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 #include "llvm/Transforms/IPO.h"
19 #include "llvm/Constants.h"
20 #include "llvm/Module.h"
21 #include "llvm/Pass.h"
22 #include "Support/Statistic.h"
28 Statistic<> NumFunctions("globaldce","Number of functions removed");
29 Statistic<> NumVariables("globaldce","Number of global variables removed");
30 Statistic<> NumCPRs("globaldce", "Number of const pointer refs removed");
32 struct GlobalDCE : public Pass {
33 // run - Do the GlobalDCE pass on the specified module, optionally updating
34 // the specified callgraph to reflect the changes.
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 RemoveUnusedConstantPointerRef(GlobalValue &GV);
47 bool SafeToDestroyConstant(Constant *C);
49 RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
52 Pass *createGlobalDCEPass() { return new GlobalDCE(); }
54 bool GlobalDCE::run(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 |= RemoveUnusedConstantPointerRef(*I);
59 // Functions with external linkage are needed if they have a body
60 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
65 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
66 Changed |= RemoveUnusedConstantPointerRef(*I);
67 // Externally visible & appending globals are needed, if they have an
69 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
75 // Now that all globals which are needed are in the AliveGlobals set, we loop
76 // through the program, deleting those which are not alive.
79 // The first pass is to drop initializers of global variables which are dead.
80 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
81 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I)
82 if (!AliveGlobals.count(I)) {
83 DeadGlobalVars.push_back(I); // Keep track of dead globals
88 // The second pass drops the bodies of functions which are dead...
89 std::vector<Function*> DeadFunctions;
90 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
91 if (!AliveGlobals.count(I)) {
92 DeadFunctions.push_back(I); // Keep track of dead globals
97 if (!DeadFunctions.empty()) {
98 // Now that all interreferences have been dropped, delete the actual objects
100 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
101 RemoveUnusedConstantPointerRef(*DeadFunctions[i]);
102 M.getFunctionList().erase(DeadFunctions[i]);
104 NumFunctions += DeadFunctions.size();
108 if (!DeadGlobalVars.empty()) {
109 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
110 RemoveUnusedConstantPointerRef(*DeadGlobalVars[i]);
111 M.getGlobalList().erase(DeadGlobalVars[i]);
113 NumVariables += DeadGlobalVars.size();
117 // Make sure that all memory is released
118 AliveGlobals.clear();
122 /// MarkGlobalIsNeeded - the specific global value as needed, and
123 /// recursively mark anything that it uses as also needed.
124 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
125 std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G);
127 // If the global is already in the set, no need to reprocess it.
128 if (I != AliveGlobals.end() && *I == G) return;
130 // Otherwise insert it now, so we do not infinitely recurse
131 AliveGlobals.insert(I, G);
133 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
134 // If this is a global variable, we must make sure to add any global values
135 // referenced by the initializer to the alive set.
136 if (GV->hasInitializer())
137 MarkUsedGlobalsAsNeeded(GV->getInitializer());
139 // Otherwise this must be a function object. We have to scan the body of
140 // the function looking for constants and global values which are used as
141 // operands. Any operands of these types must be processed to ensure that
142 // any globals used will be marked as needed.
143 Function *F = cast<Function>(G);
144 // For all basic blocks...
145 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
146 // For all instructions...
147 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
148 // For all operands...
149 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
150 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
152 else if (Constant *C = dyn_cast<Constant>(*U))
153 MarkUsedGlobalsAsNeeded(C);
157 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
158 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C))
159 GlobalIsNeeded(CPR->getValue());
161 // Loop over all of the operands of the constant, adding any globals they
162 // use to the list of needed globals.
163 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
164 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
168 // RemoveUnusedConstantPointerRef - Loop over all of the uses of the specified
169 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
170 // If found, check to see if the constant pointer ref is safe to destroy, and if
171 // so, nuke it. This will reduce the reference count on the global value, which
172 // might make it deader.
174 bool GlobalDCE::RemoveUnusedConstantPointerRef(GlobalValue &GV) {
175 for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
176 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*I))
177 if (SafeToDestroyConstant(CPR)) { // Only if unreferenced...
178 CPR->destroyConstant();
186 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
187 // by constants itself. Note that constants cannot be cyclic, so this test is
188 // pretty easy to implement recursively.
190 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
191 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
192 if (Constant *User = dyn_cast<Constant>(*I)) {
193 if (!SafeToDestroyConstant(User)) return false;
201 } // End llvm namespace