llvm/lib/Transforms/IPO/Internalize.cpp

   1 //===-- Internalize.cpp - Mark functions internal -------------------------===//
   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 pass loops over all of the functions and variables in the input module.
  10 // If the function or variable does not need to be preserved according to the
  11 // client supplied callback, it is marked as internal.
  12 //
  13 // This transformation would not be legal in a regular compilation, but it gets
  14 // extra information from the linker about what is safe.
  15 //
  16 // For example: Internalizing a function with external linkage. Only if we are
  17 // told it is only used from within this module, it is safe to do it.
  18 //
  19 //===----------------------------------------------------------------------===//
  20
  21 #include "llvm/Transforms/IPO/Internalize.h"
  22 #include "llvm/ADT/SmallPtrSet.h"
  23 #include "llvm/ADT/Statistic.h"
  24 #include "llvm/ADT/StringSet.h"
  25 #include "llvm/ADT/Triple.h"
  26 #include "llvm/Analysis/CallGraph.h"
  27 #include "llvm/IR/Module.h"
  28 #include "llvm/InitializePasses.h"
  29 #include "llvm/Pass.h"
  30 #include "llvm/Support/CommandLine.h"
  31 #include "llvm/Support/Debug.h"
  32 #include "llvm/Support/LineIterator.h"
  33 #include "llvm/Support/MemoryBuffer.h"
  34 #include "llvm/Support/raw_ostream.h"
  35 #include "llvm/Transforms/IPO.h"
  36 #include "llvm/Transforms/Utils/GlobalStatus.h"
  37 #include "llvm/Transforms/Utils/ModuleUtils.h"
  38 using namespace llvm;
  39
  40 #define DEBUG_TYPE "internalize"
  41
  42 STATISTIC(NumAliases, "Number of aliases internalized");
  43 STATISTIC(NumFunctions, "Number of functions internalized");
  44 STATISTIC(NumGlobals, "Number of global vars internalized");
  45
  46 // APIFile - A file which contains a list of symbols that should not be marked
  47 // external.
  48 static cl::opt<std::string>
  49     APIFile("internalize-public-api-file", cl::value_desc("filename"),
  50             cl::desc("A file containing list of symbol names to preserve"));
  51
  52 // APIList - A list of symbols that should not be marked internal.
  53 static cl::list<std::string>
  54     APIList("internalize-public-api-list", cl::value_desc("list"),
  55             cl::desc("A list of symbol names to preserve"), cl::CommaSeparated);
  56
  57 namespace {
  58 // Helper to load an API list to preserve from file and expose it as a functor
  59 // for internalization.
  60 class PreserveAPIList {
  61 public:
  62   PreserveAPIList() {
  63     if (!APIFile.empty())
  64       LoadFile(APIFile);
  65     ExternalNames.insert(APIList.begin(), APIList.end());
  66   }
  67
  68   bool operator()(const GlobalValue &GV) {
  69     return ExternalNames.count(GV.getName());
  70   }
  71
  72 private:
  73   // Contains the set of symbols loaded from file
  74   StringSet<> ExternalNames;
  75
  76   void LoadFile(StringRef Filename) {
  77     // Load the APIFile...
  78     ErrorOr<std::unique_ptr<MemoryBuffer>> Buf =
  79         MemoryBuffer::getFile(Filename);
  80     if (!Buf) {
  81       errs() << "WARNING: Internalize couldn't load file '" << Filename
  82              << "'! Continuing as if it's empty.\n";
  83       return; // Just continue as if the file were empty
  84     }
  85     for (line_iterator I(*Buf->get(), true), E; I != E; ++I)
  86       ExternalNames.insert(*I);
  87   }
  88 };
  89 } // end anonymous namespace
  90
  91 bool InternalizePass::shouldPreserveGV(const GlobalValue &GV) {
  92   // Function must be defined here
  93   if (GV.isDeclaration())
  94     return true;
  95
  96   // Available externally is really just a "declaration with a body".
  97   if (GV.hasAvailableExternallyLinkage())
  98     return true;
  99
 100   // Assume that dllexported symbols are referenced elsewhere
 101   if (GV.hasDLLExportStorageClass())
 102     return true;
 103
 104   // As the name suggests, externally initialized variables need preserving as
 105   // they would be initialized elsewhere externally.
 106   if (const auto *G = dyn_cast<GlobalVariable>(&GV))
 107     if (G->isExternallyInitialized())
 108       return true;
 109
 110   // Already local, has nothing to do.
