1//===- SplitModule.cpp - Split a module into partitions -------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file defines the function llvm::SplitModule, which splits a module
11// into multiple linkable partitions. It can be used to implement parallel code
12// generation for link-time optimization.
13//
14//===----------------------------------------------------------------------===//
15
16#define DEBUG_TYPE "split-module"
17
18#include "llvm/Transforms/Utils/SplitModule.h"
19#include "llvm/ADT/EquivalenceClasses.h"
20#include "llvm/ADT/Hashing.h"
21#include "llvm/ADT/MapVector.h"
22#include "llvm/ADT/SetVector.h"
23#include "llvm/IR/Constants.h"
24#include "llvm/IR/Function.h"
25#include "llvm/IR/GlobalAlias.h"
26#include "llvm/IR/GlobalObject.h"
27#include "llvm/IR/GlobalValue.h"
28#include "llvm/IR/Module.h"
29#include "llvm/Support/Debug.h"
30#include "llvm/Support/MD5.h"
31#include "llvm/Support/raw_ostream.h"
32#include "llvm/Transforms/Utils/Cloning.h"
33#include <queue>
34
35using namespace llvm;
36
37namespace {
38typedef EquivalenceClasses<const GlobalValue *> ClusterMapType;
39typedef DenseMap<const Comdat *, const GlobalValue *> ComdatMembersType;
40typedef DenseMap<const GlobalValue *, unsigned> ClusterIDMapType;
41}
42
43static void addNonConstUser(ClusterMapType &GVtoClusterMap,
44                            const GlobalValue *GV, const User *U) {
45  assert((!isa<Constant>(U) || isa<GlobalValue>(U)) && "Bad user");
46
47  if (const Instruction *I = dyn_cast<Instruction>(U)) {
48    const GlobalValue *F = I->getParent()->getParent();
49    GVtoClusterMap.unionSets(GV, F);
50  } else if (isa<GlobalIndirectSymbol>(U) || isa<Function>(U) ||
51             isa<GlobalVariable>(U)) {
52    GVtoClusterMap.unionSets(GV, cast<GlobalValue>(U));
53  } else {
54    llvm_unreachable("Underimplemented use case");
55  }
56}
57
58// Adds all GlobalValue users of V to the same cluster as GV.
59static void addAllGlobalValueUsers(ClusterMapType &GVtoClusterMap,
60                                   const GlobalValue *GV, const Value *V) {
61  for (auto *U : V->users()) {
62    SmallVector<const User *, 4> Worklist;
63    Worklist.push_back(U);
64    while (!Worklist.empty()) {
65      const User *UU = Worklist.pop_back_val();
66      // For each constant that is not a GV (a pure const) recurse.
67      if (isa<Constant>(UU) && !isa<GlobalValue>(UU)) {
68        Worklist.append(UU->user_begin(), UU->user_end());
69        continue;
70      }
71      addNonConstUser(GVtoClusterMap, GV, UU);
72    }
73  }
74}
75
76// Find partitions for module in the way that no locals need to be
77// globalized.
78// Try to balance pack those partitions into N files since this roughly equals
79// thread balancing for the backend codegen step.
80static void findPartitions(Module *M, ClusterIDMapType &ClusterIDMap,
81                           unsigned N) {
82  // At this point module should have the proper mix of globals and locals.
83  // As we attempt to partition this module, we must not change any
84  // locals to globals.
85  DEBUG(dbgs() << "Partition module with (" << M->size() << ")functions\n");
86  ClusterMapType GVtoClusterMap;
87  ComdatMembersType ComdatMembers;
88
89  auto recordGVSet = [&GVtoClusterMap, &ComdatMembers](GlobalValue &GV) {
90    if (GV.isDeclaration())
91      return;
92
93    if (!GV.hasName())
94      GV.setName("__llvmsplit_unnamed");
95
96    // Comdat groups must not be partitioned. For comdat groups that contain
97    // locals, record all their members here so we can keep them together.
98    // Comdat groups that only contain external globals are already handled by
99    // the MD5-based partitioning.
100    if (const Comdat *C = GV.getComdat()) {
101      auto &Member = ComdatMembers[C];
102      if (Member)
103        GVtoClusterMap.unionSets(Member, &GV);
104      else
105        Member = &GV;
106    }
107
108    // For aliases we should not separate them from their aliasees regardless
109    // of linkage.
110    if (auto *GIS = dyn_cast<GlobalIndirectSymbol>(&GV)) {
111      if (const GlobalObject *Base = GIS->getBaseObject())
112        GVtoClusterMap.unionSets(&GV, Base);
113    }
114
115    if (const Function *F = dyn_cast<Function>(&GV)) {
116      for (const BasicBlock &BB : *F) {
117        BlockAddress *BA = BlockAddress::lookup(&BB);
118        if (!BA || !BA->isConstantUsed())
119          continue;
120        addAllGlobalValueUsers(GVtoClusterMap, F, BA);
121      }
122    }
123
124    if (GV.hasLocalLinkage())
125      addAllGlobalValueUsers(GVtoClusterMap, &GV, &GV);
126  };
127
128  std::for_each(M->begin(), M->end(), recordGVSet);
129  std::for_each(M->global_begin(), M->global_end(), recordGVSet);
130  std::for_each(M->alias_begin(), M->alias_end(), recordGVSet);
131
132  // Assigned all GVs to merged clusters while balancing number of objects in
133  // each.
