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      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/DebugInfo.h"
     27 #include "llvm/IR/Constants.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     virtual bool runOnModule(Module &M);
     48 
     49     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     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     virtual bool runOnModule(Module &M);
     63 
     64     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     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     virtual bool runOnModule(Module &M);
     78 
     79     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     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     virtual bool runOnModule(Module &M);
     93 
     94     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     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(Value::use_iterator I = V->use_begin(), E = V->use_end(); I != E; ++I) {
    136     User *U = *I;
    137     if (U != Usr)
    138       return false;
    139   }
    140   return true;
    141 }
    142 
    143 static void RemoveDeadConstant(Constant *C) {
    144   assert(C->use_empty() && "Constant is not dead!");
    145   SmallPtrSet<Constant*, 4> Operands;
    146   for (unsigned i = 0, e = C->getNumOperands(); i != e; ++i)
    147     if (OnlyUsedBy(C->getOperand(i), C))
    148       Operands.insert(cast<Constant>(C->getOperand(i)));
    149   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
    150     if (!GV->hasLocalLinkage()) return;   // Don't delete non static globals.
    151     GV->eraseFromParent();
    152   }
    153   else if (!isa<Function>(C))
    154     if (isa<CompositeType>(C->getType()))
    155       C->destroyConstant();
    156 
    157   // If the constant referenced anything, see if we can delete it as well.
    158   for (SmallPtrSet<Constant*, 4>::iterator OI = Operands.begin(),
    159          OE = Operands.end(); OI != OE; ++OI)
    160     RemoveDeadConstant(*OI);
    161 }
    162 
    163 // Strip the symbol table of its names.
    164 //
    165 static void StripSymtab(ValueSymbolTable &ST, bool PreserveDbgInfo) {
    166   for (ValueSymbolTable::iterator VI = ST.begin(), VE = ST.end(); VI != VE; ) {
    167     Value *V = VI->getValue();
    168     ++VI;
    169     if (!isa<GlobalValue>(V) || cast<GlobalValue>(V)->hasLocalLinkage()) {
    170       if (!PreserveDbgInfo || !V->getName().startswith("llvm.dbg"))
    171         // Set name to "", removing from symbol table!
    172         V->setName("");
    173     }
    174   }
    175 }
    176 
    177 // Strip any named types of their names.
    178 static void StripTypeNames(Module &M, bool PreserveDbgInfo) {
    179   TypeFinder StructTypes;
    180   StructTypes.run(M, false);
    181 
    182   for (unsigned i = 0, e = StructTypes.size(); i != e; ++i) {
    183     StructType *STy = StructTypes[i];
    184     if (STy->isLiteral() || STy->getName().empty()) continue;
    185 
    186     if (PreserveDbgInfo && STy->getName().startswith("llvm.dbg"))
    187       continue;
    188 
    189     STy->setName("");
    190   }
    191 }
    192 
    193 /// Find values that are marked as llvm.used.
    194 static void findUsedValues(GlobalVariable *LLVMUsed,
    195                            SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
    196   if (LLVMUsed == 0) return;
    197   UsedValues.insert(LLVMUsed);
    198 
    199   ConstantArray *Inits = dyn_cast<ConstantArray>(LLVMUsed->getInitializer());
    200   if (Inits == 0) return;
    201 
    202   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
    203     if (GlobalValue *GV =
    204           dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
    205       UsedValues.insert(GV);
    206 }
    207 
    208 /// StripSymbolNames - Strip symbol names.
    209 static bool StripSymbolNames(Module &M, bool PreserveDbgInfo) {
    210 
    211   SmallPtrSet<const GlobalValue*, 8> llvmUsedValues;
    212   findUsedValues(M.getGlobalVariable("llvm.used"), llvmUsedValues);
    213   findUsedValues(M.getGlobalVariable("llvm.compiler.used"), llvmUsedValues);
    214 
    215   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
    216        I != E; ++I) {
    217     if (I->hasLocalLinkage() && llvmUsedValues.count(I) == 0)
    218       if (!PreserveDbgInfo || !I->getName().startswith("llvm.dbg"))
    219         I->setName("");     // Internal symbols can't participate in linkage
    220   }
    221 
    222   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
    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     StripSymtab(I->getValueSymbolTable(), PreserveDbgInfo);
    227   }
    228 
    229   // Remove all names from types.
