src/llvm/lib/Transforms/IPO/StripSymbols.cpp

   1 //===- StripSymbols.cpp - Strip symbols and debug info from a module ------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // The StripSymbols transformation implements code stripping. Specifically, it
  11 // can delete:
  12 //
  13 //   * names for virtual registers
  14 //   * symbols for internal globals and functions
  15 //   * debug information
  16 //
  17 // Note that this transformation makes code much less readable, so it should
  18 // only be used in situations where the 'strip' utility would be used, such as
  19 // reducing code size or making it harder to reverse engineer code.
  20 //
  21 //===----------------------------------------------------------------------===//
  22
  23 #include "llvm/Transforms/IPO.h"
  24 #include "llvm/ADT/DenseMap.h"
  25 #include "llvm/ADT/SmallPtrSet.h"
  26 #include "llvm/IR/Constants.h"
  27 #include "llvm/IR/DebugInfo.h"
  28 #include "llvm/IR/DerivedTypes.h"
  29 #include "llvm/IR/Instructions.h"
  30 #include "llvm/IR/Module.h"
  31 #include "llvm/IR/TypeFinder.h"
  32 #include "llvm/IR/ValueSymbolTable.h"
  33 #include "llvm/Pass.h"
  34 #include "llvm/Transforms/Utils/Local.h"
  35 using namespace llvm;
  36
  37 namespace {
  38   class StripSymbols : public ModulePass {
  39     bool OnlyDebugInfo;
  40   public:
  41     static char ID; // Pass identification, replacement for typeid
  42     explicit StripSymbols(bool ODI = false)
  43       : ModulePass(ID), OnlyDebugInfo(ODI) {
  44         initializeStripSymbolsPass(*PassRegistry::getPassRegistry());
  45       }
  46
  47     bool runOnModule(Module &M) override;
  48
  49     void getAnalysisUsage(AnalysisUsage &AU) const override {
  50       AU.setPreservesAll();
  51     }
  52   };
  53
  54   class StripNonDebugSymbols : public ModulePass {
  55   public:
  56     static char ID; // Pass identification, replacement for typeid
  57     explicit StripNonDebugSymbols()
  58       : ModulePass(ID) {
  59         initializeStripNonDebugSymbolsPass(*PassRegistry::getPassRegistry());
  60       }
  61
  62     bool runOnModule(Module &M) override;
  63
  64     void getAnalysisUsage(AnalysisUsage &AU) const override {
  65       AU.setPreservesAll();
  66     }
  67   };
  68
  69   class StripDebugDeclare : public ModulePass {
  70   public:
  71     static char ID; // Pass identification, replacement for typeid
  72     explicit StripDebugDeclare()
  73       : ModulePass(ID) {
  74         initializeStripDebugDeclarePass(*PassRegistry::getPassRegistry());
  75       }
  76
  77     bool runOnModule(Module &M) override;
  78
  79     void getAnalysisUsage(AnalysisUsage &AU) const override {
  80       AU.setPreservesAll();
  81     }
  82   };
  83
  84   class StripDeadDebugInfo : public ModulePass {
  85   public:
  86     static char ID; // Pass identification, replacement for typeid
  87     explicit StripDeadDebugInfo()
  88       : ModulePass(ID) {
  89         initializeStripDeadDebugInfoPass(*PassRegistry::getPassRegistry());
  90       }
  91
  92     bool runOnModule(Module &M) override;
  93
  94     void getAnalysisUsage(AnalysisUsage &AU) const override {
  95       AU.setPreservesAll();
  96     }
  97   };
  98 }
  99
 100 char StripSymbols::ID = 0;
 101 INITIALIZE_PASS(StripSymbols, "strip",
 102                 "Strip all symbols from a module", false, false)
 103
 104 ModulePass *llvm::createStripSymbolsPass(bool OnlyDebugInfo) {
 105   return new StripSymbols(OnlyDebugInfo);
 106 }
 107
 108 char StripNonDebugSymbols::ID = 0;
 109 INITIALIZE_PASS(StripNonDebugSymbols, "strip-nondebug",
 110                 "Strip all symbols, except dbg symbols, from a module",
 111                 false, false)
 112
 113 ModulePass *llvm::createStripNonDebugSymbolsPass() {
 114   return new StripNonDebugSymbols();
 115 }
 116
 117 char StripDebugDeclare::ID = 0;
 118 INITIALIZE_PASS(StripDebugDeclare, "strip-debug-declare",
 119                 "Strip all llvm.dbg.declare intrinsics", false, false)
 120
 121 ModulePass *llvm::createStripDebugDeclarePass() {
 122   return new StripDebugDeclare();
 123 }
 124
 125 char StripDeadDebugInfo::ID = 0;
 126 INITIALIZE_PASS(StripDeadDebugInfo, "strip-dead-debug-info",
 127                 "Strip debug info for unused symbols", false, false)
 128
 129 ModulePass *llvm::createStripDeadDebugInfoPass() {
 130   return new StripDeadDebugInfo();
 131 }
 132
 133 /// OnlyUsedBy - Return true if V is only used by Usr.
