1//===- SplitModule.cpp - Split a module into partitions -------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file defines the function llvm::SplitModule, which splits a module
10// into multiple linkable partitions. It can be used to implement parallel code
11// generation for link-time optimization.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/Transforms/Utils/SplitModule.h"
16#include "llvm/ADT/DenseMap.h"
17#include "llvm/ADT/EquivalenceClasses.h"
18#include "llvm/ADT/SmallPtrSet.h"
19#include "llvm/ADT/SmallVector.h"
20#include "llvm/ADT/StringRef.h"
21#include "llvm/IR/Comdat.h"
22#include "llvm/IR/Constant.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/GlobalIndirectSymbol.h"
28#include "llvm/IR/GlobalValue.h"
29#include "llvm/IR/GlobalVariable.h"
30#include "llvm/IR/Instruction.h"
31#include "llvm/IR/Module.h"
32#include "llvm/IR/User.h"
33#include "llvm/IR/Value.h"
34#include "llvm/Support/Casting.h"
35#include "llvm/Support/Debug.h"
36#include "llvm/Support/ErrorHandling.h"
37#include "llvm/Support/MD5.h"
38#include "llvm/Support/raw_ostream.h"
39#include "llvm/Transforms/Utils/Cloning.h"
40#include "llvm/Transforms/Utils/ValueMapper.h"
41#include <algorithm>
42#include <cassert>
43#include <iterator>
44#include <memory>
45#include <queue>
46#include <utility>
47#include <vector>
48
49using namespace llvm;
50
51#define DEBUG_TYPE "split-module"
52
53namespace {
54
55using ClusterMapType = EquivalenceClasses<const GlobalValue *>;
56using ComdatMembersType = DenseMap<const Comdat *, const GlobalValue *>;
57using ClusterIDMapType = DenseMap<const GlobalValue *, unsigned>;
58
59} // end anonymous namespace
60
61static void addNonConstUser(ClusterMapType &GVtoClusterMap,
62                            const GlobalValue *GV, const User *U) {
63  assert((!isa<Constant>(U) || isa<GlobalValue>(U)) && "Bad user");
64
65  if (const Instruction *I = dyn_cast<Instruction>(U)) {
66    const GlobalValue *F = I->getParent()->getParent();
67    GVtoClusterMap.unionSets(GV, F);
68  } else if (isa<GlobalIndirectSymbol>(U) || isa<Function>(U) ||
69             isa<GlobalVariable>(U)) {
70    GVtoClusterMap.unionSets(GV, cast<GlobalValue>(U));
71  } else {
72    llvm_unreachable("Underimplemented use case");
73  }
74}
75
76// Adds all GlobalValue users of V to the same cluster as GV.
77static void addAllGlobalValueUsers(ClusterMapType &GVtoClusterMap,
78                                   const GlobalValue *GV, const Value *V) {
79  for (auto *U : V->users()) {
80    SmallVector<const User *, 4> Worklist;
81    Worklist.push_back(U);
82    while (!Worklist.empty()) {
83      const User *UU = Worklist.pop_back_val();
84      // For each constant that is not a GV (a pure const) recurse.
85      if (isa<Constant>(UU) && !isa<GlobalValue>(UU)) {
86        Worklist.append(UU->user_begin(), UU->user_end());
87        continue;
88      }
89      addNonConstUser(GVtoClusterMap, GV, UU);
90    }
91  }
92}
93
94// Find partitions for module in the way that no locals need to be
95// globalized.
96// Try to balance pack those partitions into N files since this roughly equals
97// thread balancing for the backend codegen step.
98static void findPartitions(Module *M, ClusterIDMapType &ClusterIDMap,
99                           unsigned N) {
100  // At this point module should have the proper mix of globals and locals.
101  // As we attempt to partition this module, we must not change any
102  // locals to globals.
103  LLVM_DEBUG(dbgs() << "Partition module with (" << M->size()
104                    << ")functions\n");
105  ClusterMapType GVtoClusterMap;
106  ComdatMembersType ComdatMembers;
107
108  auto recordGVSet = [&GVtoClusterMap, &ComdatMembers](GlobalValue &GV) {
109    if (GV.isDeclaration())
110      return;
111
112    if (!GV.hasName())
113      GV.setName("__llvmsplit_unnamed");
114
115    // Comdat groups must not be partitioned. For comdat groups that contain
116    // locals, record all their members here so we can keep them together.
117    // Comdat groups that only contain external globals are already handled by
118    // the MD5-based partitioning.
119    if (const Comdat *C = GV.getComdat()) {
120      auto &Member = ComdatMembers[C];
121      if (Member)
122        GVtoClusterMap.unionSets(Member, &GV);
123      else
124        Member = &GV;
125    }
126
127    // For aliases we should not separate them from their aliasees regardless
128    // of linkage.
129    if (auto *GIS = dyn_cast<GlobalIndirectSymbol>(&GV)) {
130      if (const GlobalObject *Base = GIS->getBaseObject())
131        GVtoClusterMap.unionSets(&GV, Base);
132    }
133
134    if (const Function *F = dyn_cast<Function>(&GV)) {
135      for (const BasicBlock &BB : *F) {
136        BlockAddress *BA = BlockAddress::lookup(&BB);
137        if (!BA || !BA->isConstantUsed())
138          continue;
139        addAllGlobalValueUsers(GVtoClusterMap, F, BA);
140      }
141    }
142
143    if (GV.hasLocalLinkage())
144      addAllGlobalValueUsers(GVtoClusterMap, &GV, &GV);
145  };
146
147  llvm::for_each(M->functions(), recordGVSet);
148  llvm::for_each(M->globals(), recordGVSet);
149  llvm::for_each(M->aliases(), recordGVSet);
150
151  // Assigned all GVs to merged clusters while balancing number of objects in
152  // each.
