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/Pass.h"
26using namespace llvm;
27
28#define DEBUG_TYPE "globaldce"
29
30STATISTIC(NumAliases  , "Number of global aliases removed");
31STATISTIC(NumFunctions, "Number of functions removed");
32STATISTIC(NumVariables, "Number of global variables removed");
33
34namespace {
35  struct GlobalDCE : public ModulePass {
36    static char ID; // Pass identification, replacement for typeid
37    GlobalDCE() : ModulePass(ID) {
38      initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
39    }
40
41    // run - Do the GlobalDCE pass on the specified module, optionally updating
42    // the specified callgraph to reflect the changes.
43    //
44    bool runOnModule(Module &M) override;
45
46  private:
47    SmallPtrSet<GlobalValue*, 32> AliveGlobals;
48    SmallPtrSet<Constant *, 8> SeenConstants;
49
50    /// GlobalIsNeeded - mark the specific global value as needed, and
51    /// recursively mark anything that it uses as also needed.
52    void GlobalIsNeeded(GlobalValue *GV);
53    void MarkUsedGlobalsAsNeeded(Constant *C);
54
55    bool RemoveUnusedGlobalValue(GlobalValue &GV);
56  };
57}
58
59/// Returns true if F contains only a single "ret" instruction.
60static bool isEmptyFunction(Function *F) {
61  BasicBlock &Entry = F->getEntryBlock();
62  if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
63    return false;
64  ReturnInst &RI = cast<ReturnInst>(Entry.front());
65  return RI.getReturnValue() == nullptr;
66}
67
68char GlobalDCE::ID = 0;
69INITIALIZE_PASS(GlobalDCE, "globaldce",
70                "Dead Global Elimination", false, false)
71
72ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
73
74bool GlobalDCE::runOnModule(Module &M) {
75  bool Changed = false;
76
77  // Remove empty functions from the global ctors list.
78  Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
79
80  typedef std::multimap<const Comdat *, GlobalValue *> ComdatGVPairsTy;
81  ComdatGVPairsTy ComdatGVPairs;
82
83  // Loop over the module, adding globals which are obviously necessary.
84  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
85    Changed |= RemoveUnusedGlobalValue(*I);
86    // Functions with external linkage are needed if they have a body
87    if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
88      if (!I->isDiscardableIfUnused())
89        GlobalIsNeeded(I);
90      else if (const Comdat *C = I->getComdat())
91        ComdatGVPairs.insert(std::make_pair(C, I));
92    }
93  }
94
95  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
96       I != E; ++I) {
97    Changed |= RemoveUnusedGlobalValue(*I);
98    // Externally visible & appending globals are needed, if they have an
99    // initializer.
100    if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
101      if (!I->isDiscardableIfUnused())
102        GlobalIsNeeded(I);
103      else if (const Comdat *C = I->getComdat())
104        ComdatGVPairs.insert(std::make_pair(C, I));
105    }
106  }
107
108  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
109       I != E; ++I) {
110    Changed |= RemoveUnusedGlobalValue(*I);
111    // Externally visible aliases are needed.
112    if (!I->isDiscardableIfUnused()) {
113      GlobalIsNeeded(I);
114    } else if (const Comdat *C = I->getComdat()) {
115      ComdatGVPairs.insert(std::make_pair(C, I));
116    }
117  }
118
119  for (ComdatGVPairsTy::iterator I = ComdatGVPairs.begin(),
120                                 E = ComdatGVPairs.end();
121       I != E;) {
122    ComdatGVPairsTy::iterator UB = ComdatGVPairs.upper_bound(I->first);
123    bool CanDiscard = std::all_of(I, UB, [](ComdatGVPairsTy::value_type Pair) {
124      return Pair.second->isDiscardableIfUnused();
125    });
126    if (!CanDiscard) {
127      std::for_each(I, UB, [this](ComdatGVPairsTy::value_type Pair) {
128        GlobalIsNeeded(Pair.second);
129      });
130    }
131    I = UB;
132  }
133
134  // Now that all globals which are needed are in the AliveGlobals set, we loop
135  // through the program, deleting those which are not alive.
136  //
137
138  // The first pass is to drop initializers of global variables which are dead.
