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/SmallPtrSet.h"
24 #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 SmallPtrSet<GlobalValue*, 32> 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();
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 // The third pass drops targets of aliases which are dead...
111 std::vector<GlobalAlias*> DeadAliases;
112 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
114 if (!AliveGlobals.count(I)) {
115 DeadAliases.push_back(I);
119 if (!DeadFunctions.empty()) {
120 // Now that all interferences have been dropped, delete the actual objects
122 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
123 RemoveUnusedGlobalValue(*DeadFunctions[i]);
124 M.getFunctionList().erase(DeadFunctions[i]);
126 NumFunctions += DeadFunctions.size();
130 if (!DeadGlobalVars.empty()) {
131 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
132 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
133 M.getGlobalList().erase(DeadGlobalVars[i]);
135 NumVariables += DeadGlobalVars.size();
139 // Now delete any dead aliases.
140 if (!DeadAliases.empty()) {
141 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
142 RemoveUnusedGlobalValue(*DeadAliases[i]);
143 M.getAliasList().erase(DeadAliases[i]);
145 NumAliases += DeadAliases.size();
149 // Make sure that all memory is released
150 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 // If the global is already in the set, no need to reprocess it.
159 if (!AliveGlobals.insert(G))
162 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
163 // If this is a global variable, we must make sure to add any global values
164 // referenced by the initializer to the alive set.
165 if (GV->hasInitializer())
166 MarkUsedGlobalsAsNeeded(GV->getInitializer());
167 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
168 // The target of a global alias is needed.
169 MarkUsedGlobalsAsNeeded(GA->getAliasee());
171 // Otherwise this must be a function object. We have to scan the body of
172 // the function looking for constants and global values which are used as
173 // operands. Any operands of these types must be processed to ensure that
174 // any globals used will be marked as needed.
175 Function *F = cast<Function>(G);
177 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
178 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
179 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
180 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
182 else if (Constant *C = dyn_cast<Constant>(*U))
183 MarkUsedGlobalsAsNeeded(C);
187 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
188 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
189 return GlobalIsNeeded(GV);
191 // Loop over all of the operands of the constant, adding any globals they
192 // use to the list of needed globals.
193 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
194 if (Constant *OpC = dyn_cast<Constant>(*I))
195 MarkUsedGlobalsAsNeeded(OpC);
198 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
199 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
200 // If found, check to see if the constant pointer ref is safe to destroy, and if
201 // so, nuke it. This will reduce the reference count on the global value, which
202 // might make it deader.
204 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
205 if (GV.use_empty()) return false;
206 GV.removeDeadConstantUsers();
207 return GV.use_empty();