GV, DL))
return true;
Value *StoredOnceValue = GS.getStoredOnceValue();
if (GS.StoredType == GlobalStatus::StoredOnce && StoredOnceValue) {
Function &StoreFn =
const_cast<Function &>(*GS.StoredOnceStore->getFunction());
bool CanHaveNonUndefGlobalInitializer =
// If the initial value for the global was an undef value, and if only
// one other value was stored into it, we can just change the
// initializer to be the stored value, then delete all stores to the
// global. This allows us to mark it constant.
// This is restricted to address spaces that allow globals to have
// initializers. NVPTX, for example, does not support initializers for
// shared memory (AS 3).
auto *SOVConstant = dyn_cast<Constant>(StoredOnceValue);
if (SOVConstant && isa<UndefValue>(GV->getInitializer()) &&
DL.getTypeAllocSize(SOVConstant->getType()) ==
DL.getTypeAllocSize(GV->getValueType()) &&
CanHaveNonUndefGlobalInitializer) {
if (SOVConstant->getType() == GV->getValueType()) {
// Change the initializer in place.
} else {
// Create a new global with adjusted type.
auto *NGV = new GlobalVariable(
*GV->getParent(), SOVConstant->getType(), GV->isConstant(),
GV->getLinkage(), SOVConstant, "", GV, GV->getThreadLocalMode(),
// Clean up any obviously simplifiable users now.
CleanupConstantGlobalUsers(GV, DL);
if (GV->use_empty()) {
LLVM_DEBUG(dbgs() << " *** Substituting initializer allowed us to "
<< "simplify all users and delete global!\n");
return true;
// Try to optimize globals based on the knowledge that only one value
// (besides its initializer) is ever stored to the global.
if (optimizeOnceStoredGlobal(GV, StoredOnceValue, DL, GetTLI))
return true;
// Try to forward the store to any loads. If we have more than one store, we
// may have a store of the initializer between StoredOnceStore and a load.
if (GS.NumStores == 1)
if (forwardStoredOnceStore(GV, GS.StoredOnceStore, LookupDomTree))
return true;
// Otherwise, if the global was not a boolean, we can shrink it to be a
// boolean. Skip this optimization for AS that doesn't allow an initializer.
if (SOVConstant && GS.Ordering == AtomicOrdering::NotAtomic &&
(!isa<UndefValue>(GV->getInitializer()) ||
CanHaveNonUndefGlobalInitializer)) {
if (TryToShrinkGlobalToBoolean(GV, SOVConstant)) {
return true;
return Changed;
/// Analyze the specified global variable and optimize it if possible. If we
/// make a change, return true.
static bool
processGlobal(GlobalValue &GV,
function_ref<TargetTransformInfo &(Function &)> GetTTI,
function_ref<TargetLibraryInfo &(Function &)> GetTLI,
function_ref<DominatorTree &(Function &)> LookupDomTree) {
if (GV.getName().starts_with("llvm."))
return false;
GlobalStatus GS;
if (GlobalStatus::analyzeGlobal(&GV, GS))
return false;
bool Changed = false;
if (!GS.IsCompared && !GV.hasGlobalUnnamedAddr()) {
auto NewUnnamedAddr = GV.hasLocalLinkage() ? GlobalValue::UnnamedAddr::Global
: GlobalValue::UnnamedAddr::Local;
if (NewUnnamedAddr != GV.getUnnamedAddr()) {
Changed = true;
// Do more involved optimizations if the global is internal.
if (!GV.hasLocalLinkage())
return Changed;
auto *GVar = dyn_cast<GlobalVariable>(&GV);
if (!GVar)
return Changed;
if (GVar->isConstant() || !GVar->hasInitializer())
return Changed;
return processInternalGlobal(GVar, GS, GetTTI, GetTLI, LookupDomTree) ||
/// Walk all of the direct calls of the specified function, changing them to
/// FastCC.