134  auto CompareClusters = [](const std::pair<unsigned, unsigned> &a,
135                            const std::pair<unsigned, unsigned> &b) {
136    if (a.second || b.second)
137      return a.second > b.second;
138    else
139      return a.first > b.first;
140  };
141
142  std::priority_queue<std::pair<unsigned, unsigned>,
143                      std::vector<std::pair<unsigned, unsigned>>,
144                      decltype(CompareClusters)>
145      BalancinQueue(CompareClusters);
146  // Pre-populate priority queue with N slot blanks.
147  for (unsigned i = 0; i < N; ++i)
148    BalancinQueue.push(std::make_pair(i, 0));
149
150  typedef std::pair<unsigned, ClusterMapType::iterator> SortType;
151  SmallVector<SortType, 64> Sets;
152  SmallPtrSet<const GlobalValue *, 32> Visited;
153
154  // To guarantee determinism, we have to sort SCC according to size.
155  // When size is the same, use leader's name.
156  for (ClusterMapType::iterator I = GVtoClusterMap.begin(),
157                                E = GVtoClusterMap.end(); I != E; ++I)
158    if (I->isLeader())
159      Sets.push_back(
160          std::make_pair(std::distance(GVtoClusterMap.member_begin(I),
161                                       GVtoClusterMap.member_end()), I));
162
163  std::sort(Sets.begin(), Sets.end(), [](const SortType &a, const SortType &b) {
164    if (a.first == b.first)
165      return a.second->getData()->getName() > b.second->getData()->getName();
166    else
167      return a.first > b.first;
168  });
169
170  for (auto &I : Sets) {
171    unsigned CurrentClusterID = BalancinQueue.top().first;
172    unsigned CurrentClusterSize = BalancinQueue.top().second;
173    BalancinQueue.pop();
174
175    DEBUG(dbgs() << "Root[" << CurrentClusterID << "] cluster_size(" << I.first
176                 << ") ----> " << I.second->getData()->getName() << "\n");
177
178    for (ClusterMapType::member_iterator MI =
179             GVtoClusterMap.findLeader(I.second);
180         MI != GVtoClusterMap.member_end(); ++MI) {
181      if (!Visited.insert(*MI).second)
182        continue;
183      DEBUG(dbgs() << "----> " << (*MI)->getName()
184                   << ((*MI)->hasLocalLinkage() ? " l " : " e ") << "\n");
185      Visited.insert(*MI);
186      ClusterIDMap[*MI] = CurrentClusterID;
187      CurrentClusterSize++;
188    }
189    // Add this set size to the number of entries in this cluster.
190    BalancinQueue.push(std::make_pair(CurrentClusterID, CurrentClusterSize));
191  }
192}
193
194static void externalize(GlobalValue *GV) {
195  if (GV->hasLocalLinkage()) {
196    GV->setLinkage(GlobalValue::ExternalLinkage);
197    GV->setVisibility(GlobalValue::HiddenVisibility);
198  }
199
200  // Unnamed entities must be named consistently between modules. setName will
201  // give a distinct name to each such entity.
202  if (!GV->hasName())
203    GV->setName("__llvmsplit_unnamed");
204}
205
206// Returns whether GV should be in partition (0-based) I of N.
207static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) {
208  if (auto *GIS = dyn_cast<GlobalIndirectSymbol>(GV))
209    if (const GlobalObject *Base = GIS->getBaseObject())
210      GV = Base;
211
212  StringRef Name;
213  if (const Comdat *C = GV->getComdat())
214    Name = C->getName();
215  else
216    Name = GV->getName();
217
218  // Partition by MD5 hash. We only need a few bits for evenness as the number
219  // of partitions will generally be in the 1-2 figure range; the low 16 bits
220  // are enough.
221  MD5 H;
222  MD5::MD5Result R;
223  H.update(Name);
224  H.final(R);
225  return (R[0] | (R[1] << 8)) % N == I;
226}
227
228void llvm::SplitModule(
229    std::unique_ptr<Module> M, unsigned N,
230    function_ref<void(std::unique_ptr<Module> MPart)> ModuleCallback,
231    bool PreserveLocals) {
232  if (!PreserveLocals) {
233    for (Function &F : *M)
234      externalize(&F);
235    for (GlobalVariable &GV : M->globals())
236      externalize(&GV);
237    for (GlobalAlias &GA : M->aliases())
238      externalize(&GA);
239    for (GlobalIFunc &GIF : M->ifuncs())
240      externalize(&GIF);
241  }
242
243  // This performs splitting without a need for externalization, which might not
244  // always be possible.
245  ClusterIDMapType ClusterIDMap;
246  findPartitions(M.get(), ClusterIDMap, N);
247
248  // FIXME: We should be able to reuse M as the last partition instead of
249  // cloning it.
250  for (unsigned I = 0; I < N; ++I) {
251    ValueToValueMapTy VMap;
252    std::unique_ptr<Module> MPart(
253        CloneModule(M.get(), VMap, [&](const GlobalValue *GV) {
254          if (ClusterIDMap.count(GV))
255            return (ClusterIDMap[GV] == I);
256          else
257            return isInPartition(GV, I, N);
258        }));
259    if (I != 0)
260      MPart->setModuleInlineAsm("");
261    ModuleCallback(std::move(MPart));
262  }
263}
264