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"
26 using namespace llvm;
27 
28 #define DEBUG_TYPE "globaldce"
29 
30 STATISTIC(NumAliases  , "Number of global aliases removed");
31 STATISTIC(NumFunctions, "Number of functions removed");
32 STATISTIC(NumVariables, "Number of global variables removed");
33 
34 namespace {
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.
60 static 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() == NULL;
66 }
67 
68 char GlobalDCE::ID = 0;
69 INITIALIZE_PASS(GlobalDCE, "globaldce",
70                 "Dead Global Elimination", false, false)
71 
72 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
73 
74 bool 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   // Loop over the module, adding globals which are obviously necessary.
81   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
82     Changed |= RemoveUnusedGlobalValue(*I);
83     // Functions with external linkage are needed if they have a body
84     if (!I->isDiscardableIfUnused() &&
85         !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
86       GlobalIsNeeded(I);
87   }
88 
89   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
90        I != E; ++I) {
91     Changed |= RemoveUnusedGlobalValue(*I);
92     // Externally visible & appending globals are needed, if they have an
93     // initializer.
94     if (!I->isDiscardableIfUnused() &&
95         !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
96       GlobalIsNeeded(I);
97   }
98 
99   for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
100        I != E; ++I) {
101     Changed |= RemoveUnusedGlobalValue(*I);
102     // Externally visible aliases are needed.
103     if (!I->isDiscardableIfUnused())
104       GlobalIsNeeded(I);
105   }
106 
107   // Now that all globals which are needed are in the AliveGlobals set, we loop
108   // through the program, deleting those which are not alive.
109   //
110 
111   // The first pass is to drop initializers of global variables which are dead.
112   std::vector<GlobalVariable*> DeadGlobalVars;   // Keep track of dead globals
113   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
114        I != E; ++I)
115     if (!AliveGlobals.count(I)) {
116       DeadGlobalVars.push_back(I);         // Keep track of dead globals
117       I->setInitializer(nullptr);
118     }
119 
120   // The second pass drops the bodies of functions which are dead...
121   std::vector<Function*> DeadFunctions;
122   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
123     if (!AliveGlobals.count(I)) {
124       DeadFunctions.push_back(I);         // Keep track of dead globals
125       if (!I->isDeclaration())
126         I->deleteBody();
127     }
128 
129   // The third pass drops targets of aliases which are dead...
130   std::vector<GlobalAlias*> DeadAliases;
131   for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
132        ++I)
133     if (!AliveGlobals.count(I)) {
134       DeadAliases.push_back(I);
135       I->setAliasee(nullptr);
136     }
137 
138   if (!DeadFunctions.empty()) {
139     // Now that all interferences have been dropped, delete the actual objects
140     // themselves.
141     for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
142       RemoveUnusedGlobalValue(*DeadFunctions[i]);
143       M.getFunctionList().erase(DeadFunctions[i]);
144     }
145     NumFunctions += DeadFunctions.size();
146     Changed = true;
147   }
148 
149   if (!DeadGlobalVars.empty()) {
150     for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
151       RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
152       M.getGlobalList().erase(DeadGlobalVars[i]);
153     }
154     NumVariables += DeadGlobalVars.size();
155     Changed = true;
156   }
157 
158   // Now delete any dead aliases.
159   if (!DeadAliases.empty()) {
160     for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
161       RemoveUnusedGlobalValue(*DeadAliases[i]);
162       M.getAliasList().erase(DeadAliases[i]);
163     }
164     NumAliases += DeadAliases.size();
165     Changed = true;
166   }
167 
168   // Make sure that all memory is released
169   AliveGlobals.clear();
170   SeenConstants.clear();
171 
172   return Changed;
173 }
174 
175 /// GlobalIsNeeded - the specific global value as needed, and
176 /// recursively mark anything that it uses as also needed.
177 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
178   // If the global is already in the set, no need to reprocess it.
179   if (!AliveGlobals.insert(G))
180     return;
181 
182   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
183     // If this is a global variable, we must make sure to add any global values
184     // referenced by the initializer to the alive set.
185     if (GV->hasInitializer())
186       MarkUsedGlobalsAsNeeded(GV->getInitializer());
187   } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
188     // The target of a global alias is needed.
189     MarkUsedGlobalsAsNeeded(GA->getAliasee());
190   } else {
191     // Otherwise this must be a function object.  We have to scan the body of
192     // the function looking for constants and global values which are used as
193     // operands.  Any operands of these types must be processed to ensure that
194     // any globals used will be marked as needed.
195     Function *F = cast<Function>(G);
196 
197     if (F->hasPrefixData())
198       MarkUsedGlobalsAsNeeded(F->getPrefixData());
199 
200     for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
201       for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
202         for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
203           if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
204             GlobalIsNeeded(GV);
205           else if (Constant *C = dyn_cast<Constant>(*U))
206             MarkUsedGlobalsAsNeeded(C);
207   }
208 }
209 
210 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
211   if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
212     return GlobalIsNeeded(GV);
213 
214   // Loop over all of the operands of the constant, adding any globals they
215   // use to the list of needed globals.
216   for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
217     // If we've already processed this constant there's no need to do it again.
218     Constant *Op = dyn_cast<Constant>(*I);
219     if (Op && SeenConstants.insert(Op))
220       MarkUsedGlobalsAsNeeded(Op);
221   }
222 }
223 
224 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
225 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
226 // If found, check to see if the constant pointer ref is safe to destroy, and if
227 // so, nuke it.  This will reduce the reference count on the global value, which
228 // might make it deader.
229 //
230 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
231   if (GV.use_empty()) return false;
232   GV.removeDeadConstantUsers();
233   return GV.use_empty();
234 }
235