static void ChangeCalleesToFastCall(Function *F) {
for (User *U : F->users()) {
if (isa<BlockAddress>(U))
static AttributeList StripAttr(LLVMContext &C, AttributeList Attrs,
Attribute::AttrKind A) {
unsigned AttrIndex;
if (Attrs.hasAttrSomewhere(A, &AttrIndex))
return Attrs.removeAttributeAtIndex(C, AttrIndex, A);
return Attrs;
static void RemoveAttribute(Function *F, Attribute::AttrKind A) {
F->setAttributes(StripAttr(F->getContext(), F->getAttributes(), A));
for (User *U : F->users()) {
if (isa<BlockAddress>(U))
CallBase *CB = cast<CallBase>(U);
CB->setAttributes(StripAttr(F->getContext(), CB->getAttributes(), A));
/// Return true if this is a calling convention that we'd like to change. The
/// idea here is that we don't want to mess with the convention if the user
/// explicitly requested something with performance implications like coldcc,
/// GHC, or anyregcc.
static bool hasChangeableCCImpl(Function *F) {
CallingConv::ID CC = F->getCallingConv();
// FIXME: Is it worth transforming x86_stdcallcc and x86_fastcallcc?
if (CC != CallingConv::C && CC != CallingConv::X86_ThisCall)
return false;
if (F->isVarArg())
return false;
// FIXME: Change CC for the whole chain of musttail calls when possible.
// Can't change CC of the function that either has musttail calls, or is a
// musttail callee itself
for (User *U : F->users()) {
if (isa<BlockAddress>(U))
CallInst* CI = dyn_cast<CallInst>(U);
if (!CI)
if (CI->isMustTailCall())
return false;
for (BasicBlock &BB : *F)
if (BB.getTerminatingMustTailCall())
return false;
return !F->hasAddressTaken();
using ChangeableCCCacheTy = SmallDenseMap<Function *, bool, 8>;
static bool hasChangeableCC(Function *F,
ChangeableCCCacheTy &ChangeableCCCache) {
auto Res = ChangeableCCCache.try_emplace(F, false);
if (Res.second)
Res.first->second = hasChangeableCCImpl(F);
return Res.first->second;
/// Return true if the block containing the call site has a BlockFrequency of
/// less than ColdCCRelFreq% of the entry block.
static bool isColdCallSite(CallBase &CB, BlockFrequencyInfo &CallerBFI) {
const BranchProbability ColdProb(ColdCCRelFreq, 100);
auto *CallSiteBB = CB.getParent();
auto CallSiteFreq = CallerBFI.getBlockFreq(CallSiteBB);
auto CallerEntryFreq =
return CallSiteFreq < CallerEntryFreq * ColdProb;
// This function checks if the input function F is cold at all call sites. It
// also looks each call site's containing function, returning false if the
// caller function contains other non cold calls. The input vector AllCallsCold
// contains a list of functions that only have call sites in cold blocks.
static bool
isValidCandidateForColdCC(Function &F,
function_ref<BlockFrequencyInfo &(Function &)> GetBFI,
const std::vector<Function *> &AllCallsCold) {
if (F.user_empty())
return false;
for (User *U : F.users()) {
if (isa<BlockAddress>(U))
CallBase &CB = cast<CallBase>(*U);
Function *CallerFunc = CB.getParent()->getParent();
BlockFrequencyInfo &CallerBFI = GetBFI(*CallerFunc);
if (!isColdCallSite(CB, CallerBFI))
return false;
if (!llvm::is_contained(AllCallsCold, CallerFunc))
return false;
return true;
static void changeCallSitesToColdCC(Function *F) {
for (User *U : F->users()) {
if (isa<BlockAddress>(U))
// This function iterates over all the call instructions in the input Function
// and checks that all call sites are in cold blocks and are allowed to use the
// coldcc calling convention.
static bool
hasOnlyColdCalls(Function &F,
function_ref<BlockFrequencyInfo &(Function &)> GetBFI,
ChangeableCCCacheTy &ChangeableCCCache) {
for (BasicBlock &BB : F) {
for (Instruction &I : BB) {
if (CallInst *CI = dyn_cast<CallInst>(&I)) {
// Skip over isline asm instructions since they aren't function calls.
if (CI->isInlineAsm())
Function *CalledFn = CI->getCalledFunction();
if (!CalledFn)
return false;
// Skip over intrinsics since they won't remain as function calls.