139  std::vector<GlobalVariable*> DeadGlobalVars;   // Keep track of dead globals
140  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
141       I != E; ++I)
142    if (!AliveGlobals.count(I)) {
143      DeadGlobalVars.push_back(I);         // Keep track of dead globals
144      I->setInitializer(nullptr);
145    }
146
147  // The second pass drops the bodies of functions which are dead...
148  std::vector<Function*> DeadFunctions;
149  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
150    if (!AliveGlobals.count(I)) {
151      DeadFunctions.push_back(I);         // Keep track of dead globals
152      if (!I->isDeclaration())
153        I->deleteBody();
154    }
155
156  // The third pass drops targets of aliases which are dead...
157  std::vector<GlobalAlias*> DeadAliases;
158  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
159       ++I)
160    if (!AliveGlobals.count(I)) {
161      DeadAliases.push_back(I);
162      I->setAliasee(nullptr);
163    }
164
165  if (!DeadFunctions.empty()) {
166    // Now that all interferences have been dropped, delete the actual objects
167    // themselves.
168    for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
169      RemoveUnusedGlobalValue(*DeadFunctions[i]);
170      M.getFunctionList().erase(DeadFunctions[i]);
171    }
172    NumFunctions += DeadFunctions.size();
173    Changed = true;
174  }
175
176  if (!DeadGlobalVars.empty()) {
177    for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
178      RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
179      M.getGlobalList().erase(DeadGlobalVars[i]);
180    }
181    NumVariables += DeadGlobalVars.size();
182    Changed = true;
183  }
184
185  // Now delete any dead aliases.
186  if (!DeadAliases.empty()) {
187    for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
188      RemoveUnusedGlobalValue(*DeadAliases[i]);
189      M.getAliasList().erase(DeadAliases[i]);
190    }
191    NumAliases += DeadAliases.size();
192    Changed = true;
193  }
194
195  // Make sure that all memory is released
196  AliveGlobals.clear();
197  SeenConstants.clear();
198
199  return Changed;
200}
201
202/// GlobalIsNeeded - the specific global value as needed, and
203/// recursively mark anything that it uses as also needed.
204void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
205  // If the global is already in the set, no need to reprocess it.
206  if (!AliveGlobals.insert(G))
207    return;
208
209  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
210    // If this is a global variable, we must make sure to add any global values
211    // referenced by the initializer to the alive set.
212    if (GV->hasInitializer())
213      MarkUsedGlobalsAsNeeded(GV->getInitializer());
214  } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
215    // The target of a global alias is needed.
216    MarkUsedGlobalsAsNeeded(GA->getAliasee());
217  } else {
218    // Otherwise this must be a function object.  We have to scan the body of
219    // the function looking for constants and global values which are used as
220    // operands.  Any operands of these types must be processed to ensure that
221    // any globals used will be marked as needed.
222    Function *F = cast<Function>(G);
223
224    if (F->hasPrefixData())
225      MarkUsedGlobalsAsNeeded(F->getPrefixData());
226
227    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
228      for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
229        for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
230          if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
231            GlobalIsNeeded(GV);
232          else if (Constant *C = dyn_cast<Constant>(*U))
233            MarkUsedGlobalsAsNeeded(C);
234  }
235}
236
237void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
238  if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
239    return GlobalIsNeeded(GV);
240
241  // Loop over all of the operands of the constant, adding any globals they
242  // use to the list of needed globals.
243  for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
244    // If we've already processed this constant there's no need to do it again.
245    Constant *Op = dyn_cast<Constant>(*I);
246    if (Op && SeenConstants.insert(Op))
247      MarkUsedGlobalsAsNeeded(Op);
248  }
249}
250
251// RemoveUnusedGlobalValue - Loop over all of the uses of the specified
252// GlobalValue, looking for the constant pointer ref that may be pointing to it.
253// If found, check to see if the constant pointer ref is safe to destroy, and if
254// so, nuke it.  This will reduce the reference count on the global value, which
255// might make it deader.
256//
257bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
258  if (GV.use_empty()) return false;
259  GV.removeDeadConstantUsers();
260  return GV.use_empty();
261}
262