1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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/Statistic.h"
24 #include "llvm/Support/Compiler.h"
28 STATISTIC(NumFunctions, "Number of functions removed");
29 STATISTIC(NumVariables, "Number of global variables removed");
32 struct VISIBILITY_HIDDEN GlobalDCE : public ModulePass {
33 // run - Do the GlobalDCE pass on the specified module, optionally updating
34 // the specified callgraph to reflect the changes.
36 bool runOnModule(Module &M);
39 std::set<GlobalValue*> AliveGlobals;
41 /// MarkGlobalIsNeeded - the specific global value as needed, and
42 /// recursively mark anything that it uses as also needed.
43 void GlobalIsNeeded(GlobalValue *GV);
44 void MarkUsedGlobalsAsNeeded(Constant *C);
46 bool SafeToDestroyConstant(Constant* C);
47 bool RemoveUnusedGlobalValue(GlobalValue &GV);
49 RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
52 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
54 bool GlobalDCE::runOnModule(Module &M) {
56 // Loop over the module, adding globals which are obviously necessary.
57 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
58 Changed |= RemoveUnusedGlobalValue(*I);
59 // Functions with external linkage are needed if they have a body
60 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
65 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
67 Changed |= RemoveUnusedGlobalValue(*I);
68 // Externally visible & appending globals are needed, if they have an
70 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
76 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
78 // Aliases are always needed even if they are not used.
79 MarkUsedGlobalsAsNeeded(I->getAliasee());
82 // Now that all globals which are needed are in the AliveGlobals set, we loop
83 // through the program, deleting those which are not alive.
86 // The first pass is to drop initializers of global variables which are dead.
87 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
88 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I)
89 if (!AliveGlobals.count(I)) {
90 DeadGlobalVars.push_back(I); // Keep track of dead globals
95 // The second pass drops the bodies of functions which are dead...
96 std::vector<Function*> DeadFunctions;
97 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
98 if (!AliveGlobals.count(I)) {
99 DeadFunctions.push_back(I); // Keep track of dead globals
100 if (!I->isDeclaration())
104 if (!DeadFunctions.empty()) {
105 // Now that all interreferences have been dropped, delete the actual objects
107 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
108 RemoveUnusedGlobalValue(*DeadFunctions[i]);
109 M.getFunctionList().erase(DeadFunctions[i]);
111 NumFunctions += DeadFunctions.size();
115 if (!DeadGlobalVars.empty()) {
116 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
117 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
118 M.getGlobalList().erase(DeadGlobalVars[i]);
120 NumVariables += DeadGlobalVars.size();
124 // Make sure that all memory is released
125 AliveGlobals.clear();
129 /// MarkGlobalIsNeeded - the specific global value as needed, and
130 /// recursively mark anything that it uses as also needed.
131 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
132 std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G);
134 // If the global is already in the set, no need to reprocess it.
135 if (I != AliveGlobals.end() && *I == G) return;
137 // Otherwise insert it now, so we do not infinitely recurse
138 AliveGlobals.insert(I, G);
140 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
141 // If this is a global variable, we must make sure to add any global values
142 // referenced by the initializer to the alive set.
143 if (GV->hasInitializer())
144 MarkUsedGlobalsAsNeeded(GV->getInitializer());
145 } else if (!isa<GlobalAlias>(G)) {
146 // Otherwise this must be a function object. We have to scan the body of
147 // the function looking for constants and global values which are used as
148 // operands. Any operands of these types must be processed to ensure that
149 // any globals used will be marked as needed.
150 Function *F = cast<Function>(G);
151 // For all basic blocks...
152 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
153 // For all instructions...
154 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
155 // For all operands...
156 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
157 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
159 else if (Constant *C = dyn_cast<Constant>(*U))
160 MarkUsedGlobalsAsNeeded(C);
164 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
165 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
168 // Loop over all of the operands of the constant, adding any globals they
169 // use to the list of needed globals.
170 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
171 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
175 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
176 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
177 // If found, check to see if the constant pointer ref is safe to destroy, and if
178 // so, nuke it. This will reduce the reference count on the global value, which
179 // might make it deader.
181 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
182 if (GV.use_empty()) return false;
183 GV.removeDeadConstantUsers();
184 return GV.use_empty();
187 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
188 // by constants itself. Note that constants cannot be cyclic, so this test is
189 // pretty easy to implement recursively.
191 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
192 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
193 if (Constant *User = dyn_cast<Constant>(*I)) {
194 if (!SafeToDestroyConstant(User)) return false;