// Important to do this check before the linkage check below so we
// won't bail out on debug intrinsics, possibly making the generated
// code dependent on the presence of debug info.
if (CalledFn->getIntrinsicID() != Intrinsic::not_intrinsic)
if (!CalledFn->hasLocalLinkage())
return false;
// Check if it's valid to use coldcc calling convention.
if (!hasChangeableCC(CalledFn, ChangeableCCCache))
return false;
BlockFrequencyInfo &CallerBFI = GetBFI(F);
if (!isColdCallSite(*CI, CallerBFI))
return false;
return true;
static bool hasMustTailCallers(Function *F) {
for (User *U : F->users()) {
CallBase *CB = dyn_cast<CallBase>(U);
if (!CB) {
assert(isa<BlockAddress>(U) &&
"Expected either CallBase or BlockAddress");
if (CB->isMustTailCall())
return true;
return false;
static bool hasInvokeCallers(Function *F) {
for (User *U : F->users())
if (isa<InvokeInst>(U))
return true;
return false;
static void RemovePreallocated(Function *F) {
RemoveAttribute(F, Attribute::Preallocated);
auto *M = F->getParent();
IRBuilder<> Builder(M->getContext());
// Cannot modify users() while iterating over it, so make a copy.
SmallVector<User *, 4> PreallocatedCalls(F->users());
for (User *U : PreallocatedCalls) {
CallBase *CB = dyn_cast<CallBase>(U);
if (!CB)
!CB->isMustTailCall() &&
"Shouldn't call RemotePreallocated() on a musttail preallocated call");
// Create copy of call without "preallocated" operand bundle.
SmallVector<OperandBundleDef, 1> OpBundles;
CallBase *PreallocatedSetup = nullptr;
for (auto *It = OpBundles.begin(); It != OpBundles.end(); ++It) {
if (It->getTag() == "preallocated") {
PreallocatedSetup = cast<CallBase>(*It->input_begin());
assert(PreallocatedSetup && "Did not find preallocated bundle");
uint64_t ArgCount =
assert((isa<CallInst>(CB) || isa<InvokeInst>(CB)) &&
"Unknown indirect call type");
CallBase *NewCB = CallBase::Create(CB, OpBundles, CB->getIterator());
auto *StackSave = Builder.CreateStackSave();
// Replace with alloca.
// Cannot modify users() while iterating over it, so make a copy.
// can be called with the same index multiple
// times. So for each, we see if we have
// already created a Value* for the index, and if not, create an alloca and
// bitcast right after the so that it
// dominates all uses.
SmallVector<Value *, 2> ArgAllocas(ArgCount);
SmallVector<User *, 2> PreallocatedArgs(PreallocatedSetup->users());
for (auto *User : PreallocatedArgs) {
auto *UseCall = cast<CallBase>(User);
assert(UseCall->getCalledFunction()->getIntrinsicID() ==
Intrinsic::call_preallocated_arg &&
"preallocated token use was not a");
uint64_t AllocArgIndex =
Value *AllocaReplacement = ArgAllocas[AllocArgIndex];
if (!AllocaReplacement) {
auto AddressSpace = UseCall->getType()->getPointerAddressSpace();
auto *ArgType =
auto *InsertBefore = PreallocatedSetup->getNextNonDebugInstruction();
auto *Alloca =
Builder.CreateAlloca(ArgType, AddressSpace, nullptr, "paarg");
ArgAllocas[AllocArgIndex] = Alloca;
AllocaReplacement = Alloca;
// Remove
static bool
OptimizeFunctions(Module &M,
function_ref<TargetLibraryInfo &(Function &)> GetTLI,
function_ref<TargetTransformInfo &(Function &)> GetTTI,
function_ref<BlockFrequencyInfo &(Function &)> GetBFI,
function_ref<DominatorTree &(Function &)> LookupDomTree,
SmallPtrSetImpl<const Comdat *> &NotDiscardableComdats,
function_ref<void(Function &F)> ChangedCFGCallback,
function_ref<void(Function &F)> DeleteFnCallback) {
bool Changed = false;
ChangeableCCCacheTy ChangeableCCCache;
std::vector<Function *> AllCallsCold;
for (Function &F : llvm::make_early_inc_range(M))
if (hasOnlyColdCalls(F, GetBFI, ChangeableCCCache))
// Optimize functions.
