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//===- SplitModule.cpp - Split a module into partitions -------------------===//
// 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 function llvm::SplitModule, which splits a module
// into multiple linkable partitions. It can be used to implement parallel code
// generation for link-time optimization.
#include "llvm/Transforms/Utils/SplitModule.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/EquivalenceClasses.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/IR/Comdat.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalAlias.h"
#include "llvm/IR/GlobalObject.h"
#include "llvm/IR/GlobalIndirectSymbol.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MD5.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <memory>
#include <queue>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "split-module"
namespace {
using ClusterMapType = EquivalenceClasses<const GlobalValue *>;
using ComdatMembersType = DenseMap<const Comdat *, const GlobalValue *>;
using ClusterIDMapType = DenseMap<const GlobalValue *, unsigned>;
} // end anonymous namespace
static void addNonConstUser(ClusterMapType &GVtoClusterMap,
const GlobalValue *GV, const User *U) {
assert((!isa<Constant>(U) || isa<GlobalValue>(U)) && "Bad user");
if (const Instruction *I = dyn_cast<Instruction>(U)) {
const GlobalValue *F = I->getParent()->getParent();
GVtoClusterMap.unionSets(GV, F);
} else if (isa<GlobalIndirectSymbol>(U) || isa<Function>(U) ||
isa<GlobalVariable>(U)) {
GVtoClusterMap.unionSets(GV, cast<GlobalValue>(U));
} else {
llvm_unreachable("Underimplemented use case");
// Adds all GlobalValue users of V to the same cluster as GV.
static void addAllGlobalValueUsers(ClusterMapType &GVtoClusterMap,
const GlobalValue *GV, const Value *V) {
for (auto *U : V->users()) {
SmallVector<const User *, 4> Worklist;
while (!Worklist.empty()) {
const User *UU = Worklist.pop_back_val();
// For each constant that is not a GV (a pure const) recurse.
if (isa<Constant>(UU) && !isa<GlobalValue>(UU)) {
Worklist.append(UU->user_begin(), UU->user_end());
addNonConstUser(GVtoClusterMap, GV, UU);
// Find partitions for module in the way that no locals need to be
// globalized.
// Try to balance pack those partitions into N files since this roughly equals
// thread balancing for the backend codegen step.
static void findPartitions(Module &M, ClusterIDMapType &ClusterIDMap,
unsigned N) {
// At this point module should have the proper mix of globals and locals.
// As we attempt to partition this module, we must not change any
// locals to globals.
LLVM_DEBUG(dbgs() << "Partition module with (" << M.size() << ")functions\n");
ClusterMapType GVtoClusterMap;
ComdatMembersType ComdatMembers;
auto recordGVSet = [&GVtoClusterMap, &ComdatMembers](GlobalValue &GV) {
if (GV.isDeclaration())
if (!GV.hasName())
// Comdat groups must not be partitioned. For comdat groups that contain
// locals, record all their members here so we can keep them together.
// Comdat groups that only contain external globals are already handled by
// the MD5-based partitioning.
if (const Comdat *C = GV.getComdat()) {
auto &Member = ComdatMembers[C];
if (Member)
GVtoClusterMap.unionSets(Member, &GV);
Member = &GV;
// For aliases we should not separate them from their aliasees regardless
// of linkage.
if (auto *GIS = dyn_cast<GlobalIndirectSymbol>(&GV)) {
if (const GlobalObject *Base = GIS->getBaseObject())
GVtoClusterMap.unionSets(&GV, Base);
if (const Function *F = dyn_cast<Function>(&GV)) {
for (const BasicBlock &BB : *F) {
BlockAddress *BA = BlockAddress::lookup(&BB);
if (!BA || !BA->isConstantUsed())
addAllGlobalValueUsers(GVtoClusterMap, F, BA);
if (GV.hasLocalLinkage())
addAllGlobalValueUsers(GVtoClusterMap, &GV, &GV);
llvm::for_each(M.functions(), recordGVSet);
llvm::for_each(M.globals(), recordGVSet);
llvm::for_each(M.aliases(), recordGVSet);
// Assigned all GVs to merged clusters while balancing number of objects in
// each.
