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"
24 #include "llvm/Support/Compiler.h"
28 STATISTIC(NumAliases , "Number of global aliases removed");
29 STATISTIC(NumFunctions, "Number of functions removed");
30 STATISTIC(NumVariables, "Number of global variables removed");
33 struct VISIBILITY_HIDDEN GlobalDCE : public ModulePass {
34 static char ID; // Pass identification, replacement for typeid
35 GlobalDCE() : ModulePass(&ID) {}
37 // run - Do the GlobalDCE pass on the specified module, optionally updating
38 // the specified callgraph to reflect the changes.
40 bool runOnModule(Module &M);
43 std::set<GlobalValue*> AliveGlobals;
45 /// GlobalIsNeeded - mark the specific global value as needed, and
46 /// recursively mark anything that it uses as also needed.
47 void GlobalIsNeeded(GlobalValue *GV);
48 void MarkUsedGlobalsAsNeeded(Constant *C);
50 bool SafeToDestroyConstant(Constant* C);
51 bool RemoveUnusedGlobalValue(GlobalValue &GV);
55 char GlobalDCE::ID = 0;
56 static RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
58 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
60 bool GlobalDCE::runOnModule(Module &M) {
62 // Loop over the module, adding globals which are obviously necessary.
63 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
64 Changed |= RemoveUnusedGlobalValue(*I);
65 // Functions with external linkage are needed if they have a body
66 if ((!I->hasLocalLinkage() && !I->hasLinkOnceLinkage()) &&
71 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
73 Changed |= RemoveUnusedGlobalValue(*I);
74 // Externally visible & appending globals are needed, if they have an
76 if ((!I->hasLocalLinkage() && !I->hasLinkOnceLinkage()) &&
81 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
83 Changed |= RemoveUnusedGlobalValue(*I);
84 // Externally visible aliases are needed.
85 if (!I->hasInternalLinkage() && !I->hasLinkOnceLinkage())
89 // Now that all globals which are needed are in the AliveGlobals set, we loop
90 // through the program, deleting those which are not alive.
93 // The first pass is to drop initializers of global variables which are dead.
94 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
95 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I)
96 if (!AliveGlobals.count(I)) {
97 DeadGlobalVars.push_back(I); // Keep track of dead globals
101 // The second pass drops the bodies of functions which are dead...
102 std::vector<Function*> DeadFunctions;
103 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
104 if (!AliveGlobals.count(I)) {
105 DeadFunctions.push_back(I); // Keep track of dead globals
106 if (!I->isDeclaration())
110 if (!DeadFunctions.empty()) {
111 // Now that all interferences have been dropped, delete the actual objects
113 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
114 RemoveUnusedGlobalValue(*DeadFunctions[i]);
115 M.getFunctionList().erase(DeadFunctions[i]);
117 NumFunctions += DeadFunctions.size();
121 if (!DeadGlobalVars.empty()) {
122 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
123 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
124 M.getGlobalList().erase(DeadGlobalVars[i]);
126 NumVariables += DeadGlobalVars.size();
130 // Now delete any dead aliases.
131 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;) {
132 Module::alias_iterator J = I++;
133 if (!AliveGlobals.count(J)) {
134 RemoveUnusedGlobalValue(*J);
135 M.getAliasList().erase(J);
141 // Make sure that all memory is released
142 AliveGlobals.clear();
146 /// GlobalIsNeeded - the specific global value as needed, and
147 /// recursively mark anything that it uses as also needed.
148 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
149 std::set<GlobalValue*>::iterator I = AliveGlobals.find(G);
151 // If the global is already in the set, no need to reprocess it.
152 if (I != AliveGlobals.end()) return;
154 // Otherwise insert it now, so we do not infinitely recurse
155 AliveGlobals.insert(I, G);
157 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
158 // If this is a global variable, we must make sure to add any global values
159 // referenced by the initializer to the alive set.
160 if (GV->hasInitializer())
161 MarkUsedGlobalsAsNeeded(GV->getInitializer());
162 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
163 // The target of a global alias is needed.
164 MarkUsedGlobalsAsNeeded(GA->getAliasee());
166 // Otherwise this must be a function object. We have to scan the body of
167 // the function looking for constants and global values which are used as
168 // operands. Any operands of these types must be processed to ensure that
169 // any globals used will be marked as needed.
170 Function *F = cast<Function>(G);
171 // For all basic blocks...
172 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
173 // For all instructions...
174 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
175 // For all operands...
176 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
177 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
179 else if (Constant *C = dyn_cast<Constant>(*U))
180 MarkUsedGlobalsAsNeeded(C);
184 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
185 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
188 // Loop over all of the operands of the constant, adding any globals they
189 // use to the list of needed globals.
190 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
191 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
195 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
196 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
197 // If found, check to see if the constant pointer ref is safe to destroy, and if
198 // so, nuke it. This will reduce the reference count on the global value, which
199 // might make it deader.
201 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
202 if (GV.use_empty()) return false;
203 GV.removeDeadConstantUsers();
204 return GV.use_empty();
207 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
208 // by constants itself. Note that constants cannot be cyclic, so this test is
209 // pretty easy to implement recursively.
211 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
212 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
213 if (Constant *User = dyn_cast<Constant>(*I)) {
214 if (!SafeToDestroyConstant(User)) return false;