1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
3 // This transform is designed to eliminate unreachable internal globals from the
4 // program. It uses an aggressive algorithm, searching out globals that are
5 // known to be alive. After it finds all of the globals which are needed, it
6 // deletes whatever is left over. This allows it to delete recursive chunks of
7 // the program which are unreachable.
9 //===----------------------------------------------------------------------===//
11 #include "llvm/Transforms/IPO.h"
12 #include "llvm/Constants.h"
13 #include "llvm/Module.h"
14 #include "llvm/Pass.h"
15 #include "Support/Statistic.h"
19 Statistic<> NumFunctions("globaldce","Number of functions removed");
20 Statistic<> NumVariables("globaldce","Number of global variables removed");
21 Statistic<> NumCPRs("globaldce", "Number of const pointer refs removed");
23 struct GlobalDCE : public Pass {
24 // run - Do the GlobalDCE pass on the specified module, optionally updating
25 // the specified callgraph to reflect the changes.
30 std::set<GlobalValue*> AliveGlobals;
32 /// MarkGlobalIsNeeded - the specific global value as needed, and
33 /// recursively mark anything that it uses as also needed.
34 void GlobalIsNeeded(GlobalValue *GV);
35 void MarkUsedGlobalsAsNeeded(Constant *C);
37 bool RemoveUnusedConstantPointerRef(GlobalValue &GV);
38 bool SafeToDestroyConstant(Constant *C);
40 RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
43 Pass *createGlobalDCEPass() { return new GlobalDCE(); }
45 bool GlobalDCE::run(Module &M) {
47 // Loop over the module, adding globals which are obviously necessary.
48 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
49 Changed |= RemoveUnusedConstantPointerRef(*I);
50 // Functions with external linkage are needed if they have a body
51 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
56 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
57 Changed |= RemoveUnusedConstantPointerRef(*I);
58 // Externally visible & appending globals are needed, if they have an
60 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
66 // Now that all globals which are needed are in the AliveGlobals set, we loop
67 // through the program, deleting those which are not alive.
70 // The first pass is to drop initializers of global variables which are dead.
71 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
72 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I)
73 if (!AliveGlobals.count(I)) {
74 DeadGlobalVars.push_back(I); // Keep track of dead globals
79 // The second pass drops the bodies of functions which are dead...
80 std::vector<Function*> DeadFunctions;
81 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
82 if (!AliveGlobals.count(I)) {
83 DeadFunctions.push_back(I); // Keep track of dead globals
88 if (!DeadFunctions.empty()) {
89 // Now that all interreferences have been dropped, delete the actual objects
91 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
92 RemoveUnusedConstantPointerRef(*DeadFunctions[i]);
93 M.getFunctionList().erase(DeadFunctions[i]);
95 NumFunctions += DeadFunctions.size();
99 if (!DeadGlobalVars.empty()) {
100 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
101 RemoveUnusedConstantPointerRef(*DeadGlobalVars[i]);
102 M.getGlobalList().erase(DeadGlobalVars[i]);
104 NumVariables += DeadGlobalVars.size();
108 // Make sure that all memory is released
109 AliveGlobals.clear();
113 /// MarkGlobalIsNeeded - the specific global value as needed, and
114 /// recursively mark anything that it uses as also needed.
115 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
116 std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G);
118 // If the global is already in the set, no need to reprocess it.
119 if (I != AliveGlobals.end() && *I == G) return;
121 // Otherwise insert it now, so we do not infinitely recurse
122 AliveGlobals.insert(I, G);
124 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
125 // If this is a global variable, we must make sure to add any global values
126 // referenced by the initializer to the alive set.
127 if (GV->hasInitializer())
128 MarkUsedGlobalsAsNeeded(GV->getInitializer());
130 // Otherwise this must be a function object. We have to scan the body of
131 // the function looking for constants and global values which are used as
132 // operands. Any operands of these types must be processed to ensure that
133 // any globals used will be marked as needed.
134 Function *F = cast<Function>(G);
135 // For all basic blocks...
136 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
137 // For all instructions...
138 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
139 // For all operands...
140 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
141 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
143 else if (Constant *C = dyn_cast<Constant>(*U))
144 MarkUsedGlobalsAsNeeded(C);
148 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
149 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C))
150 GlobalIsNeeded(CPR->getValue());
152 // Loop over all of the operands of the constant, adding any globals they
153 // use to the list of needed globals.
154 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
155 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
159 // RemoveUnusedConstantPointerRef - Loop over all of the uses of the specified
160 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
161 // If found, check to see if the constant pointer ref is safe to destroy, and if
162 // so, nuke it. This will reduce the reference count on the global value, which
163 // might make it deader.
165 bool GlobalDCE::RemoveUnusedConstantPointerRef(GlobalValue &GV) {
166 for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
167 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*I))
168 if (SafeToDestroyConstant(CPR)) { // Only if unreferenced...
169 CPR->destroyConstant();
177 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
178 // by constants itself. Note that constants cannot be cyclic, so this test is
179 // pretty easy to implement recursively.
181 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
182 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
183 if (Constant *User = dyn_cast<Constant>(*I)) {
184 if (!SafeToDestroyConstant(User)) return false;