1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
16 //===----------------------------------------------------------------------===//
18 #define DEBUG_TYPE "globaldce"
19 #include "llvm/Transforms/IPO.h"
20 #include "llvm/Constants.h"
21 #include "llvm/Module.h"
22 #include "llvm/Pass.h"
23 #include "llvm/ADT/Statistic.h"
27 STATISTIC(NumAliases , "Number of global aliases removed");
28 STATISTIC(NumFunctions, "Number of functions removed");
29 STATISTIC(NumVariables, "Number of global variables removed");
32 struct GlobalDCE : public ModulePass {
33 static char ID; // Pass identification, replacement for typeid
34 GlobalDCE() : ModulePass(&ID) {}
36 // run - Do the GlobalDCE pass on the specified module, optionally updating
37 // the specified callgraph to reflect the changes.
39 bool runOnModule(Module &M);
42 std::set<GlobalValue*> AliveGlobals;
44 /// GlobalIsNeeded - mark the specific global value as needed, and
45 /// recursively mark anything that it uses as also needed.
46 void GlobalIsNeeded(GlobalValue *GV);
47 void MarkUsedGlobalsAsNeeded(Constant *C);
49 bool RemoveUnusedGlobalValue(GlobalValue &GV);
53 char GlobalDCE::ID = 0;
54 static RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
56 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
58 bool GlobalDCE::runOnModule(Module &M) {
61 // Loop over the module, adding globals which are obviously necessary.
62 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
63 Changed |= RemoveUnusedGlobalValue(*I);
64 // Functions with external linkage are needed if they have a body
65 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
66 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
70 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
72 Changed |= RemoveUnusedGlobalValue(*I);
73 // Externally visible & appending globals are needed, if they have an
75 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
76 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
80 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
82 Changed |= RemoveUnusedGlobalValue(*I);
83 // Externally visible aliases are needed.
84 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage())
88 // Now that all globals which are needed are in the AliveGlobals set, we loop
89 // through the program, deleting those which are not alive.
92 // The first pass is to drop initializers of global variables which are dead.
93 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
94 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I)
95 if (!AliveGlobals.count(I)) {
96 DeadGlobalVars.push_back(I); // Keep track of dead globals
100 // The second pass drops the bodies of functions which are dead...
101 std::vector<Function*> DeadFunctions;
102 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
103 if (!AliveGlobals.count(I)) {
104 DeadFunctions.push_back(I); // Keep track of dead globals
105 if (!I->isDeclaration())
109 // The third pass drops targets of aliases which are dead...
110 std::vector<GlobalAlias*> DeadAliases;
111 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
113 if (!AliveGlobals.count(I)) {
114 DeadAliases.push_back(I);
118 if (!DeadFunctions.empty()) {
119 // Now that all interferences have been dropped, delete the actual objects
121 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
122 RemoveUnusedGlobalValue(*DeadFunctions[i]);
123 M.getFunctionList().erase(DeadFunctions[i]);
125 NumFunctions += DeadFunctions.size();
129 if (!DeadGlobalVars.empty()) {
130 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
131 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
132 M.getGlobalList().erase(DeadGlobalVars[i]);
134 NumVariables += DeadGlobalVars.size();
138 // Now delete any dead aliases.
139 if (!DeadAliases.empty()) {
140 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
141 RemoveUnusedGlobalValue(*DeadAliases[i]);
142 M.getAliasList().erase(DeadAliases[i]);
144 NumAliases += DeadAliases.size();
148 // Make sure that all memory is released
149 AliveGlobals.clear();
154 /// GlobalIsNeeded - the specific global value as needed, and
155 /// recursively mark anything that it uses as also needed.
156 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
157 std::set<GlobalValue*>::iterator I = AliveGlobals.find(G);
159 // If the global is already in the set, no need to reprocess it.
160 if (I != AliveGlobals.end()) return;
162 // Otherwise insert it now, so we do not infinitely recurse
163 AliveGlobals.insert(I, G);
165 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
166 // If this is a global variable, we must make sure to add any global values
167 // referenced by the initializer to the alive set.
168 if (GV->hasInitializer())
169 MarkUsedGlobalsAsNeeded(GV->getInitializer());
170 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
171 // The target of a global alias is needed.
172 MarkUsedGlobalsAsNeeded(GA->getAliasee());
174 // Otherwise this must be a function object. We have to scan the body of
175 // the function looking for constants and global values which are used as
176 // operands. Any operands of these types must be processed to ensure that
177 // any globals used will be marked as needed.
178 Function *F = cast<Function>(G);
179 // For all basic blocks...
180 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
181 // For all instructions...
182 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
183 // For all operands...
184 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
185 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
187 else if (Constant *C = dyn_cast<Constant>(*U))
188 MarkUsedGlobalsAsNeeded(C);
192 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
193 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
196 // Loop over all of the operands of the constant, adding any globals they
197 // use to the list of needed globals.
198 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
199 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
203 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
204 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
205 // If found, check to see if the constant pointer ref is safe to destroy, and if
206 // so, nuke it. This will reduce the reference count on the global value, which
207 // might make it deader.
209 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
210 if (GV.use_empty()) return false;
211 GV.removeDeadConstantUsers();
212 return GV.use_empty();