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//===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
// 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 file defines the interface to a pass that merges duplicate global
// constants together into a single constant that is shared. This is useful
// because some passes (ie TraceValues) insert a lot of string constants into
// the program, regardless of whether or not an existing string is available.
// Algorithm: ConstantMerge is designed to build up a map of available constants
// and eliminate duplicates when it is initialized.
#include "llvm/Transforms/IPO/ConstantMerge.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Transforms/IPO.h"
#include <algorithm>
#include <cassert>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "constmerge"
STATISTIC(NumIdenticalMerged, "Number of identical global constants merged");
/// Find values that are marked as llvm.used.
static void FindUsedValues(GlobalVariable *LLVMUsed,
SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
if (!LLVMUsed) return;
ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
Value *Operand = Inits->getOperand(i)->stripPointerCasts();
GlobalValue *GV = cast<GlobalValue>(Operand);
// True if A is better than B.
static bool IsBetterCanonical(const GlobalVariable &A,
const GlobalVariable &B) {
if (!A.hasLocalLinkage() && B.hasLocalLinkage())
return true;
if (A.hasLocalLinkage() && !B.hasLocalLinkage())
return false;
return A.hasGlobalUnnamedAddr();
static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {
SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
for (const auto &V : MDs)
if (V.first != LLVMContext::MD_dbg)
return true;
return false;
static void copyDebugLocMetadata(const GlobalVariable *From,
GlobalVariable *To) {
SmallVector<DIGlobalVariableExpression *, 1> MDs;
for (auto MD : MDs)
static Align getAlign(GlobalVariable *GV) {
return GV->getAlign().getValueOr(
static bool
isUnmergeableGlobal(GlobalVariable *GV,
const SmallPtrSetImpl<const GlobalValue *> &UsedGlobals) {
// Only process constants with initializers in the default address space.
return !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
// Don't touch values marked with attribute(used).
enum class CanMerge { No, Yes };
static CanMerge makeMergeable(GlobalVariable *Old, GlobalVariable *New) {
if (!Old->hasGlobalUnnamedAddr() && !New->hasGlobalUnnamedAddr())
return CanMerge::No;
if (hasMetadataOtherThanDebugLoc(Old))
return CanMerge::No;
if (!Old->hasGlobalUnnamedAddr())
return CanMerge::Yes;
static void replace(Module &M, GlobalVariable *Old, GlobalVariable *New) {
Constant *NewConstant = New;
LLVM_DEBUG(dbgs() << "Replacing global: @" << Old->getName() << " -> @"
<< New->getName() << "\n");
// Bump the alignment if necessary.
if (Old->getAlign() || New->getAlign())
New->setAlignment(std::max(getAlign(Old), getAlign(New)));
copyDebugLocMetadata(Old, New);
// Delete the global value from the module.
assert(Old->hasLocalLinkage() &&
"Refusing to delete an externally visible global variable.");
static bool mergeConstants(Module &M) {
// Find all the globals that are marked "used". These cannot be merged.
SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
// Map unique constants to globals.
DenseMap<Constant *, GlobalVariable *> CMap;
SmallVector<std::pair<GlobalVariable *, GlobalVariable *>, 32>
size_t ChangesMade = 0;
size_t OldChangesMade = 0;
// Iterate constant merging while we are still making progress. Merging two
// constants together may allow us to merge other constants together if the
// second level constants have initializers which point to the globals that
// were just merged.
while (true) {
// Find the canonical constants others will be merged with.
for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
GVI != E; ) {
GlobalVariable *GV = &*GVI++;
// If this GV is dead, remove it.
if (GV->use_empty() && GV->hasLocalLinkage()) {
if (isUnmergeableGlobal(GV, UsedGlobals))
// This transformation is legal for weak ODR globals in the sense it
// doesn't change semantics, but we really don't want to perform it
// anyway; it's likely to pessimize code generation, and some tools
// (like the Darwin linker in cases involving CFString) don't expect it.
if (GV->isWeakForLinker())
// Don't touch globals with metadata other then !dbg.
if (hasMetadataOtherThanDebugLoc(GV))
Constant *Init = GV->getInitializer();
// Check to see if the initializer is already known.
GlobalVariable *&Slot = CMap[Init];
// If this is the first constant we find or if the old one is local,
// replace with the current one. If the current is externally visible
// it cannot be replace, but can be the canonical constant we merge with.
bool FirstConstantFound = !Slot;
if (FirstConstantFound || IsBetterCanonical(*GV, *Slot)) {
Slot = GV;
LLVM_DEBUG(dbgs() << "Cmap[" << *Init << "] = " << GV->getName()
<< (FirstConstantFound ? "\n" : " (updated)\n"));
// Identify all globals that can be merged together, filling in the
// SameContentReplacements vector. We cannot do the replacement in this pass
// because doing so may cause initializers of other globals to be rewritten,
// invalidating the Constant* pointers in CMap.
for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
GVI != E; ) {
GlobalVariable *GV = &*GVI++;
if (isUnmergeableGlobal(GV, UsedGlobals))
// We can only replace constant with local linkage.
if (!GV->hasLocalLinkage())
Constant *Init = GV->getInitializer();
// Check to see if the initializer is already known.
auto Found = CMap.find(Init);
if (Found == CMap.end())
GlobalVariable *Slot = Found->second;
if (Slot == GV)
if (makeMergeable(GV, Slot) == CanMerge::No)
// Make all uses of the duplicate constant use the canonical version.
LLVM_DEBUG(dbgs() << "Will replace: @" << GV->getName() << " -> @"
<< Slot->getName() << "\n");
SameContentReplacements.push_back(std::make_pair(GV, Slot));
// Now that we have figured out which replacements must be made, do them all
// now. This avoid invalidating the pointers in CMap, which are unneeded
// now.
for (unsigned i = 0, e = SameContentReplacements.size(); i != e; ++i) {
GlobalVariable *Old = SameContentReplacements[i].first;
GlobalVariable *New = SameContentReplacements[i].second;
replace(M, Old, New);
if (ChangesMade == OldChangesMade)
OldChangesMade = ChangesMade;
return ChangesMade;
PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {
if (!mergeConstants(M))
return PreservedAnalyses::all();
return PreservedAnalyses::none();
namespace {
struct ConstantMergeLegacyPass : public ModulePass {
static char ID; // Pass identification, replacement for typeid
ConstantMergeLegacyPass() : ModulePass(ID) {
// For this pass, process all of the globals in the module, eliminating
// duplicate constants.
bool runOnModule(Module &M) override {
if (skipModule(M))
return false;
return mergeConstants(M);
} // end anonymous namespace
char ConstantMergeLegacyPass::ID = 0;
INITIALIZE_PASS(ConstantMergeLegacyPass, "constmerge",
"Merge Duplicate Global Constants", false, false)
ModulePass *llvm::createConstantMergePass() {
return new ConstantMergeLegacyPass();