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 #include "llvm/Transforms/Utils/SplitModule.h"
17 #include "llvm/ADT/DenseMap.h"
18 #include "llvm/ADT/EquivalenceClasses.h"
19 #include "llvm/ADT/SmallPtrSet.h"
20 #include "llvm/ADT/SmallVector.h"
21 #include "llvm/ADT/StringRef.h"
22 #include "llvm/IR/Comdat.h"
23 #include "llvm/IR/Constant.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/Function.h"
26 #include "llvm/IR/GlobalAlias.h"
27 #include "llvm/IR/GlobalObject.h"
28 #include "llvm/IR/GlobalIndirectSymbol.h"
29 #include "llvm/IR/GlobalValue.h"
30 #include "llvm/IR/GlobalVariable.h"
31 #include "llvm/IR/Instruction.h"
32 #include "llvm/IR/Module.h"
33 #include "llvm/IR/User.h"
34 #include "llvm/IR/Value.h"
35 #include "llvm/Support/Casting.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/ErrorHandling.h"
38 #include "llvm/Support/MD5.h"
39 #include "llvm/Support/raw_ostream.h"
40 #include "llvm/Transforms/Utils/Cloning.h"
41 #include "llvm/Transforms/Utils/ValueMapper.h"
42 #include <algorithm>
43 #include <cassert>
44 #include <iterator>
45 #include <memory>
46 #include <queue>
47 #include <utility>
48 #include <vector>
49 
50 using namespace llvm;
51 
52 #define DEBUG_TYPE "split-module"
53 
54 namespace {
55 
56 using ClusterMapType = EquivalenceClasses<const GlobalValue *>;
57 using ComdatMembersType = DenseMap<const Comdat *, const GlobalValue *>;
58 using ClusterIDMapType = DenseMap<const GlobalValue *, unsigned>;
59 
60 } // end anonymous namespace
61 
62 static void addNonConstUser(ClusterMapType &GVtoClusterMap,
63                             const GlobalValue *GV, const User *U) {
64   assert((!isa<Constant>(U) || isa<GlobalValue>(U)) && "Bad user");
65 
66   if (const Instruction *I = dyn_cast<Instruction>(U)) {
67     const GlobalValue *F = I->getParent()->getParent();
68     GVtoClusterMap.unionSets(GV, F);
69   } else if (isa<GlobalIndirectSymbol>(U) || isa<Function>(U) ||
70              isa<GlobalVariable>(U)) {
71     GVtoClusterMap.unionSets(GV, cast<GlobalValue>(U));
72   } else {
73     llvm_unreachable("Underimplemented use case");
74   }
75 }
76 
77 // Adds all GlobalValue users of V to the same cluster as GV.
78 static void addAllGlobalValueUsers(ClusterMapType &GVtoClusterMap,
79                                    const GlobalValue *GV, const Value *V) {
80   for (auto *U : V->users()) {
81     SmallVector<const User *, 4> Worklist;
82     Worklist.push_back(U);
83     while (!Worklist.empty()) {
84       const User *UU = Worklist.pop_back_val();
85       // For each constant that is not a GV (a pure const) recurse.
86       if (isa<Constant>(UU) && !isa<GlobalValue>(UU)) {
87         Worklist.append(UU->user_begin(), UU->user_end());
88         continue;
89       }
90       addNonConstUser(GVtoClusterMap, GV, UU);
91     }
92   }
93 }
94 
95 // Find partitions for module in the way that no locals need to be
96 // globalized.
97 // Try to balance pack those partitions into N files since this roughly equals
98 // thread balancing for the backend codegen step.
99 static void findPartitions(Module *M, ClusterIDMapType &ClusterIDMap,
100                            unsigned N) {
101   // At this point module should have the proper mix of globals and locals.
102   // As we attempt to partition this module, we must not change any
103   // locals to globals.