for (Function &F : llvm::make_early_inc_range(M)) {
// Don't perform global opt pass on naked functions; we don't want fast
// calling conventions for naked functions.
if (F.hasFnAttribute(Attribute::Naked))
// Functions without names cannot be referenced outside this module.
if (!F.hasName() && !F.isDeclaration() && !F.hasLocalLinkage())
if (deleteIfDead(F, NotDiscardableComdats, DeleteFnCallback)) {
Changed = true;
// LLVM's definition of dominance allows instructions that are cyclic
// in unreachable blocks, e.g.:
// %pat = select i1 %condition, @global, i16* %pat
// because any instruction dominates an instruction in a block that's
// not reachable from entry.
// So, remove unreachable blocks from the function, because a) there's
// no point in analyzing them and b) GlobalOpt should otherwise grow
// some more complicated logic to break these cycles.
// Notify the analysis manager that we've modified the function's CFG.
if (!F.isDeclaration()) {
if (removeUnreachableBlocks(F)) {
Changed = true;
Changed |= processGlobal(F, GetTTI, GetTLI, LookupDomTree);
if (!F.hasLocalLinkage())
// If we have an inalloca parameter that we can safely remove the
// inalloca attribute from, do so. This unlocks optimizations that
// wouldn't be safe in the presence of inalloca.
// FIXME: We should also hoist alloca affected by this to the entry
// block if possible.
if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca) &&
!F.hasAddressTaken() && !hasMustTailCallers(&F) && !F.isVarArg()) {
RemoveAttribute(&F, Attribute::InAlloca);
Changed = true;
// FIXME: handle invokes
// FIXME: handle musttail
if (F.getAttributes().hasAttrSomewhere(Attribute::Preallocated)) {
if (!F.hasAddressTaken() && !hasMustTailCallers(&F) &&
!hasInvokeCallers(&F)) {
Changed = true;
if (hasChangeableCC(&F, ChangeableCCCache)) {
TargetTransformInfo &TTI = GetTTI(F);
// Change the calling convention to coldcc if either stress testing is
// enabled or the target would like to use coldcc on functions which are
// cold at all call sites and the callers contain no other non coldcc
// calls.
if (EnableColdCCStressTest ||
(TTI.useColdCCForColdCall(F) &&
isValidCandidateForColdCC(F, GetBFI, AllCallsCold))) {
Changed = true;
if (hasChangeableCC(&F, ChangeableCCCache)) {
// If this function has a calling convention worth changing, is not a
// varargs function, and is only called directly, promote it to use the
// Fast calling convention.
Changed = true;
if (F.getAttributes().hasAttrSomewhere(Attribute::Nest) &&
!F.hasAddressTaken()) {
// The function is not used by a trampoline intrinsic, so it is safe
// to remove the 'nest' attribute.
RemoveAttribute(&F, Attribute::Nest);
Changed = true;
return Changed;
static bool
OptimizeGlobalVars(Module &M,
function_ref<TargetTransformInfo &(Function &)> GetTTI,
function_ref<TargetLibraryInfo &(Function &)> GetTLI,
function_ref<DominatorTree &(Function &)> LookupDomTree,
SmallPtrSetImpl<const Comdat *> &NotDiscardableComdats) {
bool Changed = false;
for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {
// Global variables without names cannot be referenced outside this module.
if (!GV.hasName() && !GV.isDeclaration() && !GV.hasLocalLinkage())
// Simplify the initializer.
if (GV.hasInitializer())
if (auto *C = dyn_cast<Constant>(GV.getInitializer())) {
auto &DL = M.getDataLayout();
// TLI is not used in the case of a Constant, so use default nullptr
// for that optional parameter, since we don't have a Function to
// provide GetTLI anyway.
