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 
49 using namespace llvm;
50 
51 #define DEBUG_TYPE "split-module"
52 
53 namespace {
54 
55 using ClusterMapType = EquivalenceClasses<const GlobalValue *>;
56 using ComdatMembersType = DenseMap<const Comdat *, const GlobalValue *>;
57 using ClusterIDMapType = DenseMap<const GlobalValue *, unsigned>;
58 
59 } // end anonymous namespace
60 
61 static 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.
77 static 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.
98 static 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 
215 static 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.
228 static 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 
249 void 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