    230   StripTypeNames(M, PreserveDbgInfo);
    231 
    232   return true;
    233 }
    234 
    235 // StripDebugInfo - Strip debug info in the module if it exists.
    236 // To do this, we remove llvm.dbg.func.start, llvm.dbg.stoppoint, and
    237 // llvm.dbg.region.end calls, and any globals they point to if now dead.
    238 static bool StripDebugInfo(Module &M) {
    239 
    240   bool Changed = false;
    241 
    242   // Remove all of the calls to the debugger intrinsics, and remove them from
    243   // the module.
    244   if (Function *Declare = M.getFunction("llvm.dbg.declare")) {
    245     while (!Declare->use_empty()) {
    246       CallInst *CI = cast<CallInst>(Declare->use_back());
    247       CI->eraseFromParent();
    248     }
    249     Declare->eraseFromParent();
    250     Changed = true;
    251   }
    252 
    253   if (Function *DbgVal = M.getFunction("llvm.dbg.value")) {
    254     while (!DbgVal->use_empty()) {
    255       CallInst *CI = cast<CallInst>(DbgVal->use_back());
    256       CI->eraseFromParent();
    257     }
    258     DbgVal->eraseFromParent();
    259     Changed = true;
    260   }
    261 
    262   for (Module::named_metadata_iterator NMI = M.named_metadata_begin(),
    263          NME = M.named_metadata_end(); NMI != NME;) {
    264     NamedMDNode *NMD = NMI;
    265     ++NMI;
    266     if (NMD->getName().startswith("llvm.dbg.")) {
    267       NMD->eraseFromParent();
    268       Changed = true;
    269     }
    270   }
    271 
    272   for (Module::iterator MI = M.begin(), ME = M.end(); MI != ME; ++MI)
    273     for (Function::iterator FI = MI->begin(), FE = MI->end(); FI != FE;
    274          ++FI)
    275       for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE;
    276            ++BI) {
    277         if (!BI->getDebugLoc().isUnknown()) {
    278           Changed = true;
    279           BI->setDebugLoc(DebugLoc());
    280         }
    281       }
    282 
    283   return Changed;
    284 }
    285 
    286 bool StripSymbols::runOnModule(Module &M) {
    287   bool Changed = false;
    288   Changed |= StripDebugInfo(M);
    289   if (!OnlyDebugInfo)
    290     Changed |= StripSymbolNames(M, false);
    291   return Changed;
    292 }
    293 
    294 bool StripNonDebugSymbols::runOnModule(Module &M) {
    295   return StripSymbolNames(M, true);
    296 }
    297 
    298 bool StripDebugDeclare::runOnModule(Module &M) {
    299 
    300   Function *Declare = M.getFunction("llvm.dbg.declare");
    301   std::vector<Constant*> DeadConstants;
    302 
    303   if (Declare) {
    304     while (!Declare->use_empty()) {
    305       CallInst *CI = cast<CallInst>(Declare->use_back());
    306       Value *Arg1 = CI->getArgOperand(0);
    307       Value *Arg2 = CI->getArgOperand(1);
    308       assert(CI->use_empty() && "llvm.dbg intrinsic should have void result");
    309       CI->eraseFromParent();
    310       if (Arg1->use_empty()) {
    311         if (Constant *C = dyn_cast<Constant>(Arg1))
    312           DeadConstants.push_back(C);
    313         else
    314           RecursivelyDeleteTriviallyDeadInstructions(Arg1);
    315       }
    316       if (Arg2->use_empty())
    317         if (Constant *C = dyn_cast<Constant>(Arg2))
    318           DeadConstants.push_back(C);
    319     }
    320     Declare->eraseFromParent();
    321   }
    322 
    323   while (!DeadConstants.empty()) {
    324     Constant *C = DeadConstants.back();
    325     DeadConstants.pop_back();
    326     if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C)) {
    327       if (GV->hasLocalLinkage())
    328         RemoveDeadConstant(GV);
    329     } else
    330       RemoveDeadConstant(C);
    331   }
    332 
    333   return true;
    334 }
    335 
    336 /// getRealLinkageName - If special LLVM prefix that is used to inform the asm
    337 /// printer to not emit usual symbol prefix before the symbol name is used then
    338 /// return linkage name after skipping this special LLVM prefix.
