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 <cassert> 41 #include <iterator> 42 #include <memory> 43 #include <queue> 44 #include <utility> 45 #include <vector> 46 47 using namespace llvm; 48 49 #define DEBUG_TYPE "split-module" 50 51 namespace { 52 53 using ClusterMapType = EquivalenceClasses<const GlobalValue *>; 54 using ComdatMembersType = DenseMap<const Comdat *, const GlobalValue *>; 55 using ClusterIDMapType = DenseMap<const GlobalValue *, unsigned>; 56 57 bool compareClusters(const std::pair<unsigned, unsigned> &A, 58 const std::pair<unsigned, unsigned> &B) { 59 if (A.second || B.second) 60 return A.second > B.second; 61 return A.first > B.first; 62 } 63 64 using BalancingQueueType = 65 std::priority_queue<std::pair<unsigned, unsigned>, 66 std::vector<std::pair<unsigned, unsigned>>, 67 decltype(compareClusters) *>; 68 69 } // end anonymous namespace 70 71 static void addNonConstUser(ClusterMapType &GVtoClusterMap, 72 const GlobalValue *GV, const User *U) { 73 assert((!isa<Constant>(U) || isa<GlobalValue>(U)) && "Bad user"); 74 75 if (const Instruction *I = dyn_cast<Instruction>(U)) { 76 const GlobalValue *F = I->getParent()->getParent(); 77 GVtoClusterMap.unionSets(GV, F); 78 } else if (const GlobalValue *GVU = dyn_cast<GlobalValue>(U)) { 79 GVtoClusterMap.unionSets(GV, GVU); 80 } else { 81 llvm_unreachable("Underimplemented use case"); 82 } 83 } 84 85 // Adds all GlobalValue users of V to the same cluster as GV. 86 static void addAllGlobalValueUsers(ClusterMapType &GVtoClusterMap, 87 const GlobalValue *GV, const Value *V) { 88 for (const auto *U : V->users()) { 89 SmallVector<const User *, 4> Worklist; 90 Worklist.push_back(U); 91 while (!Worklist.empty()) { 92 const User *UU = Worklist.pop_back_val(); 93 // For each constant that is not a GV (a pure const) recurse. 94 if (isa<Constant>(UU) && !isa<GlobalValue>(UU)) { 95 Worklist.append(UU->user_begin(), UU->user_end()); 96 continue; 97 } 98 addNonConstUser(GVtoClusterMap, GV, UU); 99 } 100 } 101 } 102 103 static const GlobalObject *getGVPartitioningRoot(const GlobalValue *GV) { 104 const GlobalObject *GO = GV->getAliaseeObject(); 105 if (const auto *GI = dyn_cast_or_null<GlobalIFunc>(GO)) 106 GO = GI->getResolverFunction(); 107 return GO; 108 } 109 110 // Find partitions for module in the way that no locals need to be 111 // globalized. 112 // Try to balance pack those partitions into N files since this roughly equals 113 // thread balancing for the backend codegen step. 114 static void findPartitions(Module &M, ClusterIDMapType &ClusterIDMap, 115 unsigned N) { 116 // At this point module should have the proper mix of globals and locals. 117 // As we attempt to partition this module, we must not change any 118 // locals to globals. 119 LLVM_DEBUG(dbgs() << "Partition module with (" << M.size() 120 << ") functions\n"); 121 ClusterMapType GVtoClusterMap; 122 ComdatMembersType ComdatMembers; 123 124 auto recordGVSet = [&GVtoClusterMap, &ComdatMembers](GlobalValue &GV) { 125 if (GV.isDeclaration()) 126 return; 127 128 if (!GV.hasName()) 129 GV.setName("__llvmsplit_unnamed"); 130 131 // Comdat groups must not be partitioned. For comdat groups that contain 132 // locals, record all their members here so we can keep them together. 133 // Comdat groups that only contain external globals are already handled by 134 // the MD5-based partitioning. 135 if (const Comdat *C = GV.getComdat()) { 136 auto &Member = ComdatMembers[C]; 137 if (Member) 138 GVtoClusterMap.unionSets(Member, &GV); 139 else 140 Member = &GV; 141 } 142 143 // Aliases should not be separated from their aliasees and ifuncs should 144 // not be separated from their resolvers regardless of linkage. 145 if (const GlobalObject *Root = getGVPartitioningRoot(&GV)) 146 if (&GV != Root) 147 GVtoClusterMap.unionSets(&GV, Root); 148 149 if (const Function *F = dyn_cast<Function>(&GV)) { 150 for (const BasicBlock &BB : *F) { 151 BlockAddress *BA = BlockAddress::lookup(&BB); 152 if (!BA || !BA->isConstantUsed()) 153 continue; 154 addAllGlobalValueUsers(GVtoClusterMap, F, BA); 155 } 156 } 157 158 if (GV.hasLocalLinkage()) 159 addAllGlobalValueUsers(GVtoClusterMap, &GV, &GV); 160 }; 161 162 llvm::for_each(M.functions(), recordGVSet); 163 llvm::for_each(M.globals(), recordGVSet); 164 llvm::for_each(M.aliases(), recordGVSet); 165 166 // Assigned all GVs to merged clusters while balancing number of objects in 167 // each. 168 BalancingQueueType BalancingQueue(compareClusters); 169 // Pre-populate priority queue with N slot blanks. 