llvm/lib/Transforms/IPO/StripSymbols.cpp

   1 //===- StripSymbols.cpp - Strip symbols and debug info from a module ------===//
   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 // The StripSymbols transformation implements code stripping. Specifically, it
  10 // can delete:
  11 //
  12 //   * names for virtual registers
  13 //   * symbols for internal globals and functions
  14 //   * debug information
  15 //
  16 // Note that this transformation makes code much less readable, so it should
  17 // only be used in situations where the 'strip' utility would be used, such as
  18 // reducing code size or making it harder to reverse engineer code.
  19 //
  20 //===----------------------------------------------------------------------===//
  21
  22 #include "llvm/Transforms/IPO/StripSymbols.h"
  23 #include "llvm/ADT/SmallPtrSet.h"
  24 #include "llvm/IR/Constants.h"
  25 #include "llvm/IR/DebugInfo.h"
  26 #include "llvm/IR/DerivedTypes.h"
  27 #include "llvm/IR/Instructions.h"
  28 #include "llvm/IR/Module.h"
  29 #include "llvm/IR/PassManager.h"
  30 #include "llvm/IR/TypeFinder.h"
  31 #include "llvm/IR/ValueSymbolTable.h"
  32 #include "llvm/InitializePasses.h"
  33 #include "llvm/Pass.h"
  34 #include "llvm/Transforms/IPO.h"
  35 #include "llvm/Transforms/Utils/Local.h"
  36
  37 using namespace llvm;
  38
  39 namespace {
  40   class StripSymbols : public ModulePass {
  41     bool OnlyDebugInfo;
  42   public:
  43     static char ID; // Pass identification, replacement for typeid
  44     explicit StripSymbols(bool ODI = false)
  45       : ModulePass(ID), OnlyDebugInfo(ODI) {
  46         initializeStripSymbolsPass(*PassRegistry::getPassRegistry());
  47       }
  48
  49     bool runOnModule(Module &M) override;
  50
  51     void getAnalysisUsage(AnalysisUsage &AU) const override {
  52       AU.setPreservesAll();
  53     }
  54   };
  55
  56   class StripNonDebugSymbols : public ModulePass {
  57   public:
  58     static char ID; // Pass identification, replacement for typeid
  59     explicit StripNonDebugSymbols()
  60       : ModulePass(ID) {
  61         initializeStripNonDebugSymbolsPass(*PassRegistry::getPassRegistry());
  62       }
  63
  64     bool runOnModule(Module &M) override;
  65
  66     void getAnalysisUsage(AnalysisUsage &AU) const override {
  67       AU.setPreservesAll();
  68     }
  69   };
  70
  71   class StripDebugDeclare : public ModulePass {
  72   public:
  73     static char ID; // Pass identification, replacement for typeid
  74     explicit StripDebugDeclare()
  75       : ModulePass(ID) {
  76         initializeStripDebugDeclarePass(*PassRegistry::getPassRegistry());
  77       }
  78
  79     bool runOnModule(Module &M) override;
  80
  81     void getAnalysisUsage(AnalysisUsage &AU) const override {
  82       AU.setPreservesAll();
  83     }
  84   };
  85
  86   class StripDeadDebugInfo : public ModulePass {
  87   public:
  88     static char ID; // Pass identification, replacement for typeid
  89     explicit StripDeadDebugInfo()
  90       : ModulePass(ID) {
  91         initializeStripDeadDebugInfoPass(*PassRegistry::getPassRegistry());
  92       }
  93
  94     bool runOnModule(Module &M) override;
  95
  96     void getAnalysisUsage(AnalysisUsage &AU) const override {
  97       AU.setPreservesAll();
  98     }
  99   };
 100 }
 101
 102 char StripSymbols::ID = 0;
 103 INITIALIZE_PASS(StripSymbols, "strip",
 104                 "Strip all symbols from a module", false, false)
 105
 106 ModulePass *llvm::createStripSymbolsPass(bool OnlyDebugInfo) {
 107   return new StripSymbols(OnlyDebugInfo);
 108 }
 109
 110 char StripNonDebugSymbols::ID = 0;
 111 INITIALIZE_PASS(StripNonDebugSymbols, "strip-nondebug",
 112                 "Strip all symbols, except dbg symbols, from a module",
 113                 false, false)
 114
 115 ModulePass *llvm::createStripNonDebugSymbolsPass() {
 116   return new StripNonDebugSymbols();
 117 }
 118
 119 char StripDebugDeclare::ID = 0;
 120 INITIALIZE_PASS(StripDebugDeclare, "strip-debug-declare",
 121                 "Strip all llvm.dbg.declare intrinsics", false, false)
 122
 123 ModulePass *llvm::createStripDebugDeclarePass() {
 124   return new StripDebugDeclare();
 125 }
 126
 127 char StripDeadDebugInfo::ID = 0;
 128 INITIALIZE_PASS(StripDeadDebugInfo, "strip-dead-debug-info",
 129                 "Strip debug info for unused symbols", false, false)
 130
 131 ModulePass *llvm::createStripDeadDebugInfoPass() {
 132   return new StripDeadDebugInfo();
 133 }
 134
 135 /// OnlyUsedBy - Return true if V is only used by Usr.