104   DEBUG(dbgs() << "Partition module with (" << M->size() << ")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.begin(), Sets.end(),
184              [](const SortType &a, const SortType &b) {
185                if (a.first == b.first)
186                  return a.second->getData()->getName() >
187                         b.second->getData()->getName();
188                else
189                  return a.first > b.first;
190              });
191 
192   for (auto &I : Sets) {
193     unsigned CurrentClusterID = BalancinQueue.top().first;
194     unsigned CurrentClusterSize = BalancinQueue.top().second;
195     BalancinQueue.pop();
196 
197     DEBUG(dbgs() << "Root[" << CurrentClusterID << "] cluster_size(" << I.first
198                  << ") ----> " << I.second->getData()->getName() << "\n");
199 
200     for (ClusterMapType::member_iterator MI =
201              GVtoClusterMap.findLeader(I.second);
202          MI != GVtoClusterMap.member_end(); ++MI) {
203       if (!Visited.insert(*MI).second)
204         continue;
205       DEBUG(dbgs() << "----> " << (*MI)->getName()
206                    << ((*MI)->hasLocalLinkage() ? " l " : " e ") << "\n");
207       Visited.insert(*MI);
208       ClusterIDMap[*MI] = CurrentClusterID;
209       CurrentClusterSize++;
210     }
211     // Add this set size to the number of entries in this cluster.
212     BalancinQueue.push(std::make_pair(CurrentClusterID, CurrentClusterSize));
213   }
214 }
215 
216 static void externalize(GlobalValue *GV) {
217   if (GV->hasLocalLinkage()) {
218     GV->setLinkage(GlobalValue::ExternalLinkage);
219     GV->setVisibility(GlobalValue::HiddenVisibility);
220   }
221 
222   // Unnamed entities must be named consistently between modules. setName will
223   // give a distinct name to each such entity.
224   if (!GV->hasName())
225     GV->setName("__llvmsplit_unnamed");
226 }
227 
228 // Returns whether GV should be in partition (0-based) I of N.
229 static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) {
230   if (auto *GIS = dyn_cast<GlobalIndirectSymbol>(GV))
231     if (const GlobalObject *Base = GIS->getBaseObject())
232       GV = Base;
233 
234   StringRef Name;
235   if (const Comdat *C = GV->getComdat())
236     Name = C->getName();
237   else
238     Name = GV->getName();
239 
240   // Partition by MD5 hash. We only need a few bits for evenness as the number
241   // of partitions will generally be in the 1-2 figure range; the low 16 bits
242   // are enough.
243   MD5 H;
244   MD5::MD5Result R;
245   H.update(Name);
246   H.final(R);
247   return (R[0] | (R[1] << 8)) % N == I;
248 }
249 
250 void llvm::SplitModule(
251     std::unique_ptr<Module> M, unsigned N,
252     function_ref<void(std::unique_ptr<Module> MPart)> ModuleCallback,
253     bool PreserveLocals) {
254   if (!PreserveLocals) {
255     for (Function &F : *M)
256       externalize(&F);
257     for (GlobalVariable &GV : M->globals())
258       externalize(&GV);
259     for (GlobalAlias &GA : M->aliases())
260       externalize(&GA);
261     for (GlobalIFunc &GIF : M->ifuncs())
262       externalize(&GIF);
263   }
264 
265   // This performs splitting without a need for externalization, which might not
266   // always be possible.
267   ClusterIDMapType ClusterIDMap;
268   findPartitions(M.get(), ClusterIDMap, N);
269 
270   // FIXME: We should be able to reuse M as the last partition instead of
271   // cloning it.
272   for (unsigned I = 0; I < N; ++I) {
273     ValueToValueMapTy VMap;
274     std::unique_ptr<Module> MPart(
275         CloneModule(*M, VMap, [&](const GlobalValue *GV) {
276           if (ClusterIDMap.count(GV))
277             return (ClusterIDMap[GV] == I);
278           else
279             return isInPartition(GV, I, N);
280         }));
281     if (I != 0)
282       MPart->setModuleInlineAsm("");
283     ModuleCallback(std::move(MPart));
284   }
285 }
286