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

   1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
   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 // This transform is designed to eliminate unreachable internal globals from the
  11 // program.  It uses an aggressive algorithm, searching out globals that are
  12 // known to be alive.  After it finds all of the globals which are needed, it
  13 // deletes whatever is left over.  This allows it to delete recursive chunks of
  14 // the program which are unreachable.
  15 //
  16 //===----------------------------------------------------------------------===//
  17
  18 #include "llvm/Transforms/IPO.h"
  19 #include "llvm/ADT/SmallPtrSet.h"
  20 #include "llvm/ADT/Statistic.h"
  21 #include "llvm/IR/Constants.h"
  22 #include "llvm/IR/Instructions.h"
  23 #include "llvm/IR/Module.h"
  24 #include "llvm/Transforms/Utils/CtorUtils.h"
  25 #include "llvm/Transforms/Utils/GlobalStatus.h"
  26 #include "llvm/Pass.h"
  27 using namespace llvm;
  28
  29 #define DEBUG_TYPE "globaldce"
  30
  31 STATISTIC(NumAliases  , "Number of global aliases removed");
  32 STATISTIC(NumFunctions, "Number of functions removed");
  33 STATISTIC(NumVariables, "Number of global variables removed");
  34
  35 namespace {
  36   struct GlobalDCE : public ModulePass {
  37     static char ID; // Pass identification, replacement for typeid
  38     GlobalDCE() : ModulePass(ID) {
  39       initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
  40     }
  41
  42     // run - Do the GlobalDCE pass on the specified module, optionally updating
  43     // the specified callgraph to reflect the changes.
  44     //
  45     bool runOnModule(Module &M) override;
  46
  47   private:
  48     SmallPtrSet<GlobalValue*, 32> AliveGlobals;
  49     SmallPtrSet<Constant *, 8> SeenConstants;
  50
  51     /// GlobalIsNeeded - mark the specific global value as needed, and
  52     /// recursively mark anything that it uses as also needed.
  53     void GlobalIsNeeded(GlobalValue *GV);
  54     void MarkUsedGlobalsAsNeeded(Constant *C);
  55
  56     bool RemoveUnusedGlobalValue(GlobalValue &GV);
  57   };
  58 }
  59
  60 /// Returns true if F contains only a single "ret" instruction.
  61 static bool isEmptyFunction(Function *F) {
  62   BasicBlock &Entry = F->getEntryBlock();
  63   if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
  64     return false;
  65   ReturnInst &RI = cast<ReturnInst>(Entry.front());
  66   return RI.getReturnValue() == nullptr;
  67 }
  68
  69 char GlobalDCE::ID = 0;
  70 INITIALIZE_PASS(GlobalDCE, "globaldce",
  71                 "Dead Global Elimination", false, false)
  72
  73 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
  74
  75 bool GlobalDCE::runOnModule(Module &M) {
  76   bool Changed = false;
  77
  78   // Remove empty functions from the global ctors list.
  79   Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
  80
  81   // Loop over the module, adding globals which are obviously necessary.
  82   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
  83     Changed |= RemoveUnusedGlobalValue(*I);
  84     // Functions with external linkage are needed if they have a body
  85     if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
  86       if (!I->isDiscardableIfUnused())
  87         GlobalIsNeeded(I);
  88     }
  89   }
  90
  91   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
  92        I != E; ++I) {
  93     Changed |= RemoveUnusedGlobalValue(*I);
  94     // Externally visible & appending globals are needed, if they have an
  95     // initializer.
  96     if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
  97       if (!I->isDiscardableIfUnused())
  98         GlobalIsNeeded(I);
  99     }
 100   }
 101
 102   for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
 103        I != E; ++I) {
 104     Changed |= RemoveUnusedGlobalValue(*I);
 105     // Externally visible aliases are needed.
 106     if (!I->isDiscardableIfUnused()) {
 107       GlobalIsNeeded(I);
 108     }
 109   }
 110
 111   // Now that all globals which are needed are in the AliveGlobals set, we loop
 112   // through the program, deleting those which are not alive.
 113   //
 114
 115   // The first pass is to drop initializers of global variables which are dead.
 116   std::vector<GlobalVariable*> DeadGlobalVars;   // Keep track of dead globals
 117   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
 118        I != E; ++I)
 119     if (!AliveGlobals.count(I)) {
 120       DeadGlobalVars.push_back(I);         // Keep track of dead globals
 121       if (I->hasInitializer()) {
 122         Constant *Init = I->getInitializer();
 123         I->setInitializer(nullptr);
 124         if (isSafeToDestroyConstant(Init))
 125           Init->destroyConstant();
 126       }
 127     }
 128
 129   // The second pass drops the bodies of functions which are dead...