 134 static bool OnlyUsedBy(Value *V, Value *Usr) {
 135   for (User *U : V->users())
 136     if (U != Usr)
 137       return false;
 138
 139   return true;
 140 }
 141
 142 static void RemoveDeadConstant(Constant *C) {
 143   assert(C->use_empty() && "Constant is not dead!");
 144   SmallPtrSet<Constant*, 4> Operands;
 145   for (unsigned i = 0, e = C->getNumOperands(); i != e; ++i)
 146     if (OnlyUsedBy(C->getOperand(i), C))
 147       Operands.insert(cast<Constant>(C->getOperand(i)));
 148   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
 149     if (!GV->hasLocalLinkage()) return;   // Don't delete non-static globals.
 150     GV->eraseFromParent();
 151   }
 152   else if (!isa<Function>(C))
 153     if (isa<CompositeType>(C->getType()))
 154       C->destroyConstant();
 155
 156   // If the constant referenced anything, see if we can delete it as well.
 157   for (Constant *O : Operands)
 158     RemoveDeadConstant(O);
 159 }
 160
 161 // Strip the symbol table of its names.
 162 //
 163 static void StripSymtab(ValueSymbolTable &ST, bool PreserveDbgInfo) {
 164   for (ValueSymbolTable::iterator VI = ST.begin(), VE = ST.end(); VI != VE; ) {
 165     Value *V = VI->getValue();
 166     ++VI;
 167     if (!isa<GlobalValue>(V) || cast<GlobalValue>(V)->hasLocalLinkage()) {
 168       if (!PreserveDbgInfo || !V->getName().startswith("llvm.dbg"))
 169         // Set name to "", removing from symbol table!
 170         V->setName("");
 171     }
 172   }
 173 }
 174
 175 // Strip any named types of their names.
 176 static void StripTypeNames(Module &M, bool PreserveDbgInfo) {
 177   TypeFinder StructTypes;
 178   StructTypes.run(M, false);
 179
 180   for (unsigned i = 0, e = StructTypes.size(); i != e; ++i) {
 181     StructType *STy = StructTypes[i];
 182     if (STy->isLiteral() || STy->getName().empty()) continue;
 183
 184     if (PreserveDbgInfo && STy->getName().startswith("llvm.dbg"))
 185       continue;
 186
 187     STy->setName("");
 188   }
 189 }
 190
 191 /// Find values that are marked as llvm.used.
 192 static void findUsedValues(GlobalVariable *LLVMUsed,
 193                            SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
 194   if (!LLVMUsed) return;
 195   UsedValues.insert(LLVMUsed);
 196
 197   ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
 198
 199   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
 200     if (GlobalValue *GV =
 201           dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
 202       UsedValues.insert(GV);
 203 }
 204
 205 /// StripSymbolNames - Strip symbol names.
 206 static bool StripSymbolNames(Module &M, bool PreserveDbgInfo) {
 207
 208   SmallPtrSet<const GlobalValue*, 8> llvmUsedValues;
 209   findUsedValues(M.getGlobalVariable("llvm.used"), llvmUsedValues);
 210   findUsedValues(M.getGlobalVariable("llvm.compiler.used"), llvmUsedValues);
 211
 212   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
 213        I != E; ++I) {
 214     if (I->hasLocalLinkage() && llvmUsedValues.count(I) == 0)
 215       if (!PreserveDbgInfo || !I->getName().startswith("llvm.dbg"))
 216         I->setName("");     // Internal symbols can't participate in linkage
 217   }
 218
 219   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
 220     if (I->hasLocalLinkage() && llvmUsedValues.count(I) == 0)
 221       if (!PreserveDbgInfo || !I->getName().startswith("llvm.dbg"))
 222         I->setName("");     // Internal symbols can't participate in linkage
 223     StripSymtab(I->getValueSymbolTable(), PreserveDbgInfo);
 224   }
 225
 226   // Remove all names from types.
