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) {
35 initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
38 // run - Do the GlobalDCE pass on the specified module, optionally updating
39 // the specified callgraph to reflect the changes.
41 bool runOnModule(Module &M);
44 SmallPtrSet<GlobalValue*, 32> AliveGlobals;
46 /// GlobalIsNeeded - mark the specific global value as needed, and
47 /// recursively mark anything that it uses as also needed.
48 void GlobalIsNeeded(GlobalValue *GV);
49 void MarkUsedGlobalsAsNeeded(Constant *C);
51 bool RemoveUnusedGlobalValue(GlobalValue &GV);
55 char GlobalDCE::ID = 0;
56 INITIALIZE_PASS(GlobalDCE, "globaldce",
57 "Dead Global Elimination", false, false)
59 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
61 bool GlobalDCE::runOnModule(Module &M) {
64 // Loop over the module, adding globals which are obviously necessary.
65 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
66 Changed |= RemoveUnusedGlobalValue(*I);
67 // Functions with external linkage are needed if they have a body
68 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
69 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
73 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
75 Changed |= RemoveUnusedGlobalValue(*I);
76 // Externally visible & appending globals are needed, if they have an
78 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
79 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
83 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
85 Changed |= RemoveUnusedGlobalValue(*I);
86 // Externally visible aliases are needed.
87 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage())
91 // Now that all globals which are needed are in the AliveGlobals set, we loop
92 // through the program, deleting those which are not alive.
95 // The first pass is to drop initializers of global variables which are dead.
96 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
97 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
99 if (!AliveGlobals.count(I)) {
100 DeadGlobalVars.push_back(I); // Keep track of dead globals
101 I->setInitializer(0);
104 // The second pass drops the bodies of functions which are dead...
105 std::vector<Function*> DeadFunctions;
106 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
107 if (!AliveGlobals.count(I)) {
108 DeadFunctions.push_back(I); // Keep track of dead globals
109 if (!I->isDeclaration())
113 // The third pass drops targets of aliases which are dead...
114 std::vector<GlobalAlias*> DeadAliases;
115 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
117 if (!AliveGlobals.count(I)) {
118 DeadAliases.push_back(I);
122 if (!DeadFunctions.empty()) {
123 // Now that all interferences have been dropped, delete the actual objects
125 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
126 RemoveUnusedGlobalValue(*DeadFunctions[i]);
127 M.getFunctionList().erase(DeadFunctions[i]);
129 NumFunctions += DeadFunctions.size();
133 if (!DeadGlobalVars.empty()) {
134 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
135 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
136 M.getGlobalList().erase(DeadGlobalVars[i]);
138 NumVariables += DeadGlobalVars.size();
142 // Now delete any dead aliases.
143 if (!DeadAliases.empty()) {
144 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
145 RemoveUnusedGlobalValue(*DeadAliases[i]);
146 M.getAliasList().erase(DeadAliases[i]);
148 NumAliases += DeadAliases.size();
152 // Make sure that all memory is released
153 AliveGlobals.clear();
158 /// GlobalIsNeeded - the specific global value as needed, and
159 /// recursively mark anything that it uses as also needed.
160 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
161 // If the global is already in the set, no need to reprocess it.
162 if (!AliveGlobals.insert(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);
180 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
181 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
182 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
183 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
185 else if (Constant *C = dyn_cast<Constant>(*U))
186 MarkUsedGlobalsAsNeeded(C);
190 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
191 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
192 return GlobalIsNeeded(GV);
194 // Loop over all of the operands of the constant, adding any globals they
195 // use to the list of needed globals.
196 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
197 if (Constant *OpC = dyn_cast<Constant>(*I))
198 MarkUsedGlobalsAsNeeded(OpC);
201 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
202 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
203 // If found, check to see if the constant pointer ref is safe to destroy, and if
204 // so, nuke it. This will reduce the reference count on the global value, which
205 // might make it deader.
207 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
208 if (GV.use_empty()) return false;
209 GV.removeDeadConstantUsers();
210 return GV.use_empty();