1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
3 // This transform is designed to eliminate unreachable internal globals
4 // FIXME: GlobalDCE should update the callgraph, not destroy it!
6 //===----------------------------------------------------------------------===//
8 #include "llvm/Transforms/IPO.h"
9 #include "llvm/Module.h"
10 #include "llvm/Constants.h"
11 #include "llvm/DerivedTypes.h"
12 #include "llvm/Analysis/CallGraph.h"
13 #include "Support/DepthFirstIterator.h"
14 #include "Support/Statistic.h"
18 Statistic<> NumFunctions("globaldce","Number of functions removed");
19 Statistic<> NumVariables("globaldce","Number of global variables removed");
20 Statistic<> NumCPRs("globaldce", "Number of const pointer refs removed");
21 Statistic<> NumConsts("globaldce", "Number of init constants removed");
23 bool RemoveUnreachableFunctions(Module &M, CallGraph &CallGraph) {
24 // Calculate which functions are reachable from the external functions in
27 std::set<CallGraphNode*> ReachableNodes(df_begin(&CallGraph),
30 // Loop over the functions in the module twice. The first time is used to
31 // drop references that functions have to each other before they are
32 // deleted. The second pass removes the functions that need to be removed.
34 std::vector<CallGraphNode*> FunctionsToDelete; // Track unused functions
35 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
36 CallGraphNode *N = CallGraph[I];
38 if (!ReachableNodes.count(N)) { // Not reachable??
39 I->dropAllReferences();
40 N->removeAllCalledFunctions();
41 FunctionsToDelete.push_back(N);
46 // Nothing to do if no unreachable functions have been found...
47 if (FunctionsToDelete.empty()) return false;
49 // Unreachable functions have been found and should have no references to
50 // them, delete them now.
52 for (std::vector<CallGraphNode*>::iterator I = FunctionsToDelete.begin(),
53 E = FunctionsToDelete.end(); I != E; ++I)
54 delete CallGraph.removeFunctionFromModule(*I);
56 // Walk the function list, removing prototypes for functions which are not
58 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
59 if (I->use_size() == 0 && I->isExternal())
60 delete CallGraph.removeFunctionFromModule(I);
65 struct GlobalDCE : public Pass {
66 // run - Do the GlobalDCE pass on the specified module, optionally updating
67 // the specified callgraph to reflect the changes.
70 return RemoveUnreachableFunctions(M, getAnalysis<CallGraph>()) |
71 RemoveUnreachableGlobalVariables(M);
74 // getAnalysisUsage - This function works on the call graph of a module.
75 // It is capable of updating the call graph to reflect the new state of the
78 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
79 AU.addRequired<CallGraph>();
83 std::vector<GlobalValue*> WorkList;
85 inline bool RemoveIfDead(GlobalValue *GV);
86 void DestroyInitializer(Constant *C);
88 bool RemoveUnreachableGlobalVariables(Module &M);
89 bool RemoveUnusedConstantPointerRef(GlobalValue &GV);
90 bool SafeToDestroyConstant(Constant *C);
92 RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
95 Pass *createGlobalDCEPass() { return new GlobalDCE(); }
98 // RemoveIfDead - If this global value is dead, remove it from the current
99 // module and return true.
101 bool GlobalDCE::RemoveIfDead(GlobalValue *GV) {
102 // If there is only one use of the global value, it might be a
103 // ConstantPointerRef... which means that this global might actually be
105 if (GV->use_size() == 1)
106 RemoveUnusedConstantPointerRef(*GV);
108 if (!GV->use_empty()) return false;
110 if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV)) {
111 // Eliminate all global variables that are unused, and that are internal, or
112 // do not have an initializer.
114 if (GVar->hasInternalLinkage() || GVar->isExternal()) {
115 Constant *Init = GVar->hasInitializer() ? GVar->getInitializer() : 0;
116 GV->getParent()->getGlobalList().erase(GVar);
119 // If there was an initializer for the global variable, try to destroy it
121 if (Init) DestroyInitializer(Init);
123 // If the global variable is still on the worklist, remove it now.
