llvm/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/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 }