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/GlobalValue.h"
28 #include "llvm/IR/GlobalVariable.h"
29 #include "llvm/IR/Instruction.h"
30 #include "llvm/IR/Module.h"
31 #include "llvm/IR/User.h"
32 #include "llvm/IR/Value.h"
33 #include "llvm/Support/Casting.h"
34 #include "llvm/Support/Debug.h"
35 #include "llvm/Support/ErrorHandling.h"
36 #include "llvm/Support/MD5.h"
37 #include "llvm/Support/raw_ostream.h"
38 #include "llvm/Transforms/Utils/Cloning.h"
39 #include "llvm/Transforms/Utils/ValueMapper.h"
40 #include <algorithm>
41 #include <cassert>
42 #include <iterator>
43 #include <memory>
44 #include <queue>
45 #include <utility>
46 #include <vector>
47 
48 using namespace llvm;
49 
50 #define DEBUG_TYPE "split-module"
51 
52 namespace {
53 
54 using ClusterMapType = EquivalenceClasses<const GlobalValue *>;
55 using ComdatMembersType = DenseMap<const Comdat *, const GlobalValue *>;
56 using ClusterIDMapType = DenseMap<const GlobalValue *, unsigned>;
57 
58 } // end anonymous namespace
59 
60 static void addNonConstUser(ClusterMapType &GVtoClusterMap,
61                             const GlobalValue *GV, const User *U) {
62   assert((!isa<Constant>(U) || isa<GlobalValue>(U)) && "Bad user");
63 
64   if (const Instruction *I = dyn_cast<Instruction>(U)) {
65     const GlobalValue *F = I->getParent()->getParent();
66     GVtoClusterMap.unionSets(GV, F);
67   } else if (const GlobalValue *GVU = dyn_cast<GlobalValue>(U)) {
68     GVtoClusterMap.unionSets(GV, GVU);
69   } else {
70     llvm_unreachable("Underimplemented use case");
71   }
72 }
73 
74 // Adds all GlobalValue users of V to the same cluster as GV.
75 static void addAllGlobalValueUsers(ClusterMapType &GVtoClusterMap,
76                                    const GlobalValue *GV, const Value *V) {
77   for (const auto *U : V->users()) {
78     SmallVector<const User *, 4> Worklist;
79     Worklist.push_back(U);
80     while (!Worklist.empty()) {
81       const User *UU = Worklist.pop_back_val();
82       // For each constant that is not a GV (a pure const) recurse.
83       if (isa<Constant>(UU) && !isa<GlobalValue>(UU)) {
84         Worklist.append(UU->user_begin(), UU->user_end());
85         continue;
86       }
87       addNonConstUser(GVtoClusterMap, GV, UU);
88     }
89   }
90 }
91 
92 static const GlobalObject *getGVPartitioningRoot(const GlobalValue *GV) {
93   const GlobalObject *GO = GV->getAliaseeObject();
94   if (const auto *GI = dyn_cast_or_null<GlobalIFunc>(GO))
95     GO = GI->getResolverFunction();
96   return GO;
97 }
98 
99 // Find partitions for module in the way that no locals need to be
100 // globalized.
101 // Try to balance pack those partitions into N files since this roughly equals
102 // thread balancing for the backend codegen step.
103 static void findPartitions(Module &M, ClusterIDMapType &ClusterIDMap,
104                            unsigned N) {
105   // At this point module should have the proper mix of globals and locals.
106   // As we attempt to partition this module, we must not change any
107   // locals to globals.
108   LLVM_DEBUG(dbgs() << "Partition module with (" << M.size() << ")functions\n");
109   ClusterMapType GVtoClusterMap;
110   ComdatMembersType ComdatMembers;
111 
112   auto recordGVSet = [&GVtoClusterMap, &ComdatMembers](GlobalValue &GV) {
113     if (GV.isDeclaration())
114       return;
115 
116     if (!GV.hasName())
117       GV.setName("__llvmsplit_unnamed");
118 
119     // Comdat groups must not be partitioned. For comdat groups that contain
120     // locals, record all their members here so we can keep them together.
121     // Comdat groups that only contain external globals are already handled by
122     // the MD5-based partitioning.
123     if (const Comdat *C = GV.getComdat()) {
124       auto &Member = ComdatMembers[C];
125       if (Member)
126         GVtoClusterMap.unionSets(Member, &GV);
127       else
128         Member = &GV;
129     }
130 
131     // Aliases should not be separated from their aliasees and ifuncs should
132     // not be separated from their resolvers regardless of linkage.
133     if (const GlobalObject *Root = getGVPartitioningRoot(&GV))
134       if (&GV != Root)
135         GVtoClusterMap.unionSets(&GV, Root);
136 
137     if (const Function *F = dyn_cast<Function>(&GV)) {
138       for (const BasicBlock &BB : *F) {
139         BlockAddress *BA = BlockAddress::lookup(&BB);
140         if (!BA || !BA->isConstantUsed())
141           continue;
142         addAllGlobalValueUsers(GVtoClusterMap, F, BA);
143       }
144     }
145 
146     if (GV.hasLocalLinkage())
147       addAllGlobalValueUsers(GVtoClusterMap, &GV, &GV);
148   };
149 
150   llvm::for_each(M.functions(), recordGVSet);
151   llvm::for_each(M.globals(), recordGVSet);
152   llvm::for_each(M.aliases(), recordGVSet);
153 
154   // Assigned all GVs to merged clusters while balancing number of objects in
155   // each.