170 for (unsigned i = 0; i < N; ++i) 171 BalancingQueue.push(std::make_pair(i, 0)); 172 173 SmallPtrSet<const GlobalValue *, 32> Visited; 174 175 // To guarantee determinism, we have to sort SCC according to size. 176 // When size is the same, use leader's name. 177 for (const auto &C : GVtoClusterMap) { 178 if (!C->isLeader()) 179 continue; 180 181 unsigned CurrentClusterID = BalancingQueue.top().first; 182 unsigned CurrentClusterSize = BalancingQueue.top().second; 183 BalancingQueue.pop(); 184 185 LLVM_DEBUG(dbgs() << "Root[" << CurrentClusterID << "] cluster_size(" 186 << std::distance(GVtoClusterMap.member_begin(*C), 187 GVtoClusterMap.member_end()) 188 << ") ----> " << C->getData()->getName() << "\n"); 189 190 for (ClusterMapType::member_iterator MI = GVtoClusterMap.findLeader(*C); 191 MI != GVtoClusterMap.member_end(); ++MI) { 192 if (!Visited.insert(*MI).second) 193 continue; 194 LLVM_DEBUG(dbgs() << "----> " << (*MI)->getName() 195 << ((*MI)->hasLocalLinkage() ? " l " : " e ") << "\n"); 196 Visited.insert(*MI); 197 ClusterIDMap[*MI] = CurrentClusterID; 198 CurrentClusterSize++; 199 } 200 // Add this set size to the number of entries in this cluster. 201 BalancingQueue.push(std::make_pair(CurrentClusterID, CurrentClusterSize)); 202 } 203 } 204 205 static void externalize(GlobalValue *GV) { 206 if (GV->hasLocalLinkage()) { 207 GV->setLinkage(GlobalValue::ExternalLinkage); 208 GV->setVisibility(GlobalValue::HiddenVisibility); 209 } 210 211 // Unnamed entities must be named consistently between modules. setName will 212 // give a distinct name to each such entity. 213 if (!GV->hasName()) 214 GV->setName("__llvmsplit_unnamed"); 215 } 216 217 // Returns whether GV should be in partition (0-based) I of N. 218 static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) { 219 if (const GlobalObject *Root = getGVPartitioningRoot(GV)) 220 GV = Root; 221 222 StringRef Name; 223 if (const Comdat *C = GV->getComdat()) 224 Name = C->getName(); 225 else 226 Name = GV->getName(); 227 228 // Partition by MD5 hash. We only need a few bits for evenness as the number 229 // of partitions will generally be in the 1-2 figure range; the low 16 bits 230 // are enough. 231 MD5 H; 232 MD5::MD5Result R; 233 H.update(Name); 234 H.final(R); 235 return (R[0] | (R[1] << 8)) % N == I; 236 } 237 238 void llvm::SplitModule( 239 Module &M, unsigned N, 240 function_ref<void(std::unique_ptr<Module> MPart)> ModuleCallback, 241 bool PreserveLocals, bool RoundRobin) { 242 if (!PreserveLocals) { 243 for (Function &F : M) 244 externalize(&F); 245 for (GlobalVariable &GV : M.globals()) 246 externalize(&GV); 247 for (GlobalAlias &GA : M.aliases()) 248 externalize(&GA); 249 for (GlobalIFunc &GIF : M.ifuncs()) 250 externalize(&GIF); 251 } 252 253 // This performs splitting without a need for externalization, which might not 254 // always be possible. 255 ClusterIDMapType ClusterIDMap; 256 findPartitions(M, ClusterIDMap, N); 257 258 // Find functions not mapped to modules in ClusterIDMap and count functions 259 // per module. Map unmapped functions using round-robin so that they skip 260 // being distributed by isInPartition() based on function name hashes below. 261 // This provides better uniformity of distribution of functions to modules 262 // in some cases - for example when the number of functions equals to N. 263 if (RoundRobin) { 264 DenseMap<unsigned, unsigned> ModuleFunctionCount; 265 SmallVector<const GlobalValue *> UnmappedFunctions; 266 for (const auto &F : M.functions()) { 267 if (F.isDeclaration() || 268 F.getLinkage() != GlobalValue::LinkageTypes::ExternalLinkage) 269 continue; 270 auto It = ClusterIDMap.find(&F); 271 if (It == ClusterIDMap.end()) 272 UnmappedFunctions.push_back(&F); 273 else 274 ++ModuleFunctionCount[It->second]; 275 } 276 BalancingQueueType BalancingQueue(compareClusters); 277 for (unsigned I = 0; I < N; ++I) { 278 if (auto It = ModuleFunctionCount.find(I); 279 It != ModuleFunctionCount.end()) 280 BalancingQueue.push(*It); 281 else 282 BalancingQueue.push({I, 0}); 283 } 284 for (const auto *const F : UnmappedFunctions) { 285 const unsigned I = BalancingQueue.top().first; 286 const unsigned Count = BalancingQueue.top().second; 287 BalancingQueue.pop(); 288 ClusterIDMap.insert({F, I}); 289 BalancingQueue.push({I, Count + 1}); 290 } 291 } 292 293 // FIXME: We should be able to reuse M as the last partition instead of 294 // cloning it. Note that the callers at the moment expect the module to 295 // be preserved, so will need some adjustments as well. 296 for (unsigned I = 0; I < N; ++I) { 297 ValueToValueMapTy VMap; 298 std::unique_ptr<Module> MPart( 299 CloneModule(M, VMap, [&](const GlobalValue *GV) { 300 if (auto It = ClusterIDMap.find(GV); It != ClusterIDMap.end()) 301 return It->second == I; 302 else 303 return isInPartition(GV, I, N); 304 })); 305 if (I != 0) 306 MPart->setModuleInlineAsm(""); 307 ModuleCallback(std::move(MPart)); 308 } 309 } 310