 111   if (GV.hasLocalLinkage())
 112     return false;
 113
 114   // Check some special cases
 115   if (AlwaysPreserved.count(GV.getName()))
 116     return true;
 117
 118   return MustPreserveGV(GV);
 119 }
 120
 121 bool InternalizePass::maybeInternalize(
 122     GlobalValue &GV, DenseMap<const Comdat *, ComdatInfo> &ComdatMap) {
 123   SmallString<0> ComdatName;
 124   if (Comdat *C = GV.getComdat()) {
 125     // For GlobalAlias, C is the aliasee object's comdat which may have been
 126     // redirected. So ComdatMap may not contain C.
 127     if (ComdatMap.lookup(C).External)
 128       return false;
 129
 130     if (auto *GO = dyn_cast<GlobalObject>(&GV)) {
 131       // If a comdat with one member is not externally visible, we can drop it.
 132       // Otherwise, the comdat can be used to establish dependencies among the
 133       // group of sections. Thus we have to keep the comdat but switch it to
 134       // nodeduplicate.
 135       // Note: nodeduplicate is not necessary for COFF. wasm doesn't support
 136       // nodeduplicate.
 137       ComdatInfo &Info = ComdatMap.find(C)->second;
 138       if (Info.Size == 1)
 139         GO->setComdat(nullptr);
 140       else if (!IsWasm)
 141         C->setSelectionKind(Comdat::NoDeduplicate);
 142     }
 143
 144     if (GV.hasLocalLinkage())
 145       return false;
 146   } else {
 147     if (GV.hasLocalLinkage())
 148       return false;
 149
 150     if (shouldPreserveGV(GV))
 151       return false;
 152   }
 153
 154   GV.setVisibility(GlobalValue::DefaultVisibility);
 155   GV.setLinkage(GlobalValue::InternalLinkage);
 156   return true;
 157 }
 158
 159 // If GV is part of a comdat and is externally visible, update the comdat size
 160 // and keep track of its comdat so that we don't internalize any of its members.
 161 void InternalizePass::checkComdat(
 162     GlobalValue &GV, DenseMap<const Comdat *, ComdatInfo> &ComdatMap) {
 163   Comdat *C = GV.getComdat();
 164   if (!C)
 165     return;
 166
 167   ComdatInfo &Info = ComdatMap.try_emplace(C).first->second;
 168   ++Info.Size;
 169   if (shouldPreserveGV(GV))
 170     Info.External = true;
 171 }
 172
 173 bool InternalizePass::internalizeModule(Module &M, CallGraph *CG) {
 174   bool Changed = false;
 175   CallGraphNode *ExternalNode = CG ? CG->getExternalCallingNode() : nullptr;
 176
 177   SmallVector<GlobalValue *, 4> Used;
 178   collectUsedGlobalVariables(M, Used, false);
 179
 180   // Collect comdat size and visiblity information for the module.
 181   DenseMap<const Comdat *, ComdatInfo> ComdatMap;
 182   if (!M.getComdatSymbolTable().empty()) {
 183     for (Function &F : M)
 184       checkComdat(F, ComdatMap);
 185     for (GlobalVariable &GV : M.globals())
 186       checkComdat(GV, ComdatMap);
 187     for (GlobalAlias &GA : M.aliases())
 188       checkComdat(GA, ComdatMap);
 189   }
 190
 191   // We must assume that globals in llvm.used have a reference that not even
 192   // the linker can see, so we don't internalize them.
 193   // For llvm.compiler.used the situation is a bit fuzzy. The assembler and
 194   // linker can drop those symbols. If this pass is running as part of LTO,
 195   // one might think that it could just drop llvm.compiler.used. The problem
 196   // is that even in LTO llvm doesn't see every reference. For example,
 197   // we don't see references from function local inline assembly. To be
 198   // conservative, we internalize symbols in llvm.compiler.used, but we
 199   // keep llvm.compiler.used so that the symbol is not deleted by llvm.
 200   for (GlobalValue *V : Used) {
 201     AlwaysPreserved.insert(V->getName());
 202   }
 203
 204   // Mark all functions not in the api as internal.
 205   IsWasm = Triple(M.getTargetTriple()).isOSBinFormatWasm();
 206   for (Function &I : M) {
 207     if (!maybeInternalize(I, ComdatMap))
 208       continue;
 209     Changed = true;
 210
 211     if (ExternalNode)
 212       // Remove a callgraph edge from the external node to this function.