 136 static bool OnlyUsedBy(Value *V, Value *Usr) {
 137   for (User *U : V->users())
 138     if (U != Usr)
 139       return false;
 140
 141   return true;
 142 }
 143
 144 static void RemoveDeadConstant(Constant *C) {
 145   assert(C->use_empty() && "Constant is not dead!");
 146   SmallPtrSet<Constant*, 4> Operands;
 147   for (Value *Op : C->operands())
 148     if (OnlyUsedBy(Op, C))
 149       Operands.insert(cast<Constant>(Op));
 150   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
 151     if (!GV->hasLocalLinkage()) return;   // Don't delete non-static globals.
 152     GV->eraseFromParent();
 153   } else if (!isa<Function>(C)) {
 154     // FIXME: Why does the type of the constant matter here?
 155     if (isa<StructType>(C->getType()) || isa<ArrayType>(C->getType()) ||
 156         isa<VectorType>(C->getType()))
 157       C->destroyConstant();
 158   }
 159
 160   // If the constant referenced anything, see if we can delete it as well.
 161   for (Constant *O : Operands)
 162     RemoveDeadConstant(O);
 163 }
 164
 165 // Strip the symbol table of its names.
 166 //
 167 static void StripSymtab(ValueSymbolTable &ST, bool PreserveDbgInfo) {
 168   for (ValueSymbolTable::iterator VI = ST.begin(), VE = ST.end(); VI != VE; ) {
 169     Value *V = VI->getValue();
 170     ++VI;
 171     if (!isa<GlobalValue>(V) || cast<GlobalValue>(V)->hasLocalLinkage()) {
 172       if (!PreserveDbgInfo || !V->getName().startswith("llvm.dbg"))
 173         // Set name to "", removing from symbol table!
 174         V->setName("");
 175     }
 176   }
 177 }
 178
 179 // Strip any named types of their names.
 180 static void StripTypeNames(Module &M, bool PreserveDbgInfo) {
 181   TypeFinder StructTypes;
 182   StructTypes.run(M, false);
 183
 184   for (unsigned i = 0, e = StructTypes.size(); i != e; ++i) {
 185     StructType *STy = StructTypes[i];
 186     if (STy->isLiteral() || STy->getName().empty()) continue;
 187
 188     if (PreserveDbgInfo && STy->getName().startswith("llvm.dbg"))
 189       continue;
 190
 191     STy->setName("");
 192   }
 193 }
 194
 195 /// Find values that are marked as llvm.used.
 196 static void findUsedValues(GlobalVariable *LLVMUsed,
 197                            SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
 198   if (!LLVMUsed) return;
 199   UsedValues.insert(LLVMUsed);
 200
 201   ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
 202
 203   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
 204     if (GlobalValue *GV =
 205           dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
 206       UsedValues.insert(GV);
 207 }
 208
 209 /// StripSymbolNames - Strip symbol names.
