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 // Unreachables 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);
59 struct GlobalDCE : public Pass {
60 // run - Do the GlobalDCE pass on the specified module, optionally updating
61 // the specified callgraph to reflect the changes.
64 return RemoveUnreachableFunctions(M, getAnalysis<CallGraph>()) |
65 RemoveUnreachableGlobalVariables(M);
68 // getAnalysisUsage - This function works on the call graph of a module.
69 // It is capable of updating the call graph to reflect the new state of the
72 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
73 AU.addRequired<CallGraph>();
77 std::vector<GlobalValue*> WorkList;
79 inline bool RemoveIfDead(GlobalValue *GV);
80 void DestroyInitializer(Constant *C);
82 bool RemoveUnreachableGlobalVariables(Module &M);
83 bool RemoveUnusedConstantPointerRef(GlobalValue &GV);
84 bool SafeToDestroyConstant(Constant *C);
86 RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
89 Pass *createGlobalDCEPass() { return new GlobalDCE(); }
92 // RemoveIfDead - If this global value is dead, remove it from the current
93 // module and return true.
95 bool GlobalDCE::RemoveIfDead(GlobalValue *GV) {
96 // If there is only one use of the global value, it might be a
97 // ConstantPointerRef... which means that this global might actually be
99 if (GV->use_size() == 1)
100 RemoveUnusedConstantPointerRef(*GV);
102 if (!GV->use_empty()) return false;
104 if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV)) {
105 // Eliminate all global variables that are unused, and that are internal, or
106 // do not have an initializer.
108 if (GVar->hasInternalLinkage() || GVar->isExternal()) {
109 Constant *Init = GVar->hasInitializer() ? GVar->getInitializer() : 0;
110 GV->getParent()->getGlobalList().erase(GVar);
113 // If there was an initializer for the global variable, try to destroy it
115 if (Init) DestroyInitializer(Init);
117 // If the global variable is still on the worklist, remove it now.
118 std::vector<GlobalValue*>::iterator I = std::find(WorkList.begin(),
120 while (I != WorkList.end())
121 I = std::find(WorkList.erase(I), WorkList.end(), GV);
126 Function *F = cast<Function>(GV);
133 // DestroyInitializer - A global variable was just destroyed and C is its
134 // initializer. If we can, destroy C and all of the constants it refers to.
136 void GlobalDCE::DestroyInitializer(Constant *C) {
137 // Cannot destroy constants still being used, and cannot destroy primitive
139 if (!C->use_empty() || C->getType()->isPrimitiveType()) return;
141 // If this is a CPR, the global value referred to may be dead now! Add it to
144 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C)) {
145 WorkList.push_back(CPR->getValue());
146 C->destroyConstant();
149 bool DestroyContents = true;
151 // As an optimization to the GlobalDCE algorithm, do attempt to destroy the
152 // contents of an array of primitive types, because we know that this will
153 // never succeed, and there could be a lot of them.
155 if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
156 if (CA->getType()->getElementType()->isPrimitiveType())
157 DestroyContents = false; // Nothing we can do with the subcontents
159 // All other constants refer to other constants. Destroy them if possible
162 std::vector<Value*> SubConstants;
163 if (DestroyContents) SubConstants.insert(SubConstants.end(),
164 C->op_begin(), C->op_end());
166 // Destroy the actual constant...
167 C->destroyConstant();
170 if (DestroyContents) {
171 // Remove duplicates from SubConstants, so that we do not call
172 // DestroyInitializer on the same constant twice (the first call might
173 // delete it, so this would be bad)
175 std::sort(SubConstants.begin(), SubConstants.end());
176 SubConstants.erase(std::unique(SubConstants.begin(), SubConstants.end()),
179 // Loop over the subconstants, destroying them as well.
180 for (unsigned i = 0, e = SubConstants.size(); i != e; ++i)
181 DestroyInitializer(cast<Constant>(SubConstants[i]));
186 bool GlobalDCE::RemoveUnreachableGlobalVariables(Module &M) {
187 bool Changed = false;
188 WorkList.reserve(M.gsize());
190 // Insert all of the globals into the WorkList, making sure to run
191 // RemoveUnusedConstantPointerRef at least once on all globals...
193 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
194 Changed |= RemoveUnusedConstantPointerRef(*I);
195 WorkList.push_back(I);
197 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
198 Changed |= RemoveUnusedConstantPointerRef(*I);
199 WorkList.push_back(I);
202 // Loop over the worklist, deleting global objects that we can. Whenever we
203 // delete something that might make something else dead, it gets added to the
206 while (!WorkList.empty()) {
207 GlobalValue *GV = WorkList.back();
210 Changed |= RemoveIfDead(GV);
213 // Make sure that all memory is free'd from the worklist...
214 std::vector<GlobalValue*>().swap(WorkList);
219 // RemoveUnusedConstantPointerRef - Loop over all of the uses of the specified
220 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
221 // If found, check to see if the constant pointer ref is safe to destroy, and if
222 // so, nuke it. This will reduce the reference count on the global value, which
223 // might make it deader.
225 bool GlobalDCE::RemoveUnusedConstantPointerRef(GlobalValue &GV) {
226 for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
227 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*I))
228 if (SafeToDestroyConstant(CPR)) { // Only if unreferenced...
229 CPR->destroyConstant();
237 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
238 // by constants itself. Note that constants cannot be cyclic, so this test is
239 // pretty easy to implement recursively.
241 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
242 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
243 if (Constant *User = dyn_cast<Constant>(*I)) {
244 if (!SafeToDestroyConstant(User)) return false;