auto CompareClusters = [](const std::pair<unsigned, unsigned> &a,
const std::pair<unsigned, unsigned> &b) {
if (a.second || b.second)
return a.second > b.second;
return a.first > b.first;
std::priority_queue<std::pair<unsigned, unsigned>,
std::vector<std::pair<unsigned, unsigned>>,
// Pre-populate priority queue with N slot blanks.
for (unsigned i = 0; i < N; ++i)
BalancinQueue.push(std::make_pair(i, 0));
using SortType = std::pair<unsigned, ClusterMapType::iterator>;
SmallVector<SortType, 64> Sets;
SmallPtrSet<const GlobalValue *, 32> Visited;
// To guarantee determinism, we have to sort SCC according to size.
// When size is the same, use leader's name.
for (ClusterMapType::iterator I = GVtoClusterMap.begin(),
E = GVtoClusterMap.end(); I != E; ++I)
if (I->isLeader())
GVtoClusterMap.member_end()), I));
llvm::sort(Sets, [](const SortType &a, const SortType &b) {
if (a.first == b.first)
return a.second->getData()->getName() > b.second->getData()->getName();
return a.first > b.first;
for (auto &I : Sets) {
unsigned CurrentClusterID =;
unsigned CurrentClusterSize =;
LLVM_DEBUG(dbgs() << "Root[" << CurrentClusterID << "] cluster_size("
<< I.first << ") ----> " << I.second->getData()->getName()
<< "\n");
for (ClusterMapType::member_iterator MI =
MI != GVtoClusterMap.member_end(); ++MI) {
if (!Visited.insert(*MI).second)
LLVM_DEBUG(dbgs() << "----> " << (*MI)->getName()
<< ((*MI)->hasLocalLinkage() ? " l " : " e ") << "\n");
ClusterIDMap[*MI] = CurrentClusterID;
// Add this set size to the number of entries in this cluster.
BalancinQueue.push(std::make_pair(CurrentClusterID, CurrentClusterSize));
static void externalize(GlobalValue *GV) {
if (GV->hasLocalLinkage()) {
// Unnamed entities must be named consistently between modules. setName will
// give a distinct name to each such entity.
if (!GV->hasName())
// Returns whether GV should be in partition (0-based) I of N.
static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) {
if (auto *GIS = dyn_cast<GlobalIndirectSymbol>(GV))
if (const GlobalObject *Base = GIS->getBaseObject())
GV = Base;
StringRef Name;
if (const Comdat *C = GV->getComdat())
Name = C->getName();
Name = GV->getName();
// Partition by MD5 hash. We only need a few bits for evenness as the number
// of partitions will generally be in the 1-2 figure range; the low 16 bits
// are enough.
MD5 H;
MD5::MD5Result R;
return (R[0] | (R[1] << 8)) % N == I;
void llvm::SplitModule(
Module &M, unsigned N,
function_ref<void(std::unique_ptr<Module> MPart)> ModuleCallback,
bool PreserveLocals) {
if (!PreserveLocals) {
for (Function &F : M)
for (GlobalVariable &GV : M.globals())
for (GlobalAlias &GA : M.aliases())
for (GlobalIFunc &GIF : M.ifuncs())
// This performs splitting without a need for externalization, which might not
// always be possible.
ClusterIDMapType ClusterIDMap;
findPartitions(M, ClusterIDMap, N);
// FIXME: We should be able to reuse M as the last partition instead of
// cloning it. Note that the callers at the moment expect the module to
// be preserved, so will need some adjustments as well.
for (unsigned I = 0; I < N; ++I) {
ValueToValueMapTy VMap;
std::unique_ptr<Module> MPart(
CloneModule(M, VMap, [&](const GlobalValue *GV) {
if (ClusterIDMap.count(GV))
return (ClusterIDMap[GV] == I);
return isInPartition(GV, I, N);
if (I != 0)