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->hasExternalLinkage() && !I->isExternal())
55 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
56 Changed |= RemoveUnusedConstantPointerRef(*I);
57 // Externally visible globals are needed, if they have an initializer.
58 if (I->hasExternalLinkage() && !I->isExternal())
63 // Now that all globals which are needed are in the AliveGlobals set, we loop
64 // through the program, deleting those which are not alive.
67 // The first pass is to drop initializers of global variables which are dead.
68 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
69 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I)
70 if (!AliveGlobals.count(I)) {
71 DeadGlobalVars.push_back(I); // Keep track of dead globals
76 // The second pass drops the bodies of functions which are dead...
77 std::vector<Function*> DeadFunctions;
78 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
79 if (!AliveGlobals.count(I)) {
80 DeadFunctions.push_back(I); // Keep track of dead globals
85 if (!DeadFunctions.empty()) {
86 // Now that all interreferences have been dropped, delete the actual objects
88 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
89 RemoveUnusedConstantPointerRef(*DeadFunctions[i]);
90 M.getFunctionList().erase(DeadFunctions[i]);
92 NumFunctions += DeadFunctions.size();
96 if (!DeadGlobalVars.empty()) {
97 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
98 RemoveUnusedConstantPointerRef(*DeadGlobalVars[i]);
99 M.getGlobalList().erase(DeadGlobalVars[i]);
101 NumVariables += DeadGlobalVars.size();
105 // Make sure that all memory is released
106 AliveGlobals.clear();
110 /// MarkGlobalIsNeeded - the specific global value as needed, and
111 /// recursively mark anything that it uses as also needed.
112 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
113 std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G);
115 // If the global is already in the set, no need to reprocess it.
116 if (I != AliveGlobals.end() && *I == G) return;
118 // Otherwise insert it now, so we do not infinitely recurse
119 AliveGlobals.insert(I, G);
121 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
122 // If this is a global variable, we must make sure to add any global values
123 // referenced by the initializer to the alive set.
124 if (GV->hasInitializer())
125 MarkUsedGlobalsAsNeeded(GV->getInitializer());
127 // Otherwise this must be a function object. We have to scan the body of
128 // the function looking for constants and global values which are used as
129 // operands. Any operands of these types must be processed to ensure that
130 // any globals used will be marked as needed.
131 Function *F = cast<Function>(G);
132 // For all basic blocks...
133 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
134 // For all instructions...
135 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
136 // For all operands...
137 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
138 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
140 else if (Constant *C = dyn_cast<Constant>(*U))
141 MarkUsedGlobalsAsNeeded(C);
145 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
146 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C))
147 GlobalIsNeeded(CPR->getValue());
149 // Loop over all of the operands of the constant, adding any globals they
150 // use to the list of needed globals.
151 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
152 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
156 // RemoveUnusedConstantPointerRef - Loop over all of the uses of the specified
157 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
158 // If found, check to see if the constant pointer ref is safe to destroy, and if
159 // so, nuke it. This will reduce the reference count on the global value, which
160 // might make it deader.
162 bool GlobalDCE::RemoveUnusedConstantPointerRef(GlobalValue &GV) {
163 for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
164 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*I))
165 if (SafeToDestroyConstant(CPR)) { // Only if unreferenced...
166 CPR->destroyConstant();
174 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
175 // by constants itself. Note that constants cannot be cyclic, so this test is
176 // pretty easy to implement recursively.
178 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
179 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
180 if (Constant *User = dyn_cast<Constant>(*I)) {
181 if (!SafeToDestroyConstant(User)) return false;