 213       ExternalNode->removeOneAbstractEdgeTo((*CG)[&I]);
 214
 215     ++NumFunctions;
 216     LLVM_DEBUG(dbgs() << "Internalizing func " << I.getName() << "\n");
 217   }
 218
 219   // Never internalize the llvm.used symbol.  It is used to implement
 220   // attribute((used)).
 221   // FIXME: Shouldn't this just filter on llvm.metadata section??
 222   AlwaysPreserved.insert("llvm.used");
 223   AlwaysPreserved.insert("llvm.compiler.used");
 224
 225   // Never internalize anchors used by the machine module info, else the info
 226   // won't find them.  (see MachineModuleInfo.)
 227   AlwaysPreserved.insert("llvm.global_ctors");
 228   AlwaysPreserved.insert("llvm.global_dtors");
 229   AlwaysPreserved.insert("llvm.global.annotations");
 230
 231   // Never internalize symbols code-gen inserts.
 232   // FIXME: We should probably add this (and the __stack_chk_guard) via some
 233   // type of call-back in CodeGen.
 234   AlwaysPreserved.insert("__stack_chk_fail");
 235   if (Triple(M.getTargetTriple()).isOSAIX())
 236     AlwaysPreserved.insert("__ssp_canary_word");
 237   else
 238     AlwaysPreserved.insert("__stack_chk_guard");
 239
 240   // Mark all global variables with initializers that are not in the api as
 241   // internal as well.
 242   for (auto &GV : M.globals()) {
 243     if (!maybeInternalize(GV, ComdatMap))
 244       continue;
 245     Changed = true;
 246
 247     ++NumGlobals;
 248     LLVM_DEBUG(dbgs() << "Internalized gvar " << GV.getName() << "\n");
 249   }
 250
 251   // Mark all aliases that are not in the api as internal as well.
 252   for (auto &GA : M.aliases()) {
 253     if (!maybeInternalize(GA, ComdatMap))
 254       continue;
 255     Changed = true;
 256
 257     ++NumAliases;
 258     LLVM_DEBUG(dbgs() << "Internalized alias " << GA.getName() << "\n");
 259   }
 260
 261   return Changed;
 262 }
 263
 264 InternalizePass::InternalizePass() : MustPreserveGV(PreserveAPIList()) {}
 265
 266 PreservedAnalyses InternalizePass::run(Module &M, ModuleAnalysisManager &AM) {
 267   if (!internalizeModule(M, AM.getCachedResult<CallGraphAnalysis>(M)))
 268     return PreservedAnalyses::all();
 269
 270   PreservedAnalyses PA;
 271   PA.preserve<CallGraphAnalysis>();
 272   return PA;
 273 }
 274
 275 namespace {
 276 class InternalizeLegacyPass : public ModulePass {
 277   // Client supplied callback to control wheter a symbol must be preserved.
 278   std::function<bool(const GlobalValue &)> MustPreserveGV;
 279
 280 public:
 281   static char ID; // Pass identification, replacement for typeid
 282
 283   InternalizeLegacyPass() : ModulePass(ID), MustPreserveGV(PreserveAPIList()) {}
 284
 285   InternalizeLegacyPass(std::function<bool(const GlobalValue &)> MustPreserveGV)
 286       : ModulePass(ID), MustPreserveGV(std::move(MustPreserveGV)) {
 287     initializeInternalizeLegacyPassPass(*PassRegistry::getPassRegistry());
 288   }
 289
 290   bool runOnModule(Module &M) override {
 291     if (skipModule(M))
 292       return false;
 293
 294     CallGraphWrapperPass *CGPass =
 295         getAnalysisIfAvailable<CallGraphWrapperPass>();
 296     CallGraph *CG = CGPass ? &CGPass->getCallGraph() : nullptr;
 297     return internalizeModule(M, MustPreserveGV, CG);
 298   }
 299
 300   void getAnalysisUsage(AnalysisUsage &AU) const override {
 301     AU.setPreservesCFG();
 302     AU.addPreserved<CallGraphWrapperPass>();
 303   }
 304 };
 305 }
 306
 307 char InternalizeLegacyPass::ID = 0;
 308 INITIALIZE_PASS(InternalizeLegacyPass, "internalize",
 309                 "Internalize Global Symbols", false, false)
 310
 311 ModulePass *llvm::createInternalizePass() {
 312   return new InternalizeLegacyPass();
 313 }
 314
 315 ModulePass *llvm::createInternalizePass(
 316     std::function<bool(const GlobalValue &)> MustPreserveGV) {
 317   return new InternalizeLegacyPass(std::move(MustPreserveGV));
 318 }