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/GlobalVariable.h"
13 #include "llvm/Analysis/CallGraph.h"
14 #include "Support/DepthFirstIterator.h"
15 #include "Support/StatisticReporter.h"
18 static Statistic<> NumFunctions("globaldce\t- Number of functions removed");
19 static Statistic<> NumVariables("globaldce\t- Number of global variables removed");
20 static Statistic<> NumCPRs("globaldce\t- Number of const pointer refs removed");
21 static Statistic<> NumConsts("globaldce\t- Number of init constants removed");
23 static bool RemoveUnreachableFunctions(Module &M, CallGraph &CallGraph) {
24 // Calculate which functions are reachable from the external functions in the
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 deleted.
32 // 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);
60 struct GlobalDCE : public Pass {
61 // run - Do the GlobalDCE pass on the specified module, optionally updating
62 // the specified callgraph to reflect the changes.
65 return RemoveUnreachableFunctions(M, getAnalysis<CallGraph>()) |
66 RemoveUnreachableGlobalVariables(M);
69 // getAnalysisUsage - This function works on the call graph of a module.
70 // It is capable of updating the call graph to reflect the new state of the
73 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
74 AU.addRequired<CallGraph>();
78 std::vector<GlobalValue*> WorkList;
80 inline bool RemoveIfDead(GlobalValue *GV);
81 void DestroyInitializer(Constant *C);
83 bool RemoveUnreachableGlobalVariables(Module &M);
84 bool RemoveUnusedConstantPointerRef(GlobalValue &GV);
85 bool SafeToDestroyConstant(Constant *C);
87 RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
90 Pass *createGlobalDCEPass() { return new GlobalDCE(); }
93 // RemoveIfDead - If this global value is dead, remove it from the current
94 // module and return true.
96 bool GlobalDCE::RemoveIfDead(GlobalValue *GV) {
97 // If there is only one use of the global value, it might be a
98 // ConstantPointerRef... which means that this global might actually be
100 if (GV->use_size() == 1)
101 RemoveUnusedConstantPointerRef(*GV);
103 if (!GV->use_empty()) return false;
105 if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV)) {
106 // Eliminate all global variables that are unused, and that are internal, or
107 // do not have an initializer.
109 if (!GVar->hasExternalLinkage() || !GVar->hasInitializer()) {
110 Constant *Init = GVar->hasInitializer() ? GVar->getInitializer() : 0;
111 GV->getParent()->getGlobalList().erase(GVar);
114 // If there was an initializer for the global variable, try to destroy it
116 if (Init) DestroyInitializer(Init);
118 // If the global variable is still on the worklist, remove it now.
119 std::vector<GlobalValue*>::iterator I = std::find(WorkList.begin(),
121 while (I != WorkList.end())
122 I = std::find(WorkList.erase(I), WorkList.end(), GV);
127 Function *F = cast<Function>(GV);
134 // DestroyInitializer - A global variable was just destroyed and C is its
135 // initializer. If we can, destroy C and all of the constants it refers to.
137 void GlobalDCE::DestroyInitializer(Constant *C) {
138 // Cannot destroy constants still being used, and cannot destroy primitive
140 if (!C->use_empty() || C->getType()->isPrimitiveType()) return;
142 // If this is a CPR, the global value referred to may be dead now! Add it to
145 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C)) {
146 WorkList.push_back(CPR->getValue());
147 C->destroyConstant();
150 bool DestroyContents = true;
152 // As an optimization to the GlobalDCE algorithm, do attempt to destroy the
153 // contents of an array of primitive types, because we know that this will
154 // never succeed, and there could be a lot of them.
156 if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
157 if (CA->getType()->getElementType()->isPrimitiveType())
158 DestroyContents = false; // Nothing we can do with the subcontents
160 // All other constants refer to other constants. Destroy them if possible
163 std::vector<Value*> SubConstants;
164 if (DestroyContents) SubConstants.insert(SubConstants.end(),
165 C->op_begin(), C->op_end());
167 // Destroy the actual constant...
168 C->destroyConstant();
171 if (DestroyContents) {
172 // Remove duplicates from SubConstants, so that we do not call
173 // DestroyInitializer on the same constant twice (the first call might
174 // delete it, so this would be bad)
176 std::sort(SubConstants.begin(), SubConstants.end());
177 SubConstants.erase(std::unique(SubConstants.begin(), SubConstants.end()),
180 // Loop over the subconstants, destroying them as well.
181 for (unsigned i = 0, e = SubConstants.size(); i != e; ++i)
182 DestroyInitializer(cast<Constant>(SubConstants[i]));
187 bool GlobalDCE::RemoveUnreachableGlobalVariables(Module &M) {
188 bool Changed = false;
189 WorkList.reserve(M.gsize());
191 // Insert all of the globals into the WorkList, making sure to run
192 // RemoveUnusedConstantPointerRef at least once on all globals...
194 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
195 Changed |= RemoveUnusedConstantPointerRef(*I);
196 WorkList.push_back(I);
198 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
199 Changed |= RemoveUnusedConstantPointerRef(*I);
200 WorkList.push_back(I);
203 // Loop over the worklist, deleting global objects that we can. Whenever we
204 // delete something that might make something else dead, it gets added to the
207 while (!WorkList.empty()) {
208 GlobalValue *GV = WorkList.back();
211 Changed |= RemoveIfDead(GV);
214 // Make sure that all memory is free'd from the worklist...
215 std::vector<GlobalValue*>().swap(WorkList);
220 // RemoveUnusedConstantPointerRef - Loop over all of the uses of the specified
221 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
222 // If found, check to see if the constant pointer ref is safe to destroy, and if
223 // so, nuke it. This will reduce the reference count on the global value, which
224 // might make it deader.
226 bool GlobalDCE::RemoveUnusedConstantPointerRef(GlobalValue &GV) {
227 for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
228 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*I))
229 if (SafeToDestroyConstant(CPR)) { // Only if unreferenced...
230 CPR->destroyConstant();
238 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
239 // by constants itself. Note that constants cannot be cyclic, so this test is
240 // pretty easy to implement recursively.
242 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
243 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
244 if (Constant *User = dyn_cast<Constant>(*I)) {
245 if (!SafeToDestroyConstant(User)) return false;