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 INITIALIZE_PASS(GlobalDCE, "globaldce",
55 "Dead Global Elimination", false, false);
57 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
59 bool GlobalDCE::runOnModule(Module &M) {
62 // Loop over the module, adding globals which are obviously necessary.
63 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
64 Changed |= RemoveUnusedGlobalValue(*I);
65 // Functions with external linkage are needed if they have a body
66 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
67 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
71 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
73 Changed |= RemoveUnusedGlobalValue(*I);
74 // Externally visible & appending globals are needed, if they have an
76 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
77 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
81 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
83 Changed |= RemoveUnusedGlobalValue(*I);
84 // Externally visible aliases are needed.
85 if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage())
89 // Now that all globals which are needed are in the AliveGlobals set, we loop
90 // through the program, deleting those which are not alive.
93 // The first pass is to drop initializers of global variables which are dead.
94 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
95 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
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();
156 /// GlobalIsNeeded - the specific global value as needed, and
157 /// recursively mark anything that it uses as also needed.
158 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
159 // If the global is already in the set, no need to reprocess it.
160 if (!AliveGlobals.insert(G))
163 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
164 // If this is a global variable, we must make sure to add any global values
165 // referenced by the initializer to the alive set.
166 if (GV->hasInitializer())
167 MarkUsedGlobalsAsNeeded(GV->getInitializer());
168 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
169 // The target of a global alias is needed.
170 MarkUsedGlobalsAsNeeded(GA->getAliasee());
172 // Otherwise this must be a function object. We have to scan the body of
173 // the function looking for constants and global values which are used as
174 // operands. Any operands of these types must be processed to ensure that
175 // any globals used will be marked as needed.
176 Function *F = cast<Function>(G);
178 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
179 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
180 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
181 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
183 else if (Constant *C = dyn_cast<Constant>(*U))
184 MarkUsedGlobalsAsNeeded(C);
188 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
189 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
190 return GlobalIsNeeded(GV);
192 // Loop over all of the operands of the constant, adding any globals they
193 // use to the list of needed globals.
194 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
195 if (Constant *OpC = dyn_cast<Constant>(*I))
196 MarkUsedGlobalsAsNeeded(OpC);
199 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
200 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
201 // If found, check to see if the constant pointer ref is safe to destroy, and if
202 // so, nuke it. This will reduce the reference count on the global value, which
203 // might make it deader.
205 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
206 if (GV.use_empty()) return false;
207 GV.removeDeadConstantUsers();
208 return GV.use_empty();