 130   std::vector<Function*> DeadFunctions;
 131   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
 132     if (!AliveGlobals.count(I)) {
 133       DeadFunctions.push_back(I);         // Keep track of dead globals
 134       if (!I->isDeclaration())
 135         I->deleteBody();
 136     }
 137
 138   // The third pass drops targets of aliases which are dead...
 139   std::vector<GlobalAlias*> DeadAliases;
 140   for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
 141        ++I)
 142     if (!AliveGlobals.count(I)) {
 143       DeadAliases.push_back(I);
 144       I->setAliasee(nullptr);
 145     }
 146
 147   if (!DeadFunctions.empty()) {
 148     // Now that all interferences have been dropped, delete the actual objects
 149     // themselves.
 150     for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
 151       RemoveUnusedGlobalValue(*DeadFunctions[i]);
 152       M.getFunctionList().erase(DeadFunctions[i]);
 153     }
 154     NumFunctions += DeadFunctions.size();
 155     Changed = true;
 156   }
 157
 158   if (!DeadGlobalVars.empty()) {
 159     for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
 160       RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
 161       M.getGlobalList().erase(DeadGlobalVars[i]);
 162     }
 163     NumVariables += DeadGlobalVars.size();
 164     Changed = true;
 165   }
 166
 167   // Now delete any dead aliases.
 168   if (!DeadAliases.empty()) {
 169     for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
 170       RemoveUnusedGlobalValue(*DeadAliases[i]);
 171       M.getAliasList().erase(DeadAliases[i]);
 172     }
 173     NumAliases += DeadAliases.size();
 174     Changed = true;
 175   }
 176
 177   // Make sure that all memory is released
 178   AliveGlobals.clear();
 179   SeenConstants.clear();
 180
 181   return Changed;
 182 }
 183
 184 /// GlobalIsNeeded - the specific global value as needed, and
 185 /// recursively mark anything that it uses as also needed.
 186 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
 187   // If the global is already in the set, no need to reprocess it.
 188   if (!AliveGlobals.insert(G).second)
 189     return;
 190
 191   Module *M = G->getParent();
 192   if (Comdat *C = G->getComdat()) {
 193     for (Function &F : *M)
 194       if (F.getComdat() == C)
 195         GlobalIsNeeded(&F);
 196     for (GlobalVariable &GV : M->globals())
 197       if (GV.getComdat() == C)
 198         GlobalIsNeeded(&GV);
 199     for (GlobalAlias &GA : M->aliases())
 200       if (GA.getComdat() == C)
 201         GlobalIsNeeded(&GA);
 202   }
 203
 204   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
 205     // If this is a global variable, we must make sure to add any global values
 206     // referenced by the initializer to the alive set.
 207     if (GV->hasInitializer())
 208       MarkUsedGlobalsAsNeeded(GV->getInitializer());
 209   } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
 210     // The target of a global alias is needed.
 211     MarkUsedGlobalsAsNeeded(GA->getAliasee());
 212   } else {
 213     // Otherwise this must be a function object.  We have to scan the body of
 214     // the function looking for constants and global values which are used as
 215     // operands.  Any operands of these types must be processed to ensure that
 216     // any globals used will be marked as needed.
 217     Function *F = cast<Function>(G);
 218
 219     if (F->hasPrefixData())
 220       MarkUsedGlobalsAsNeeded(F->getPrefixData());
 221
 222     if (F->hasPrologueData())
 223       MarkUsedGlobalsAsNeeded(F->getPrologueData());
 224
 225     for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
 226       for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
 227         for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
 228           if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
 229             GlobalIsNeeded(GV);
 230           else if (Constant *C = dyn_cast<Constant>(*U))
 231             MarkUsedGlobalsAsNeeded(C);
 232   }
 233 }
 234
 235 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
 236   if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
 237     return GlobalIsNeeded(GV);
 238
 239   // Loop over all of the operands of the constant, adding any globals they
 240   // use to the list of needed globals.
 241   for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
 242     // If we've already processed this constant there's no need to do it again.
 243     Constant *Op = dyn_cast<Constant>(*I);
 244     if (Op && SeenConstants.insert(Op).second)
 245       MarkUsedGlobalsAsNeeded(Op);
 246   }
 247 }
 248
 249 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
 250 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
 251 // If found, check to see if the constant pointer ref is safe to destroy, and if
 252 // so, nuke it.  This will reduce the reference count on the global value, which
 253 // might make it deader.
 254 //
 255 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
 256   if (GV.use_empty()) return false;
 257   GV.removeDeadConstantUsers();
 258   return GV.use_empty();
 259 }