Constant *New = ConstantFoldConstant(C, DL, /*TLI*/ nullptr);
if (New != C)
if (deleteIfDead(GV, NotDiscardableComdats)) {
Changed = true;
Changed |= processGlobal(GV, GetTTI, GetTLI, LookupDomTree);
return Changed;
/// Evaluate static constructors in the function, if we can. Return true if we
/// can, false otherwise.
static bool EvaluateStaticConstructor(Function *F, const DataLayout &DL,
TargetLibraryInfo *TLI) {
// Skip external functions.
if (F->isDeclaration())
return false;
// Call the function.
Evaluator Eval(DL, TLI);
Constant *RetValDummy;
bool EvalSuccess = Eval.EvaluateFunction(F, RetValDummy,
SmallVector<Constant*, 0>());
if (EvalSuccess) {
// We succeeded at evaluation: commit the result.
auto NewInitializers = Eval.getMutatedInitializers();
<< F->getName() << "' to " << NewInitializers.size()
<< " stores.\n");
for (const auto &Pair : NewInitializers)
for (GlobalVariable *GV : Eval.getInvariants())
return EvalSuccess;
static int compareNames(Constant *const *A, Constant *const *B) {
Value *AStripped = (*A)->stripPointerCasts();
Value *BStripped = (*B)->stripPointerCasts();
return AStripped->getName().compare(BStripped->getName());
static void setUsedInitializer(GlobalVariable &V,
const SmallPtrSetImpl<GlobalValue *> &Init) {
if (Init.empty()) {
// Get address space of pointers in the array of pointers.
const Type *UsedArrayType = V.getValueType();
const auto *VAT = cast<ArrayType>(UsedArrayType);
const auto *VEPT = cast<PointerType>(VAT->getArrayElementType());
// Type of pointer to the array of pointers.
PointerType *PtrTy =
PointerType::get(V.getContext(), VEPT->getAddressSpace());
SmallVector<Constant *, 8> UsedArray;
for (GlobalValue *GV : Init) {
Constant *Cast = ConstantExpr::getPointerBitCastOrAddrSpaceCast(GV, PtrTy);
// Sort to get deterministic order.
array_pod_sort(UsedArray.begin(), UsedArray.end(), compareNames);
ArrayType *ATy = ArrayType::get(PtrTy, UsedArray.size());
Module *M = V.getParent();
GlobalVariable *NV =
new GlobalVariable(*M, ATy, false, GlobalValue::AppendingLinkage,
ConstantArray::get(ATy, UsedArray), "");
delete &V;
namespace {
/// An easy to access representation of llvm.used and llvm.compiler.used.
class LLVMUsed {
SmallPtrSet<GlobalValue *, 4> Used;
SmallPtrSet<GlobalValue *, 4> CompilerUsed;
GlobalVariable *UsedV;
GlobalVariable *CompilerUsedV;
LLVMUsed(Module &M) {
SmallVector<GlobalValue *, 4> Vec;
UsedV = collectUsedGlobalVariables(M, Vec, false);
Used = {Vec.begin(), Vec.end()};
CompilerUsedV = collectUsedGlobalVariables(M, Vec, true);
CompilerUsed = {Vec.begin(), Vec.end()};
using iterator = SmallPtrSet<GlobalValue *, 4>::iterator;
using used_iterator_range = iterator_range<iterator>;
iterator usedBegin() { return Used.begin(); }
iterator usedEnd() { return Used.end(); }
used_iterator_range used() {
return used_iterator_range(usedBegin(), usedEnd());
iterator compilerUsedBegin() { return CompilerUsed.begin(); }
iterator compilerUsedEnd() { return CompilerUsed.end(); }
used_iterator_range compilerUsed() {
return used_iterator_range(compilerUsedBegin(), compilerUsedEnd());
bool usedCount(GlobalValue *GV) const { return Used.count(GV); }
bool compilerUsedCount(GlobalValue *GV) const {
return CompilerUsed.count(GV);
bool usedErase(GlobalValue *GV) { return Used.erase(GV); }
bool compilerUsedErase(GlobalValue *GV) { return CompilerUsed.erase(GV); }
bool usedInsert(GlobalValue *GV) { return Used.insert(GV).second; }
bool compilerUsedInsert(GlobalValue *GV) {
return CompilerUsed.insert(GV).second;
void syncVariablesAndSets() {
if (UsedV)
setUsedInitializer(*UsedV, Used);
if (CompilerUsedV)
setUsedInitializer(*CompilerUsedV, CompilerUsed);
} // end anonymous namespace
static bool hasUseOtherThanLLVMUsed(GlobalAlias &GA, const LLVMUsed &U) {
if (GA.use_empty()) // No use at all.