 227   StripTypeNames(M, PreserveDbgInfo);
 228
 229   return true;
 230 }
 231
 232 bool StripSymbols::runOnModule(Module &M) {
 233   bool Changed = false;
 234   Changed |= StripDebugInfo(M);
 235   if (!OnlyDebugInfo)
 236     Changed |= StripSymbolNames(M, false);
 237   return Changed;
 238 }
 239
 240 bool StripNonDebugSymbols::runOnModule(Module &M) {
 241   return StripSymbolNames(M, true);
 242 }
 243
 244 bool StripDebugDeclare::runOnModule(Module &M) {
 245
 246   Function *Declare = M.getFunction("llvm.dbg.declare");
 247   std::vector<Constant*> DeadConstants;
 248
 249   if (Declare) {
 250     while (!Declare->use_empty()) {
 251       CallInst *CI = cast<CallInst>(Declare->user_back());
 252       Value *Arg1 = CI->getArgOperand(0);
 253       Value *Arg2 = CI->getArgOperand(1);
 254       assert(CI->use_empty() && "llvm.dbg intrinsic should have void result");
 255       CI->eraseFromParent();
 256       if (Arg1->use_empty()) {
 257         if (Constant *C = dyn_cast<Constant>(Arg1))
 258           DeadConstants.push_back(C);
 259         else
 260           RecursivelyDeleteTriviallyDeadInstructions(Arg1);
 261       }
 262       if (Arg2->use_empty())
 263         if (Constant *C = dyn_cast<Constant>(Arg2))
 264           DeadConstants.push_back(C);
 265     }
 266     Declare->eraseFromParent();
 267   }
 268
 269   while (!DeadConstants.empty()) {
 270     Constant *C = DeadConstants.back();
 271     DeadConstants.pop_back();
 272     if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
 273       if (GV->hasLocalLinkage())
 274         RemoveDeadConstant(GV);
 275     } else
 276       RemoveDeadConstant(C);
 277   }
 278
 279   return true;
 280 }
 281
 282 /// Remove any debug info for global variables/functions in the given module for
 283 /// which said global variable/function no longer exists (i.e. is null).
 284 ///
 285 /// Debugging information is encoded in llvm IR using metadata. This is designed
 286 /// such a way that debug info for symbols preserved even if symbols are
 287 /// optimized away by the optimizer. This special pass removes debug info for
 288 /// such symbols.
 289 bool StripDeadDebugInfo::runOnModule(Module &M) {
 290   bool Changed = false;
 291
 292   LLVMContext &C = M.getContext();
 293
 294   // Find all debug info in F. This is actually overkill in terms of what we
 295   // want to do, but we want to try and be as resilient as possible in the face
 296   // of potential debug info changes by using the formal interfaces given to us
 297   // as much as possible.
 298   DebugInfoFinder F;
 299   F.processModule(M);
 300
 301   // For each compile unit, find the live set of global variables/functions and
 302   // replace the current list of potentially dead global variables/functions
 303   // with the live list.
 304   SmallVector<Metadata *, 64> LiveGlobalVariables;
 305   SmallVector<Metadata *, 64> LiveSubprograms;
 306   DenseSet<const MDNode *> VisitedSet;
 307
 308   for (DICompileUnit DIC : F.compile_units()) {
 309     assert(DIC.Verify() && "DIC must verify as a DICompileUnit.");
 310
 311     // Create our live subprogram list.
 312     DIArray SPs = DIC.getSubprograms();
 313     bool SubprogramChange = false;
 314     for (unsigned i = 0, e = SPs.getNumElements(); i != e; ++i) {
 315       DISubprogram DISP(SPs.getElement(i));
 316       assert(DISP.Verify() && "DISP must verify as a DISubprogram.");
 317
 318       // Make sure we visit each subprogram only once.
 319       if (!VisitedSet.insert(DISP).second)
 320         continue;
 321
 322       // If the function referenced by DISP is not null, the function is live.
 323       if (DISP.getFunction())
 324         LiveSubprograms.push_back(DISP);
 325       else
 326         SubprogramChange = true;
 327     }
 328
 329     // Create our live global variable list.
 330     DIArray GVs = DIC.getGlobalVariables();
 331     bool GlobalVariableChange = false;
 332     for (unsigned i = 0, e = GVs.getNumElements(); i != e; ++i) {
 333       DIGlobalVariable DIG(GVs.getElement(i));
 334       assert(DIG.Verify() && "DIG must verify as DIGlobalVariable.");
 335
 336       // Make sure we only visit each global variable only once.
 337       if (!VisitedSet.insert(DIG).second)
 338         continue;
 339
 340       // If the global variable referenced by DIG is not null, the global
 341       // variable is live.
 342       if (DIG.getGlobal())
 343         LiveGlobalVariables.push_back(DIG);
 344       else
 345         GlobalVariableChange = true;
 346     }
 347
 348     // If we found dead subprograms or global variables, replace the current
 349     // subprogram list/global variable list with our new live subprogram/global
 350     // variable list.
 351     if (SubprogramChange) {
 352       DIC.replaceSubprograms(DIArray(MDNode::get(C, LiveSubprograms)));
 353       Changed = true;
 354     }
 355
 356     if (GlobalVariableChange) {
 357       DIC.replaceGlobalVariables(DIArray(MDNode::get(C, LiveGlobalVariables)));
 358       Changed = true;
 359     }
 360
 361     // Reset lists for the next iteration.
 362     LiveSubprograms.clear();
 363     LiveGlobalVariables.clear();
 364   }
 365
 366   return Changed;
 367 }