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 RemoveUnusedGlobalValue(GlobalValue &GV);
54 char GlobalDCE::ID = 0;
55 static RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
57 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
59 bool GlobalDCE::runOnModule(Module &M) {
61 Context = &M.getContext();
63 // Loop over the module, adding globals which are obviously necessary.
64 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
65 Changed |= RemoveUnusedGlobalValue(*I);
66 // Functions with external linkage are needed if they have a body
67 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
68 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
72 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
74 Changed |= RemoveUnusedGlobalValue(*I);
75 // Externally visible & appending globals are needed, if they have an
77 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
78 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
82 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
84 Changed |= RemoveUnusedGlobalValue(*I);
85 // Externally visible aliases are needed.
86 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage())
90 // Now that all globals which are needed are in the AliveGlobals set, we loop
91 // through the program, deleting those which are not alive.
94 // The first pass is to drop initializers of global variables which are dead.
95 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
96 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I)
97 if (!AliveGlobals.count(I)) {
98 DeadGlobalVars.push_back(I); // Keep track of dead globals
102 // The second pass drops the bodies of functions which are dead...
103 std::vector<Function*> DeadFunctions;
104 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
105 if (!AliveGlobals.count(I)) {
106 DeadFunctions.push_back(I); // Keep track of dead globals
107 if (!I->isDeclaration())
111 // The third pass drops targets of aliases which are dead...
112 std::vector<GlobalAlias*> DeadAliases;
113 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
115 if (!AliveGlobals.count(I)) {
116 DeadAliases.push_back(I);
120 if (!DeadFunctions.empty()) {
121 // Now that all interferences have been dropped, delete the actual objects
123 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
124 RemoveUnusedGlobalValue(*DeadFunctions[i]);
125 M.getFunctionList().erase(DeadFunctions[i]);
127 NumFunctions += DeadFunctions.size();
131 if (!DeadGlobalVars.empty()) {
132 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
133 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
134 M.getGlobalList().erase(DeadGlobalVars[i]);
136 NumVariables += DeadGlobalVars.size();
140 // Now delete any dead aliases.
141 if (!DeadAliases.empty()) {
142 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
143 RemoveUnusedGlobalValue(*DeadAliases[i]);
144 M.getAliasList().erase(DeadAliases[i]);
146 NumAliases += DeadAliases.size();
150 // Make sure that all memory is released
151 AliveGlobals.clear();
155 /// GlobalIsNeeded - the specific global value as needed, and
156 /// recursively mark anything that it uses as also needed.
157 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
158 std::set<GlobalValue*>::iterator I = AliveGlobals.find(G);
160 // If the global is already in the set, no need to reprocess it.
161 if (I != AliveGlobals.end()) return;
163 // Otherwise insert it now, so we do not infinitely recurse
164 AliveGlobals.insert(I, G);
166 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
167 // If this is a global variable, we must make sure to add any global values
168 // referenced by the initializer to the alive set.
169 if (GV->hasInitializer())
170 MarkUsedGlobalsAsNeeded(GV->getInitializer());
171 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
172 // The target of a global alias is needed.
173 MarkUsedGlobalsAsNeeded(GA->getAliasee());
175 // Otherwise this must be a function object. We have to scan the body of
176 // the function looking for constants and global values which are used as
177 // operands. Any operands of these types must be processed to ensure that
178 // any globals used will be marked as needed.
179 Function *F = cast<Function>(G);
180 // For all basic blocks...
181 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
182 // For all instructions...
183 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
184 // For all operands...
185 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
186 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
188 else if (Constant *C = dyn_cast<Constant>(*U))
189 MarkUsedGlobalsAsNeeded(C);
193 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
194 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
197 // Loop over all of the operands of the constant, adding any globals they
198 // use to the list of needed globals.
199 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
200 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
204 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
205 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
206 // If found, check to see if the constant pointer ref is safe to destroy, and if
207 // so, nuke it. This will reduce the reference count on the global value, which
208 // might make it deader.
210 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
211 if (GV.use_empty()) return false;
212 GV.removeDeadConstantUsers();
213 return GV.use_empty();