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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 
35 using namespace llvm;
36 
37 namespace {
38 typedef EquivalenceClasses<const GlobalValue *> ClusterMapType;
39 typedef DenseMap<const Comdat *, const GlobalValue *> ComdatMembersType;
40 typedef DenseMap<const GlobalValue *, unsigned> ClusterIDMapType;
41 }
42 
addNonConstUser(ClusterMapType & GVtoClusterMap,const GlobalValue * GV,const User * U)43 static 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.
addAllGlobalValueUsers(ClusterMapType & GVtoClusterMap,const GlobalValue * GV,const Value * V)59 static 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.
findPartitions(Module * M,ClusterIDMapType & ClusterIDMap,unsigned N)80 static 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 
externalize(GlobalValue * GV)194 static 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.
isInPartition(const GlobalValue * GV,unsigned I,unsigned N)207 static 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 
SplitModule(std::unique_ptr<Module> M,unsigned N,function_ref<void (std::unique_ptr<Module> MPart)> ModuleCallback,bool PreserveLocals)228 void 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