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#include <unordered_map>
28using namespace llvm;
29
30#define DEBUG_TYPE "globaldce"
31
32STATISTIC(NumAliases  , "Number of global aliases removed");
33STATISTIC(NumFunctions, "Number of functions removed");
34STATISTIC(NumVariables, "Number of global variables removed");
35
36namespace {
37  struct GlobalDCE : public ModulePass {
38    static char ID; // Pass identification, replacement for typeid
39    GlobalDCE() : ModulePass(ID) {
40      initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
41    }
42
43    // run - Do the GlobalDCE pass on the specified module, optionally updating
44    // the specified callgraph to reflect the changes.
45    //
46    bool runOnModule(Module &M) override;
47
48  private:
49    SmallPtrSet<GlobalValue*, 32> AliveGlobals;
50    SmallPtrSet<Constant *, 8> SeenConstants;
51    std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
52
53    /// GlobalIsNeeded - mark the specific global value as needed, and
54    /// recursively mark anything that it uses as also needed.
55    void GlobalIsNeeded(GlobalValue *GV);
56    void MarkUsedGlobalsAsNeeded(Constant *C);
57
58    bool RemoveUnusedGlobalValue(GlobalValue &GV);
59  };
60}
61
62/// Returns true if F contains only a single "ret" instruction.
63static bool isEmptyFunction(Function *F) {
64  BasicBlock &Entry = F->getEntryBlock();
65  if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
66    return false;
67  ReturnInst &RI = cast<ReturnInst>(Entry.front());
68  return RI.getReturnValue() == nullptr;
69}
70
71char GlobalDCE::ID = 0;
72INITIALIZE_PASS(GlobalDCE, "globaldce",
73                "Dead Global Elimination", false, false)
74
75ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
76
77bool GlobalDCE::runOnModule(Module &M) {
78  bool Changed = false;
79
80  // Remove empty functions from the global ctors list.
81  Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
82
83  // Collect the set of members for each comdat.
84  for (Function &F : M)
85    if (Comdat *C = F.getComdat())
86      ComdatMembers.insert(std::make_pair(C, &F));
87  for (GlobalVariable &GV : M.globals())
88    if (Comdat *C = GV.getComdat())
89      ComdatMembers.insert(std::make_pair(C, &GV));
90  for (GlobalAlias &GA : M.aliases())
91    if (Comdat *C = GA.getComdat())
92      ComdatMembers.insert(std::make_pair(C, &GA));
93
94  // Loop over the module, adding globals which are obviously necessary.
95  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
96    Changed |= RemoveUnusedGlobalValue(*I);
97    // Functions with external linkage are needed if they have a body
98    if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
99      if (!I->isDiscardableIfUnused())
100        GlobalIsNeeded(I);
101    }
102  }
103
104  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
105       I != E; ++I) {
106    Changed |= RemoveUnusedGlobalValue(*I);
107    // Externally visible & appending globals are needed, if they have an
108    // initializer.
109    if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
110      if (!I->isDiscardableIfUnused())
111        GlobalIsNeeded(I);
112    }
113  }
114
115  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
116       I != E; ++I) {
117    Changed |= RemoveUnusedGlobalValue(*I);
118    // Externally visible aliases are needed.
119    if (!I->isDiscardableIfUnused()) {
120      GlobalIsNeeded(I);
121    }
122  }
123
124  // Now that all globals which are needed are in the AliveGlobals set, we loop
125  // through the program, deleting those which are not alive.
126  //
127
128  // The first pass is to drop initializers of global variables which are dead.
129  std::vector<GlobalVariable*> DeadGlobalVars;   // Keep track of dead globals
130  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
131       I != E; ++I)
132    if (!AliveGlobals.count(I)) {
133      DeadGlobalVars.push_back(I);         // Keep track of dead globals
134      if (I->hasInitializer()) {
135        Constant *Init = I->getInitializer();
136        I->setInitializer(nullptr);
137        if (isSafeToDestroyConstant(Init))
138          Init->destroyConstant();
139      }
140    }
141
142  // The second pass drops the bodies of functions which are dead...