return false;
assert((!U.usedCount(&GA) || !U.compilerUsedCount(&GA)) &&
"We should have removed the duplicated "
"element from llvm.compiler.used");
if (!GA.hasOneUse())
// Strictly more than one use. So at least one is not in llvm.used and
// llvm.compiler.used.
return true;
// Exactly one use. Check if it is in llvm.used or llvm.compiler.used.
return !U.usedCount(&GA) && !U.compilerUsedCount(&GA);
static bool mayHaveOtherReferences(GlobalValue &GV, const LLVMUsed &U) {
if (!GV.hasLocalLinkage())
return true;
return U.usedCount(&GV) || U.compilerUsedCount(&GV);
static bool hasUsesToReplace(GlobalAlias &GA, const LLVMUsed &U,
bool &RenameTarget) {
if (GA.isWeakForLinker())
return false;
RenameTarget = false;
bool Ret = false;
if (hasUseOtherThanLLVMUsed(GA, U))
Ret = true;
// If the alias is externally visible, we may still be able to simplify it.
if (!mayHaveOtherReferences(GA, U))
return Ret;
// If the aliasee has internal linkage and no other references (e.g.,
// @llvm.used, @llvm.compiler.used), give it the name and linkage of the
// alias, and delete the alias. This turns:
// define internal ... @f(...)
// @a = alias ... @f
// into:
// define ... @a(...)
Constant *Aliasee = GA.getAliasee();
GlobalValue *Target = cast<GlobalValue>(Aliasee->stripPointerCasts());
if (mayHaveOtherReferences(*Target, U))
return Ret;
RenameTarget = true;
return true;
static bool
OptimizeGlobalAliases(Module &M,
SmallPtrSetImpl<const Comdat *> &NotDiscardableComdats) {
bool Changed = false;
LLVMUsed Used(M);
for (GlobalValue *GV : Used.used())
// Return whether GV is explicitly or implicitly dso_local and not replaceable
// by another definition in the current linkage unit.
auto IsModuleLocal = [](GlobalValue &GV) {
return !GlobalValue::isInterposableLinkage(GV.getLinkage()) &&
(GV.isDSOLocal() || GV.isImplicitDSOLocal());
for (GlobalAlias &J : llvm::make_early_inc_range(M.aliases())) {
// Aliases without names cannot be referenced outside this module.
if (!J.hasName() && !J.isDeclaration() && !J.hasLocalLinkage())
if (deleteIfDead(J, NotDiscardableComdats)) {
Changed = true;
// If the alias can change at link time, nothing can be done - bail out.
if (!IsModuleLocal(J))
Constant *Aliasee = J.getAliasee();
GlobalValue *Target = dyn_cast<GlobalValue>(Aliasee->stripPointerCasts());
// We can't trivially replace the alias with the aliasee if the aliasee is
// non-trivial in some way. We also can't replace the alias with the aliasee
// if the aliasee may be preemptible at runtime. On ELF, a non-preemptible
// alias can be used to access the definition as if preemption did not
// happen.
// TODO: Try to handle non-zero GEPs of local aliasees.
if (!Target || !IsModuleLocal(*Target))
// Make all users of the alias use the aliasee instead.
bool RenameTarget;
if (!hasUsesToReplace(J, Used, RenameTarget))
Changed = true;
if (RenameTarget) {
// Give the aliasee the name, linkage and other attributes of the alias.
if (Used.usedErase(&J))
if (Used.compilerUsedErase(&J))
} else if (mayHaveOtherReferences(J, Used))
// Delete the alias.
