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 #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/Module.h"
23 #include "llvm/Pass.h"
26 #define DEBUG_TYPE "globaldce"
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 GlobalDCE : public ModulePass {
34 static char ID; // Pass identification, replacement for typeid
35 GlobalDCE() : ModulePass(ID) {
36 initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
39 // run - Do the GlobalDCE pass on the specified module, optionally updating
40 // the specified callgraph to reflect the changes.
42 bool runOnModule(Module &M) override;
45 SmallPtrSet<GlobalValue*, 32> AliveGlobals;
46 SmallPtrSet<Constant *, 8> SeenConstants;
48 /// GlobalIsNeeded - mark the specific global value as needed, and
49 /// recursively mark anything that it uses as also needed.
50 void GlobalIsNeeded(GlobalValue *GV);
51 void MarkUsedGlobalsAsNeeded(Constant *C);
53 bool RemoveUnusedGlobalValue(GlobalValue &GV);
57 char GlobalDCE::ID = 0;
58 INITIALIZE_PASS(GlobalDCE, "globaldce",
59 "Dead Global Elimination", false, false)
61 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
63 bool GlobalDCE::runOnModule(Module &M) {
66 // Loop over the module, adding globals which are obviously necessary.
67 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
68 Changed |= RemoveUnusedGlobalValue(*I);
69 // Functions with external linkage are needed if they have a body
70 if (!I->isDiscardableIfUnused() &&
71 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
75 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
77 Changed |= RemoveUnusedGlobalValue(*I);
78 // Externally visible & appending globals are needed, if they have an
80 if (!I->isDiscardableIfUnused() &&
81 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
85 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
87 Changed |= RemoveUnusedGlobalValue(*I);
88 // Externally visible aliases are needed.
89 if (!I->isDiscardableIfUnused())
93 // Now that all globals which are needed are in the AliveGlobals set, we loop
94 // through the program, deleting those which are not alive.
97 // The first pass is to drop initializers of global variables which are dead.
98 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
99 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
101 if (!AliveGlobals.count(I)) {
102 DeadGlobalVars.push_back(I); // Keep track of dead globals
103 I->setInitializer(0);
106 // The second pass drops the bodies of functions which are dead...
107 std::vector<Function*> DeadFunctions;
108 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
109 if (!AliveGlobals.count(I)) {
110 DeadFunctions.push_back(I); // Keep track of dead globals
111 if (!I->isDeclaration())
115 // The third pass drops targets of aliases which are dead...
116 std::vector<GlobalAlias*> DeadAliases;
117 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
119 if (!AliveGlobals.count(I)) {
120 DeadAliases.push_back(I);
124 if (!DeadFunctions.empty()) {
125 // Now that all interferences have been dropped, delete the actual objects
127 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
128 RemoveUnusedGlobalValue(*DeadFunctions[i]);
129 M.getFunctionList().erase(DeadFunctions[i]);
131 NumFunctions += DeadFunctions.size();
135 if (!DeadGlobalVars.empty()) {
136 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
137 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
138 M.getGlobalList().erase(DeadGlobalVars[i]);
140 NumVariables += DeadGlobalVars.size();
144 // Now delete any dead aliases.
145 if (!DeadAliases.empty()) {
146 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
147 RemoveUnusedGlobalValue(*DeadAliases[i]);
148 M.getAliasList().erase(DeadAliases[i]);
150 NumAliases += DeadAliases.size();
154 // Make sure that all memory is released
155 AliveGlobals.clear();
156 SeenConstants.clear();
161 /// GlobalIsNeeded - the specific global value as needed, and
162 /// recursively mark anything that it uses as also needed.
163 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
164 // If the global is already in the set, no need to reprocess it.
165 if (!AliveGlobals.insert(G))
168 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
169 // If this is a global variable, we must make sure to add any global values
170 // referenced by the initializer to the alive set.
171 if (GV->hasInitializer())
172 MarkUsedGlobalsAsNeeded(GV->getInitializer());
173 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
174 // The target of a global alias is needed.
175 MarkUsedGlobalsAsNeeded(GA->getAliasee());
177 // Otherwise this must be a function object. We have to scan the body of
178 // the function looking for constants and global values which are used as
179 // operands. Any operands of these types must be processed to ensure that
180 // any globals used will be marked as needed.
181 Function *F = cast<Function>(G);
183 if (F->hasPrefixData())
184 MarkUsedGlobalsAsNeeded(F->getPrefixData());
186 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
187 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
188 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
189 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
191 else if (Constant *C = dyn_cast<Constant>(*U))
192 MarkUsedGlobalsAsNeeded(C);
196 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
197 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
198 return GlobalIsNeeded(GV);
200 // Loop over all of the operands of the constant, adding any globals they
201 // use to the list of needed globals.
202 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
203 // If we've already processed this constant there's no need to do it again.
204 Constant *Op = dyn_cast<Constant>(*I);
205 if (Op && SeenConstants.insert(Op))
206 MarkUsedGlobalsAsNeeded(Op);
210 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
211 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
212 // If found, check to see if the constant pointer ref is safe to destroy, and if
213 // so, nuke it. This will reduce the reference count on the global value, which
214 // might make it deader.
216 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
217 if (GV.use_empty()) return false;
218 GV.removeDeadConstantUsers();
219 return GV.use_empty();