LLVM  3.7.0
GlobalDCE.cpp
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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"
26 #include "llvm/Pass.h"
27 #include <unordered_map>
28 using namespace llvm;
29 
30 #define DEBUG_TYPE "globaldce"
31 
32 STATISTIC(NumAliases , "Number of global aliases removed");
33 STATISTIC(NumFunctions, "Number of functions removed");
34 STATISTIC(NumVariables, "Number of global variables removed");
35 
36 namespace {
37  struct GlobalDCE : public ModulePass {
38  static char ID; // Pass identification, replacement for typeid
39  GlobalDCE() : ModulePass(ID) {
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.
63 static 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 
71 char GlobalDCE::ID = 0;
72 INITIALIZE_PASS(GlobalDCE, "globaldce",
73  "Dead Global Elimination", false, false)
74 
75 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
76 
77 bool GlobalDCE::runOnModule(Module &M) {
78  bool Changed = false;
79 
80  // Remove empty functions from the global ctors list.
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.
200 void 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  if (F->hasPersonalityFn())
232  MarkUsedGlobalsAsNeeded(F->getPersonalityFn());
233 
234  for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
235  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
236  for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
237  if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
238  GlobalIsNeeded(GV);
239  else if (Constant *C = dyn_cast<Constant>(*U))
240  MarkUsedGlobalsAsNeeded(C);
241  }
242 }
243 
244 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
245  if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
246  return GlobalIsNeeded(GV);
247 
248  // Loop over all of the operands of the constant, adding any globals they
249  // use to the list of needed globals.
250  for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
251  // If we've already processed this constant there's no need to do it again.
252  Constant *Op = dyn_cast<Constant>(*I);
253  if (Op && SeenConstants.insert(Op).second)
254  MarkUsedGlobalsAsNeeded(Op);
255  }
256 }
257 
258 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
259 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
260 // If found, check to see if the constant pointer ref is safe to destroy, and if
261 // so, nuke it. This will reduce the reference count on the global value, which
262 // might make it deader.
263 //
264 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
265  if (GV.use_empty()) return false;
267  return GV.use_empty();
268 }
ReturnInst - Return a value (possibly void), from a function.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
Constant * getPrologueData() const
Definition: Function.cpp:956
STATISTIC(NumFunctions,"Total number of functions")
A Module instance is used to store all the information related to an LLVM module. ...
Definition: Module.h:114
iterator end()
Definition: Function.h:459
bool hasPrologueData() const
Definition: Function.h:509
const Instruction & front() const
Definition: BasicBlock.h:243
F(f)
op_iterator op_begin()
Definition: User.h:183
Value * getReturnValue() const
Convenience accessor. Returns null if there is no return value.
A Use represents the edge between a Value definition and its users.
Definition: Use.h:69
#define G(x, y, z)
Definition: MD5.cpp:52
INITIALIZE_PASS(GlobalDCE,"globaldce","Dead Global Elimination", false, false) ModulePass *llvm
Definition: GlobalDCE.cpp:72
iterator begin()
Definition: Function.h:457
ModulePass * createGlobalDCEPass()
createGlobalDCEPass - This transform is designed to eliminate unreachable internal globals (functions...
LLVM Basic Block Representation.
Definition: BasicBlock.h:65
This is an important base class in LLVM.
Definition: Constant.h:41
This file contains the declarations for the subclasses of Constant, which represent the different fla...
bool hasPersonalityFn() const
Get the personality function associated with this function.
Definition: Function.h:132
op_iterator op_end()
Definition: User.h:185
bool isSafeToDestroyConstant(const Constant *C)
It is safe to destroy a constant iff it is only used by constants itself.
iterator_range< T > make_range(T x, T y)
Convenience function for iterating over sub-ranges.
SmallPtrSet - This class implements a set which is optimized for holding SmallSize or less elements...
Definition: SmallPtrSet.h:299
Comdat * getComdat()
Definition: Globals.cpp:116
Module.h This file contains the declarations for the Module class.
static bool isEmptyFunction(Function *F)
Returns true if F contains only a single "ret" instruction.
Definition: GlobalDCE.cpp:63
const BasicBlock & getEntryBlock() const
Definition: Function.h:442
LLVM_ATTRIBUTE_UNUSED_RESULT std::enable_if< !is_simple_type< Y >::value, typename cast_retty< X, const Y >::ret_type >::type dyn_cast(const Y &Val)
Definition: Casting.h:285
void initializeGlobalDCEPass(PassRegistry &)
Constant * getPersonalityFn() const
Definition: Function.h:133
#define I(x, y, z)
Definition: MD5.cpp:54
ModulePass class - This class is used to implement unstructured interprocedural optimizations and ana...
Definition: Pass.h:236
bool hasPrefixData() const
Definition: Function.h:502
void destroyConstant()
Called if some element of this constant is no longer valid.
Definition: Constants.cpp:279
size_t size() const
Definition: BasicBlock.h:241
Constant * getPrefixData() const
Definition: Function.cpp:927
bool use_empty() const
Definition: Value.h:275
void removeDeadConstantUsers() const
removeDeadConstantUsers - If there are any dead constant users dangling off of this constant...
Definition: Constants.cpp:487
bool optimizeGlobalCtorsList(Module &M, function_ref< bool(Function *)> ShouldRemove)
Call "ShouldRemove" for every entry in M's global_ctor list and remove the entries for which it ret...
Definition: CtorUtils.cpp:120