1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This transform is designed to eliminate unreachable internal globals from the
11 // program. It uses an aggressive algorithm, searching out globals that are
12 // known to be alive. After it finds all of the globals which are needed, it
13 // deletes whatever is left over. This allows it to delete recursive chunks of
14 // the program which are unreachable.
16 //===----------------------------------------------------------------------===//
18 #include "llvm/Transforms/IPO.h"
19 #include "llvm/Constants.h"
20 #include "llvm/Module.h"
21 #include "llvm/Pass.h"
22 #include "llvm/ADT/Statistic.h"
27 Statistic<> NumFunctions("globaldce","Number of functions removed");
28 Statistic<> NumVariables("globaldce","Number of global variables removed");
30 struct GlobalDCE : public ModulePass {
31 // run - Do the GlobalDCE pass on the specified module, optionally updating
32 // the specified callgraph to reflect the changes.
34 bool runOnModule(Module &M);
37 std::set<GlobalValue*> AliveGlobals;
39 /// MarkGlobalIsNeeded - the specific global value as needed, and
40 /// recursively mark anything that it uses as also needed.
41 void GlobalIsNeeded(GlobalValue *GV);
42 void MarkUsedGlobalsAsNeeded(Constant *C);
44 bool SafeToDestroyConstant(Constant* C);
45 bool RemoveUnusedGlobalValue(GlobalValue &GV);
47 RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
50 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
52 bool GlobalDCE::runOnModule(Module &M) {
54 // Loop over the module, adding globals which are obviously necessary.
55 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
56 Changed |= RemoveUnusedGlobalValue(*I);
57 // Functions with external linkage are needed if they have a body
58 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
63 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I) {
64 Changed |= RemoveUnusedGlobalValue(*I);
65 // Externally visible & appending globals are needed, if they have an
67 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
73 // Now that all globals which are needed are in the AliveGlobals set, we loop
74 // through the program, deleting those which are not alive.
77 // The first pass is to drop initializers of global variables which are dead.
78 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
79 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I)
80 if (!AliveGlobals.count(I)) {
81 DeadGlobalVars.push_back(I); // Keep track of dead globals
86 // The second pass drops the bodies of functions which are dead...
87 std::vector<Function*> DeadFunctions;
88 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
89 if (!AliveGlobals.count(I)) {
90 DeadFunctions.push_back(I); // Keep track of dead globals
95 if (!DeadFunctions.empty()) {
96 // Now that all interreferences have been dropped, delete the actual objects
98 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
99 RemoveUnusedGlobalValue(*DeadFunctions[i]);
100 M.getFunctionList().erase(DeadFunctions[i]);
102 NumFunctions += DeadFunctions.size();
106 if (!DeadGlobalVars.empty()) {
107 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
108 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
109 M.getGlobalList().erase(DeadGlobalVars[i]);
111 NumVariables += DeadGlobalVars.size();
115 // Make sure that all memory is released
116 AliveGlobals.clear();
120 /// MarkGlobalIsNeeded - the specific global value as needed, and
121 /// recursively mark anything that it uses as also needed.
122 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
123 std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G);
125 // If the global is already in the set, no need to reprocess it.
126 if (I != AliveGlobals.end() && *I == G) return;
128 // Otherwise insert it now, so we do not infinitely recurse
129 AliveGlobals.insert(I, G);
131 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
132 // If this is a global variable, we must make sure to add any global values
133 // referenced by the initializer to the alive set.
134 if (GV->hasInitializer())
135 MarkUsedGlobalsAsNeeded(GV->getInitializer());
137 // Otherwise this must be a function object. We have to scan the body of
138 // the function looking for constants and global values which are used as
139 // operands. Any operands of these types must be processed to ensure that
140 // any globals used will be marked as needed.
141 Function *F = cast<Function>(G);
142 // For all basic blocks...
143 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
144 // For all instructions...
145 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
146 // For all operands...
147 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
148 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
150 else if (Constant *C = dyn_cast<Constant>(*U))
151 MarkUsedGlobalsAsNeeded(C);
155 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
156 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
159 // Loop over all of the operands of the constant, adding any globals they
160 // use to the list of needed globals.
161 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
162 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
166 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
167 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
168 // If found, check to see if the constant pointer ref is safe to destroy, and if
169 // so, nuke it. This will reduce the reference count on the global value, which
170 // might make it deader.
172 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
173 if (GV.use_empty()) return false;
174 GV.removeDeadConstantUsers();
175 return GV.use_empty();
178 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
179 // by constants itself. Note that constants cannot be cyclic, so this test is
180 // pretty easy to implement recursively.
182 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
183 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
184 if (Constant *User = dyn_cast<Constant>(*I)) {
185 if (!SafeToDestroyConstant(User)) return false;