143  std::vector<Function*> DeadFunctions;
144  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
145    if (!AliveGlobals.count(I)) {
146      DeadFunctions.push_back(I);         // Keep track of dead globals
147      if (!I->isDeclaration())
148        I->deleteBody();
149    }
150
151  // The third pass drops targets of aliases which are dead...
152  std::vector<GlobalAlias*> DeadAliases;
153  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
154       ++I)
155    if (!AliveGlobals.count(I)) {
156      DeadAliases.push_back(I);
157      I->setAliasee(nullptr);
158    }
159
160  if (!DeadFunctions.empty()) {
161    // Now that all interferences have been dropped, delete the actual objects
162    // themselves.
163    for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
164      RemoveUnusedGlobalValue(*DeadFunctions[i]);
165      M.getFunctionList().erase(DeadFunctions[i]);
166    }
167    NumFunctions += DeadFunctions.size();
168    Changed = true;
169  }
170
171  if (!DeadGlobalVars.empty()) {
172    for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
173      RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
174      M.getGlobalList().erase(DeadGlobalVars[i]);
175    }
176    NumVariables += DeadGlobalVars.size();
177    Changed = true;
178  }
179
180  // Now delete any dead aliases.
181  if (!DeadAliases.empty()) {
182    for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
183      RemoveUnusedGlobalValue(*DeadAliases[i]);
184      M.getAliasList().erase(DeadAliases[i]);
185    }
186    NumAliases += DeadAliases.size();
187    Changed = true;
188  }
189
190  // Make sure that all memory is released
191  AliveGlobals.clear();
192  SeenConstants.clear();
193  ComdatMembers.clear();
194
195  return Changed;
196}
197
198/// GlobalIsNeeded - the specific global value as needed, and
199/// recursively mark anything that it uses as also needed.
200void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
201  // If the global is already in the set, no need to reprocess it.
202  if (!AliveGlobals.insert(G).second)
203    return;
204
205  if (Comdat *C = G->getComdat()) {
206    for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
207      GlobalIsNeeded(CM.second);
208  }
209
210  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
211    // If this is a global variable, we must make sure to add any global values
212    // referenced by the initializer to the alive set.
213    if (GV->hasInitializer())
214      MarkUsedGlobalsAsNeeded(GV->getInitializer());
215  } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
216    // The target of a global alias is needed.
217    MarkUsedGlobalsAsNeeded(GA->getAliasee());
218  } else {
219    // Otherwise this must be a function object.  We have to scan the body of
220    // the function looking for constants and global values which are used as
221    // operands.  Any operands of these types must be processed to ensure that
222    // any globals used will be marked as needed.
223    Function *F = cast<Function>(G);
224
225    if (F->hasPrefixData())
226      MarkUsedGlobalsAsNeeded(F->getPrefixData());
227
228    if (F->hasPrologueData())
229      MarkUsedGlobalsAsNeeded(F->getPrologueData());
230
231    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
232      for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
233        for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
234          if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
235            GlobalIsNeeded(GV);
236          else if (Constant *C = dyn_cast<Constant>(*U))
237            MarkUsedGlobalsAsNeeded(C);
238  }
239}
240
241void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
242  if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
243    return GlobalIsNeeded(GV);
244
245  // Loop over all of the operands of the constant, adding any globals they
246  // use to the list of needed globals.
247  for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
248    // If we've already processed this constant there's no need to do it again.
249    Constant *Op = dyn_cast<Constant>(*I);
250    if (Op && SeenConstants.insert(Op).second)
251      MarkUsedGlobalsAsNeeded(Op);
252  }
253}
254
255// RemoveUnusedGlobalValue - Loop over all of the uses of the specified
256// GlobalValue, looking for the constant pointer ref that may be pointing to it.
257// If found, check to see if the constant pointer ref is safe to destroy, and if
258// so, nuke it.  This will reduce the reference count on the global value, which
259// might make it deader.
260//
261bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
262  if (GV.use_empty()) return false;
263  GV.removeDeadConstantUsers();
264  return GV.use_empty();
265}
266