Changed = true;
return Changed;
static Function *
FindAtExitLibFunc(Module &M,
function_ref<TargetLibraryInfo &(Function &)> GetTLI,
LibFunc Func) {
// Hack to get a default TLI before we have actual Function.
auto FuncIter = M.begin();
if (FuncIter == M.end())
return nullptr;
auto *TLI = &GetTLI(*FuncIter);
if (!TLI->has(Func))
return nullptr;
Function *Fn = M.getFunction(TLI->getName(Func));
if (!Fn)
return nullptr;
// Now get the actual TLI for Fn.
TLI = &GetTLI(*Fn);
// Make sure that the function has the correct prototype.
LibFunc F;
if (!TLI->getLibFunc(*Fn, F) || F != Func)
return nullptr;
return Fn;
/// Returns whether the given function is an empty C++ destructor or atexit
/// handler and can therefore be eliminated. Note that we assume that other
/// optimization passes have already simplified the code so we simply check for
/// 'ret'.
static bool IsEmptyAtExitFunction(const Function &Fn) {
// FIXME: We could eliminate C++ destructors if they're readonly/readnone and
// nounwind, but that doesn't seem worth doing.
if (Fn.isDeclaration())
return false;
for (const auto &I : Fn.getEntryBlock()) {
if (I.isDebugOrPseudoInst())
if (isa<ReturnInst>(I))
return true;
return false;
static bool OptimizeEmptyGlobalAtExitDtors(Function *CXAAtExitFn, bool isCXX) {
/// Itanium C++ ABI p3.3.5:
/// After constructing a global (or local static) object, that will require
/// destruction on exit, a termination function is registered as follows:
/// extern "C" int __cxa_atexit ( void (*f)(void *), void *p, void *d );
/// This registration, e.g. __cxa_atexit(f,p,d), is intended to cause the
/// call f(p) when DSO d is unloaded, before all such termination calls
/// registered before this one. It returns zero if registration is
/// successful, nonzero on failure.
// This pass will look for calls to __cxa_atexit or atexit where the function
// is trivial and remove them.
bool Changed = false;
for (User *U : llvm::make_early_inc_range(CXAAtExitFn->users())) {
// We're only interested in calls. Theoretically, we could handle invoke
// instructions as well, but neither llvm-gcc nor clang generate invokes
// to __cxa_atexit.
CallInst *CI = dyn_cast<CallInst>(U);
if (!CI)
Function *DtorFn =
if (!DtorFn || !IsEmptyAtExitFunction(*DtorFn))
// Just remove the call.
if (isCXX)
Changed |= true;
return Changed;
static Function *hasSideeffectFreeStaticResolution(GlobalIFunc &IF) {
if (IF.isInterposable())
return nullptr;
Function *Resolver = IF.getResolverFunction();
if (!Resolver)
return nullptr;
if (Resolver->isInterposable())
return nullptr;
// Only handle functions that have been optimized into a single basic block.
auto It = Resolver->begin();
if (++It != Resolver->end())
return nullptr;
BasicBlock &BB = Resolver->getEntryBlock();
if (any_of(BB, [](Instruction &I) { return I.mayHaveSideEffects(); }))
return nullptr;
auto *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
if (!Ret)
return nullptr;
return dyn_cast<Function>(Ret->getReturnValue());
/// Find IFuncs that have resolvers that always point at the same statically
/// known callee, and replace their callers with a direct call.