153  auto CompareClusters = [](const std::pair<unsigned, unsigned> &a,
154                            const std::pair<unsigned, unsigned> &b) {
155    if (a.second || b.second)
156      return a.second > b.second;
157    else
158      return a.first > b.first;
159  };
160
161  std::priority_queue<std::pair<unsigned, unsigned>,
162                      std::vector<std::pair<unsigned, unsigned>>,
163                      decltype(CompareClusters)>
164      BalancinQueue(CompareClusters);
165  // Pre-populate priority queue with N slot blanks.
166  for (unsigned i = 0; i < N; ++i)
167    BalancinQueue.push(std::make_pair(i, 0));
168
169  using SortType = std::pair<unsigned, ClusterMapType::iterator>;
170
171  SmallVector<SortType, 64> Sets;
172  SmallPtrSet<const GlobalValue *, 32> Visited;
173
174  // To guarantee determinism, we have to sort SCC according to size.
175  // When size is the same, use leader's name.
176  for (ClusterMapType::iterator I = GVtoClusterMap.begin(),
177                                E = GVtoClusterMap.end(); I != E; ++I)
178    if (I->isLeader())
179      Sets.push_back(
180          std::make_pair(std::distance(GVtoClusterMap.member_begin(I),
181                                       GVtoClusterMap.member_end()), I));
182
183  llvm::sort(Sets, [](const SortType &a, const SortType &b) {
184    if (a.first == b.first)
185      return a.second->getData()->getName() > b.second->getData()->getName();
186    else
187      return a.first > b.first;
188  });
189
190  for (auto &I : Sets) {
191    unsigned CurrentClusterID = BalancinQueue.top().first;
192    unsigned CurrentClusterSize = BalancinQueue.top().second;
193    BalancinQueue.pop();
194
195    LLVM_DEBUG(dbgs() << "Root[" << CurrentClusterID << "] cluster_size("
196                      << I.first << ") ----> " << I.second->getData()->getName()
197                      << "\n");
198
199    for (ClusterMapType::member_iterator MI =
200             GVtoClusterMap.findLeader(I.second);
201         MI != GVtoClusterMap.member_end(); ++MI) {
202      if (!Visited.insert(*MI).second)
203        continue;
204      LLVM_DEBUG(dbgs() << "----> " << (*MI)->getName()
205                        << ((*MI)->hasLocalLinkage() ? " l " : " e ") << "\n");
206      Visited.insert(*MI);
207      ClusterIDMap[*MI] = CurrentClusterID;
208      CurrentClusterSize++;
209    }
210    // Add this set size to the number of entries in this cluster.
211    BalancinQueue.push(std::make_pair(CurrentClusterID, CurrentClusterSize));
212  }
213}
214
215static void externalize(GlobalValue *GV) {
216  if (GV->hasLocalLinkage()) {
217    GV->setLinkage(GlobalValue::ExternalLinkage);
218    GV->setVisibility(GlobalValue::HiddenVisibility);
219  }
220
221  // Unnamed entities must be named consistently between modules. setName will
222  // give a distinct name to each such entity.
223  if (!GV->hasName())
224    GV->setName("__llvmsplit_unnamed");
225}
226
227// Returns whether GV should be in partition (0-based) I of N.
228static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) {
229  if (auto *GIS = dyn_cast<GlobalIndirectSymbol>(GV))
230    if (const GlobalObject *Base = GIS->getBaseObject())
231      GV = Base;
232
233  StringRef Name;
234  if (const Comdat *C = GV->getComdat())
235    Name = C->getName();
236  else
237    Name = GV->getName();
238
239  // Partition by MD5 hash. We only need a few bits for evenness as the number
240  // of partitions will generally be in the 1-2 figure range; the low 16 bits
241  // are enough.
242  MD5 H;
243  MD5::MD5Result R;
244  H.update(Name);
245  H.final(R);
246  return (R[0] | (R[1] << 8)) % N == I;
247}
248
249void llvm::SplitModule(
250    std::unique_ptr<Module> M, unsigned N,
251    function_ref<void(std::unique_ptr<Module> MPart)> ModuleCallback,
252    bool PreserveLocals) {
253  if (!PreserveLocals) {
254    for (Function &F : *M)
255      externalize(&F);
256    for (GlobalVariable &GV : M->globals())
257      externalize(&GV);
258    for (GlobalAlias &GA : M->aliases())
259      externalize(&GA);
260    for (GlobalIFunc &GIF : M->ifuncs())
261      externalize(&GIF);
262  }
263
264  // This performs splitting without a need for externalization, which might not
265  // always be possible.
266  ClusterIDMapType ClusterIDMap;
267  findPartitions(M.get(), ClusterIDMap, N);
268
269  // FIXME: We should be able to reuse M as the last partition instead of
270  // cloning it.
271  for (unsigned I = 0; I < N; ++I) {
272    ValueToValueMapTy VMap;
273    std::unique_ptr<Module> MPart(
274        CloneModule(*M, VMap, [&](const GlobalValue *GV) {
275          if (ClusterIDMap.count(GV))
276            return (ClusterIDMap[GV] == I);
277          else
278            return isInPartition(GV, I, N);
279        }));
280    if (I != 0)
281      MPart->setModuleInlineAsm("");
282    ModuleCallback(std::move(MPart));
283  }
284}
285