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/ADT/SmallPtrSet.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/IR/Constants.h"
23 #include "llvm/IR/Module.h"
24 #include "llvm/Pass.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;
45 SmallPtrSet<Constant *, 8> SeenConstants;
47 /// GlobalIsNeeded - mark the specific global value as needed, and
48 /// recursively mark anything that it uses as also needed.
49 void GlobalIsNeeded(GlobalValue *GV);
50 void MarkUsedGlobalsAsNeeded(Constant *C);
52 bool RemoveUnusedGlobalValue(GlobalValue &GV);
56 char GlobalDCE::ID = 0;
57 INITIALIZE_PASS(GlobalDCE, "globaldce",
58 "Dead Global Elimination", false, false)
60 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
62 bool GlobalDCE::runOnModule(Module &M) {
65 // Loop over the module, adding globals which are obviously necessary.
66 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
67 Changed |= RemoveUnusedGlobalValue(*I);
68 // Functions with external linkage are needed if they have a body
69 if (!I->isDiscardableIfUnused() &&
70 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
74 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
76 Changed |= RemoveUnusedGlobalValue(*I);
77 // Externally visible & appending globals are needed, if they have an
79 if (!I->isDiscardableIfUnused() &&
80 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
84 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
86 Changed |= RemoveUnusedGlobalValue(*I);
87 // Externally visible aliases are needed.
88 if (!I->isDiscardableIfUnused())
92 // Now that all globals which are needed are in the AliveGlobals set, we loop
93 // through the program, deleting those which are not alive.
96 // The first pass is to drop initializers of global variables which are dead.
97 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
98 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
100 if (!AliveGlobals.count(I)) {
101 DeadGlobalVars.push_back(I); // Keep track of dead globals
102 I->setInitializer(0);
105 // The second pass drops the bodies of functions which are dead...
106 std::vector<Function*> DeadFunctions;
107 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
108 if (!AliveGlobals.count(I)) {
109 DeadFunctions.push_back(I); // Keep track of dead globals
110 if (!I->isDeclaration())
114 // The third pass drops targets of aliases which are dead...
115 std::vector<GlobalAlias*> DeadAliases;
116 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
118 if (!AliveGlobals.count(I)) {
119 DeadAliases.push_back(I);
123 if (!DeadFunctions.empty()) {
124 // Now that all interferences have been dropped, delete the actual objects
126 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
127 RemoveUnusedGlobalValue(*DeadFunctions[i]);
128 M.getFunctionList().erase(DeadFunctions[i]);
130 NumFunctions += DeadFunctions.size();
134 if (!DeadGlobalVars.empty()) {
135 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
136 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
137 M.getGlobalList().erase(DeadGlobalVars[i]);
139 NumVariables += DeadGlobalVars.size();
143 // Now delete any dead aliases.
144 if (!DeadAliases.empty()) {
145 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
146 RemoveUnusedGlobalValue(*DeadAliases[i]);
147 M.getAliasList().erase(DeadAliases[i]);
149 NumAliases += DeadAliases.size();
153 // Make sure that all memory is released
154 AliveGlobals.clear();
155 SeenConstants.clear();
160 /// GlobalIsNeeded - the specific global value as needed, and
161 /// recursively mark anything that it uses as also needed.
162 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
163 // If the global is already in the set, no need to reprocess it.
164 if (!AliveGlobals.insert(G))
167 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
168 // If this is a global variable, we must make sure to add any global values
169 // referenced by the initializer to the alive set.
170 if (GV->hasInitializer())
171 MarkUsedGlobalsAsNeeded(GV->getInitializer());
172 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
173 // The target of a global alias is needed.
174 MarkUsedGlobalsAsNeeded(GA->getAliasee());
176 // Otherwise this must be a function object. We have to scan the body of
177 // the function looking for constants and global values which are used as
178 // operands. Any operands of these types must be processed to ensure that
179 // any globals used will be marked as needed.
180 Function *F = cast<Function>(G);
182 if (F->hasPrefixData())
183 MarkUsedGlobalsAsNeeded(F->getPrefixData());
185 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
186 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
187 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
188 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
190 else if (Constant *C = dyn_cast<Constant>(*U))
191 MarkUsedGlobalsAsNeeded(C);
195 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
196 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
197 return GlobalIsNeeded(GV);
199 // Loop over all of the operands of the constant, adding any globals they
200 // use to the list of needed globals.
201 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
202 // If we've already processed this constant there's no need to do it again.
203 Constant *Op = dyn_cast<Constant>(*I);
204 if (Op && SeenConstants.insert(Op))
205 MarkUsedGlobalsAsNeeded(Op);
209 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
210 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
211 // If found, check to see if the constant pointer ref is safe to destroy, and if
212 // so, nuke it. This will reduce the reference count on the global value, which
213 // might make it deader.
215 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
216 if (GV.use_empty()) return false;
217 GV.removeDeadConstantUsers();
218 return GV.use_empty();