124 std::vector<GlobalValue*>::iterator I = std::find(WorkList.begin(),
126 while (I != WorkList.end()) {
127 I = WorkList.erase(I);
128 I = std::find(I, WorkList.end(), GV);
134 Function *F = cast<Function>(GV);
141 // DestroyInitializer - A global variable was just destroyed and C is its
142 // initializer. If we can, destroy C and all of the constants it refers to.
144 void GlobalDCE::DestroyInitializer(Constant *C) {
145 // Cannot destroy constants still being used, and cannot destroy primitive
147 if (!C->use_empty() || C->getType()->isPrimitiveType()) return;
149 // If this is a CPR, the global value referred to may be dead now! Add it to
152 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C)) {
153 WorkList.push_back(CPR->getValue());
154 C->destroyConstant();
157 bool DestroyContents = true;
159 // As an optimization to the GlobalDCE algorithm, do attempt to destroy the
160 // contents of an array of primitive types, because we know that this will
161 // never succeed, and there could be a lot of them.
163 if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
164 if (CA->getType()->getElementType()->isPrimitiveType())
165 DestroyContents = false; // Nothing we can do with the subcontents
167 // All other constants refer to other constants. Destroy them if possible
170 std::vector<Value*> SubConstants;
171 if (DestroyContents) SubConstants.insert(SubConstants.end(),
172 C->op_begin(), C->op_end());
174 // Destroy the actual constant...
175 C->destroyConstant();
178 if (DestroyContents) {
179 // Remove duplicates from SubConstants, so that we do not call
180 // DestroyInitializer on the same constant twice (the first call might
181 // delete it, so this would be bad)
183 std::sort(SubConstants.begin(), SubConstants.end());
184 SubConstants.erase(std::unique(SubConstants.begin(), SubConstants.end()),
187 // Loop over the subconstants, destroying them as well.
188 for (unsigned i = 0, e = SubConstants.size(); i != e; ++i)
189 DestroyInitializer(cast<Constant>(SubConstants[i]));
194 bool GlobalDCE::RemoveUnreachableGlobalVariables(Module &M) {
195 bool Changed = false;
196 WorkList.reserve(M.gsize());
198 // Insert all of the globals into the WorkList, making sure to run
199 // RemoveUnusedConstantPointerRef at least once on all globals...
201 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
202 Changed |= RemoveUnusedConstantPointerRef(*I);
203 WorkList.push_back(I);
205 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
206 Changed |= RemoveUnusedConstantPointerRef(*I);
207 WorkList.push_back(I);
210 // Loop over the worklist, deleting global objects that we can. Whenever we
211 // delete something that might make something else dead, it gets added to the
214 while (!WorkList.empty()) {
215 GlobalValue *GV = WorkList.back();
218 Changed |= RemoveIfDead(GV);
221 // Make sure that all memory is free'd from the worklist...
222 std::vector<GlobalValue*>().swap(WorkList);
227 // RemoveUnusedConstantPointerRef - Loop over all of the uses of the specified
228 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
229 // If found, check to see if the constant pointer ref is safe to destroy, and if
230 // so, nuke it. This will reduce the reference count on the global value, which
231 // might make it deader.
233 bool GlobalDCE::RemoveUnusedConstantPointerRef(GlobalValue &GV) {
234 for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
235 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*I))
236 if (SafeToDestroyConstant(CPR)) { // Only if unreferenced...
237 CPR->destroyConstant();
245 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
246 // by constants itself. Note that constants cannot be cyclic, so this test is
247 // pretty easy to implement recursively.
249 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
250 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
251 if (Constant *User = dyn_cast<Constant>(*I)) {
252 if (!SafeToDestroyConstant(User)) return false;