    339 static StringRef getRealLinkageName(StringRef LinkageName) {
    340   char One = '\1';
    341   if (LinkageName.startswith(StringRef(&One, 1)))
    342     return LinkageName.substr(1);
    343   return LinkageName;
    344 }
    345 
    346 bool StripDeadDebugInfo::runOnModule(Module &M) {
    347   bool Changed = false;
    348 
    349   // Debugging infomration is encoded in llvm IR using metadata. This is designed
    350   // such a way that debug info for symbols preserved even if symbols are
    351   // optimized away by the optimizer. This special pass removes debug info for
    352   // such symbols.
    353 
    354   // llvm.dbg.gv keeps track of debug info for global variables.
    355   if (NamedMDNode *NMD = M.getNamedMetadata("llvm.dbg.gv")) {
    356     SmallVector<MDNode *, 8> MDs;
    357     for (unsigned i = 0, e = NMD->getNumOperands(); i != e; ++i)
    358       if (DIGlobalVariable(NMD->getOperand(i)).Verify())
    359         MDs.push_back(NMD->getOperand(i));
    360       else
    361         Changed = true;
    362     NMD->eraseFromParent();
    363     NMD = NULL;
    364 
    365     for (SmallVector<MDNode *, 8>::iterator I = MDs.begin(),
    366            E = MDs.end(); I != E; ++I) {
    367       GlobalVariable *GV = DIGlobalVariable(*I).getGlobal();
    368       if (GV && M.getGlobalVariable(GV->getName(), true)) {
    369         if (!NMD)
    370           NMD = M.getOrInsertNamedMetadata("llvm.dbg.gv");
    371         NMD->addOperand(*I);
    372       }
    373       else
    374         Changed = true;
    375     }
    376   }
    377 
    378   // llvm.dbg.sp keeps track of debug info for subprograms.
    379   if (NamedMDNode *NMD = M.getNamedMetadata("llvm.dbg.sp")) {
    380     SmallVector<MDNode *, 8> MDs;
    381     for (unsigned i = 0, e = NMD->getNumOperands(); i != e; ++i)
    382       if (DISubprogram(NMD->getOperand(i)).Verify())
    383         MDs.push_back(NMD->getOperand(i));
    384       else
    385         Changed = true;
    386     NMD->eraseFromParent();
    387     NMD = NULL;
    388 
    389     for (SmallVector<MDNode *, 8>::iterator I = MDs.begin(),
    390            E = MDs.end(); I != E; ++I) {
    391       bool FnIsLive = false;
    392       if (Function *F = DISubprogram(*I).getFunction())
    393         if (M.getFunction(F->getName()))
    394           FnIsLive = true;
    395       if (FnIsLive) {
    396           if (!NMD)
    397             NMD = M.getOrInsertNamedMetadata("llvm.dbg.sp");
    398           NMD->addOperand(*I);
    399       } else {
    400         // Remove llvm.dbg.lv.fnname named mdnode which may have been used
    401         // to hold debug info for dead function's local variables.
    402         StringRef FName = DISubprogram(*I).getLinkageName();
    403         if (FName.empty())
    404           FName = DISubprogram(*I).getName();
    405         if (NamedMDNode *LVNMD =
    406             M.getNamedMetadata(Twine("llvm.dbg.lv.",
    407                                      getRealLinkageName(FName))))
    408           LVNMD->eraseFromParent();
    409       }
    410     }
    411   }
    412 
    413   return Changed;
    414 }
    415