static bool OptimizeStaticIFuncs(Module &M) {
bool Changed = false;
for (GlobalIFunc &IF : M.ifuncs())
if (Function *Callee = hasSideeffectFreeStaticResolution(IF))
if (!IF.use_empty()) {
Changed = true;
return Changed;
static bool
DeleteDeadIFuncs(Module &M,
SmallPtrSetImpl<const Comdat *> &NotDiscardableComdats) {
bool Changed = false;
for (GlobalIFunc &IF : make_early_inc_range(M.ifuncs()))
if (deleteIfDead(IF, NotDiscardableComdats)) {
Changed = true;
return Changed;
static bool
optimizeGlobalsInModule(Module &M, const DataLayout &DL,
function_ref<TargetLibraryInfo &(Function &)> GetTLI,
function_ref<TargetTransformInfo &(Function &)> GetTTI,
function_ref<BlockFrequencyInfo &(Function &)> GetBFI,
function_ref<DominatorTree &(Function &)> LookupDomTree,
function_ref<void(Function &F)> ChangedCFGCallback,
function_ref<void(Function &F)> DeleteFnCallback) {
SmallPtrSet<const Comdat *, 8> NotDiscardableComdats;
bool Changed = false;
bool LocalChange = true;
std::optional<uint32_t> FirstNotFullyEvaluatedPriority;
while (LocalChange) {
LocalChange = false;
for (const GlobalVariable &GV : M.globals())
if (const Comdat *C = GV.getComdat())
if (!GV.isDiscardableIfUnused() || !GV.use_empty())
for (Function &F : M)
if (const Comdat *C = F.getComdat())
if (!F.isDefTriviallyDead())
for (GlobalAlias &GA : M.aliases())
if (const Comdat *C = GA.getComdat())
if (!GA.isDiscardableIfUnused() || !GA.use_empty())
// Delete functions that are trivially dead, ccc -> fastcc
LocalChange |= OptimizeFunctions(M, GetTLI, GetTTI, GetBFI, LookupDomTree,
NotDiscardableComdats, ChangedCFGCallback,
// Optimize global_ctors list.
LocalChange |=
optimizeGlobalCtorsList(M, [&](uint32_t Priority, Function *F) {
if (FirstNotFullyEvaluatedPriority &&
*FirstNotFullyEvaluatedPriority != Priority)
return false;
bool Evaluated = EvaluateStaticConstructor(F, DL, &GetTLI(*F));
if (!Evaluated)
FirstNotFullyEvaluatedPriority = Priority;
return Evaluated;
// Optimize non-address-taken globals.
LocalChange |= OptimizeGlobalVars(M, GetTTI, GetTLI, LookupDomTree,
// Resolve aliases, when possible.
LocalChange |= OptimizeGlobalAliases(M, NotDiscardableComdats);
// Try to remove trivial global destructors if they are not removed
// already.
if (Function *CXAAtExitFn =
FindAtExitLibFunc(M, GetTLI, LibFunc_cxa_atexit))
LocalChange |= OptimizeEmptyGlobalAtExitDtors(CXAAtExitFn, true);
if (Function *AtExitFn = FindAtExitLibFunc(M, GetTLI, LibFunc_atexit))
LocalChange |= OptimizeEmptyGlobalAtExitDtors(AtExitFn, false);
// Optimize IFuncs whose callee's are statically known.
LocalChange |= OptimizeStaticIFuncs(M);
// Remove any IFuncs that are now dead.
LocalChange |= DeleteDeadIFuncs(M, NotDiscardableComdats);
Changed |= LocalChange;
// TODO: Move all global ctors functions to the end of the module for code
// layout.
return Changed;
PreservedAnalyses GlobalOptPass::run(Module &M, ModuleAnalysisManager &AM) {
auto &DL = M.getDataLayout();
auto &FAM =
auto LookupDomTree = [&FAM](Function &F) -> DominatorTree &{
return FAM.getResult<DominatorTreeAnalysis>(F);
auto GetTLI = [&FAM](Function &F) -> TargetLibraryInfo & {
return FAM.getResult<TargetLibraryAnalysis>(F);
auto GetTTI = [&FAM](Function &F) -> TargetTransformInfo & {
return FAM.getResult<TargetIRAnalysis>(F);
auto GetBFI = [&FAM](Function &F) -> BlockFrequencyInfo & {
return FAM.getResult<BlockFrequencyAnalysis>(F);
auto ChangedCFGCallback = [&FAM](Function &F) {
FAM.invalidate(F, PreservedAnalyses::none());
auto DeleteFnCallback = [&FAM](Function &F) { FAM.clear(F, F.getName()); };
if (!optimizeGlobalsInModule(M, DL, GetTLI, GetTTI, GetBFI, LookupDomTree,
ChangedCFGCallback, DeleteFnCallback))
return PreservedAnalyses::all();
PreservedAnalyses PA = PreservedAnalyses::none();
// We made sure to clear analyses for deleted functions.
// The only place we modify the CFG is when calling
// removeUnreachableBlocks(), but there we make sure to invalidate analyses
// for modified functions.
return PA;