156   auto CompareClusters = [](const std::pair<unsigned, unsigned> &a,
157                             const std::pair<unsigned, unsigned> &b) {
158     if (a.second || b.second)
159       return a.second > b.second;
160     else
161       return a.first > b.first;
162   };
163 
164   std::priority_queue<std::pair<unsigned, unsigned>,
165                       std::vector<std::pair<unsigned, unsigned>>,
166                       decltype(CompareClusters)>
167       BalancinQueue(CompareClusters);
168   // Pre-populate priority queue with N slot blanks.
169   for (unsigned i = 0; i < N; ++i)
170     BalancinQueue.push(std::make_pair(i, 0));
171 
172   using SortType = std::pair<unsigned, ClusterMapType::iterator>;
173 
174   SmallVector<SortType, 64> Sets;
175   SmallPtrSet<const GlobalValue *, 32> Visited;
176 
177   // To guarantee determinism, we have to sort SCC according to size.
178   // When size is the same, use leader's name.
179   for (ClusterMapType::iterator I = GVtoClusterMap.begin(),
180                                 E = GVtoClusterMap.end(); I != E; ++I)
181     if (I->isLeader())
182       Sets.push_back(
183           std::make_pair(std::distance(GVtoClusterMap.member_begin(I),
184                                        GVtoClusterMap.member_end()), I));
185 
186   llvm::sort(Sets, [](const SortType &a, const SortType &b) {
187     if (a.first == b.first)
188       return a.second->getData()->getName() > b.second->getData()->getName();
189     else
190       return a.first > b.first;
191   });
192 
193   for (auto &I : Sets) {
194     unsigned CurrentClusterID = BalancinQueue.top().first;
195     unsigned CurrentClusterSize = BalancinQueue.top().second;
196     BalancinQueue.pop();
197 
198     LLVM_DEBUG(dbgs() << "Root[" << CurrentClusterID << "] cluster_size("
199                       << I.first << ") ----> " << I.second->getData()->getName()
200                       << "\n");
201 
202     for (ClusterMapType::member_iterator MI =
203              GVtoClusterMap.findLeader(I.second);
204          MI != GVtoClusterMap.member_end(); ++MI) {
205       if (!Visited.insert(*MI).second)
206         continue;
207       LLVM_DEBUG(dbgs() << "----> " << (*MI)->getName()
208                         << ((*MI)->hasLocalLinkage() ? " l " : " e ") << "\n");
209       Visited.insert(*MI);
210       ClusterIDMap[*MI] = CurrentClusterID;
211       CurrentClusterSize++;
212     }
213     // Add this set size to the number of entries in this cluster.
214     BalancinQueue.push(std::make_pair(CurrentClusterID, CurrentClusterSize));
215   }
216 }
217 
218 static void externalize(GlobalValue *GV) {
219   if (GV->hasLocalLinkage()) {
220     GV->setLinkage(GlobalValue::ExternalLinkage);
221     GV->setVisibility(GlobalValue::HiddenVisibility);
222   }
223 
224   // Unnamed entities must be named consistently between modules. setName will
225   // give a distinct name to each such entity.
226   if (!GV->hasName())
227     GV->setName("__llvmsplit_unnamed");
228 }
229 
230 // Returns whether GV should be in partition (0-based) I of N.
231 static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) {
232   if (const GlobalObject *Root = getGVPartitioningRoot(GV))
233     GV = Root;
234 
235   StringRef Name;
236   if (const Comdat *C = GV->getComdat())
237     Name = C->getName();
238   else
239     Name = GV->getName();
240 
241   // Partition by MD5 hash. We only need a few bits for evenness as the number
242   // of partitions will generally be in the 1-2 figure range; the low 16 bits
243   // are enough.
244   MD5 H;
245   MD5::MD5Result R;
246   H.update(Name);
247   H.final(R);
248   return (R[0] | (R[1] << 8)) % N == I;
249 }
250 
251 void llvm::SplitModule(
252     Module &M, unsigned N,
253     function_ref<void(std::unique_ptr<Module> MPart)> ModuleCallback,
254     bool PreserveLocals) {
255   if (!PreserveLocals) {
256     for (Function &F : M)
257       externalize(&F);
258     for (GlobalVariable &GV : M.globals())
259       externalize(&GV);
260     for (GlobalAlias &GA : M.aliases())
261       externalize(&GA);
262     for (GlobalIFunc &GIF : M.ifuncs())
263       externalize(&GIF);
264   }
265 
266   // This performs splitting without a need for externalization, which might not
267   // always be possible.
268   ClusterIDMapType ClusterIDMap;
269   findPartitions(M, ClusterIDMap, N);
270 
271   // FIXME: We should be able to reuse M as the last partition instead of
272   // cloning it. Note that the callers at the moment expect the module to
273   // be preserved, so will need some adjustments as well.
274   for (unsigned I = 0; I < N; ++I) {
275     ValueToValueMapTy VMap;
276     std::unique_ptr<Module> MPart(
277         CloneModule(M, VMap, [&](const GlobalValue *GV) {
278           if (ClusterIDMap.count(GV))
279             return (ClusterIDMap[GV] == I);
280           else
281             return isInPartition(GV, I, N);
282         }));
283     if (I != 0)
284       MPart->setModuleInlineAsm("");
285     ModuleCallback(std::move(MPart));
286   }
287 }
288