 210 static bool StripSymbolNames(Module &M, bool PreserveDbgInfo) {
 211
 212   SmallPtrSet<const GlobalValue*, 8> llvmUsedValues;
 213   findUsedValues(M.getGlobalVariable("llvm.used"), llvmUsedValues);
 214   findUsedValues(M.getGlobalVariable("llvm.compiler.used"), llvmUsedValues);
 215
 216   for (GlobalVariable &GV : M.globals()) {
 217     if (GV.hasLocalLinkage() && llvmUsedValues.count(&GV) == 0)
 218       if (!PreserveDbgInfo || !GV.getName().startswith("llvm.dbg"))
 219         GV.setName(""); // Internal symbols can't participate in linkage
 220   }
 221
 222   for (Function &I : M) {
 223     if (I.hasLocalLinkage() && llvmUsedValues.count(&I) == 0)
 224       if (!PreserveDbgInfo || !I.getName().startswith("llvm.dbg"))
 225         I.setName(""); // Internal symbols can't participate in linkage
 226     if (auto *Symtab = I.getValueSymbolTable())
 227       StripSymtab(*Symtab, PreserveDbgInfo);
 228   }
 229
 230   // Remove all names from types.
 231   StripTypeNames(M, PreserveDbgInfo);
 232
 233   return true;
 234 }
 235
 236 bool StripSymbols::runOnModule(Module &M) {
 237   if (skipModule(M))
 238     return false;
 239
 240   bool Changed = false;
 241   Changed |= StripDebugInfo(M);
 242   if (!OnlyDebugInfo)
 243     Changed |= StripSymbolNames(M, false);
 244   return Changed;
 245 }
 246
 247 bool StripNonDebugSymbols::runOnModule(Module &M) {
 248   if (skipModule(M))
 249     return false;
 250
 251   return StripSymbolNames(M, true);
 252 }
 253
 254 static bool stripDebugDeclareImpl(Module &M) {
 255
 256   Function *Declare = M.getFunction("llvm.dbg.declare");
 257   std::vector<Constant*> DeadConstants;
 258
 259   if (Declare) {
 260     while (!Declare->use_empty()) {
 261       CallInst *CI = cast<CallInst>(Declare->user_back());
 262       Value *Arg1 = CI->getArgOperand(0);
 263       Value *Arg2 = CI->getArgOperand(1);
 264       assert(CI->use_empty() && "llvm.dbg intrinsic should have void result");
 265       CI->eraseFromParent();
 266       if (Arg1->use_empty()) {
 267         if (Constant *C = dyn_cast<Constant>(Arg1))
 268           DeadConstants.push_back(C);
 269         else
 270           RecursivelyDeleteTriviallyDeadInstructions(Arg1);
 271       }
 272       if (Arg2->use_empty())
 273         if (Constant *C = dyn_cast<Constant>(Arg2))
 274           DeadConstants.push_back(C);
 275     }
 276     Declare->eraseFromParent();
 277   }
 278
 279   while (!DeadConstants.empty()) {
 280     Constant *C = DeadConstants.back();
 281     DeadConstants.pop_back();
 282     if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
 283       if (GV->hasLocalLinkage())
 284         RemoveDeadConstant(GV);
 285     } else
 286       RemoveDeadConstant(C);
 287   }
 288
 289   return true;
 290 }
 291
 292 bool StripDebugDeclare::runOnModule(Module &M) {
 293   if (skipModule(M))
 294     return false;
 295   return stripDebugDeclareImpl(M);
 296 }
 297
 298 static bool stripDeadDebugInfoImpl(Module &M) {
 299   bool Changed = false;
 300
 301   LLVMContext &C = M.getContext();
 302
 303   // Find all debug info in F. This is actually overkill in terms of what we
 304   // want to do, but we want to try and be as resilient as possible in the face
 305   // of potential debug info changes by using the formal interfaces given to us
 306   // as much as possible.
 307   DebugInfoFinder F;
 308   F.processModule(M);
 309
 310   // For each compile unit, find the live set of global variables/functions and
 311   // replace the current list of potentially dead global variables/functions
 312   // with the live list.
