lib/Transforms/Utils/SplitModule.cpp

   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 }