 313   SmallVector<Metadata *, 64> LiveGlobalVariables;
 314   DenseSet<DIGlobalVariableExpression *> VisitedSet;
 315
 316   std::set<DIGlobalVariableExpression *> LiveGVs;
 317   for (GlobalVariable &GV : M.globals()) {
 318     SmallVector<DIGlobalVariableExpression *, 1> GVEs;
 319     GV.getDebugInfo(GVEs);
 320     for (auto *GVE : GVEs)
 321       LiveGVs.insert(GVE);
 322   }
 323
 324   std::set<DICompileUnit *> LiveCUs;
 325   // Any CU referenced from a subprogram is live.
 326   for (DISubprogram *SP : F.subprograms()) {
 327     if (SP->getUnit())
 328       LiveCUs.insert(SP->getUnit());
 329   }
 330
 331   bool HasDeadCUs = false;
 332   for (DICompileUnit *DIC : F.compile_units()) {
 333     // Create our live global variable list.
 334     bool GlobalVariableChange = false;
 335     for (auto *DIG : DIC->getGlobalVariables()) {
 336       if (DIG->getExpression() && DIG->getExpression()->isConstant())
 337         LiveGVs.insert(DIG);
 338
 339       // Make sure we only visit each global variable only once.
 340       if (!VisitedSet.insert(DIG).second)
 341         continue;
 342
 343       // If a global variable references DIG, the global variable is live.
 344       if (LiveGVs.count(DIG))
 345         LiveGlobalVariables.push_back(DIG);
 346       else
 347         GlobalVariableChange = true;
 348     }
 349
 350     if (!LiveGlobalVariables.empty())
 351       LiveCUs.insert(DIC);
 352     else if (!LiveCUs.count(DIC))
 353       HasDeadCUs = true;
 354
 355     // If we found dead global variables, replace the current global
 356     // variable list with our new live global variable list.
 357     if (GlobalVariableChange) {
 358       DIC->replaceGlobalVariables(MDTuple::get(C, LiveGlobalVariables));
 359       Changed = true;
 360     }
 361
 362     // Reset lists for the next iteration.
 363     LiveGlobalVariables.clear();
 364   }
 365
 366   if (HasDeadCUs) {
 367     // Delete the old node and replace it with a new one
 368     NamedMDNode *NMD = M.getOrInsertNamedMetadata("llvm.dbg.cu");
 369     NMD->clearOperands();
 370     if (!LiveCUs.empty()) {
 371       for (DICompileUnit *CU : LiveCUs)
 372         NMD->addOperand(CU);
 373     }
 374     Changed = true;
 375   }
 376
 377   return Changed;
 378 }
 379
 380 /// Remove any debug info for global variables/functions in the given module for
 381 /// which said global variable/function no longer exists (i.e. is null).
 382 ///
 383 /// Debugging information is encoded in llvm IR using metadata. This is designed
 384 /// such a way that debug info for symbols preserved even if symbols are
 385 /// optimized away by the optimizer. This special pass removes debug info for
 386 /// such symbols.
 387 bool StripDeadDebugInfo::runOnModule(Module &M) {
 388   if (skipModule(M))
 389     return false;
 390   return stripDeadDebugInfoImpl(M);
 391 }
 392
 393 PreservedAnalyses StripSymbolsPass::run(Module &M, ModuleAnalysisManager &AM) {
 394   StripDebugInfo(M);
 395   StripSymbolNames(M, false);
 396   return PreservedAnalyses::all();
 397 }
 398
 399 PreservedAnalyses StripNonDebugSymbolsPass::run(Module &M,
 400                                                 ModuleAnalysisManager &AM) {
 401   StripSymbolNames(M, true);
 402   return PreservedAnalyses::all();
 403 }
 404
 405 PreservedAnalyses StripDebugDeclarePass::run(Module &M,
 406                                              ModuleAnalysisManager &AM) {
 407   stripDebugDeclareImpl(M);
 408   return PreservedAnalyses::all();
 409 }
 410
 411 PreservedAnalyses StripDeadDebugInfoPass::run(Module &M,
 412                                               ModuleAnalysisManager &AM) {
 413   stripDeadDebugInfoImpl(M);
 414   return PreservedAnalyses::all();
 415 }