1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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 #define DEBUG_TYPE "globaldce"
19 #include "llvm/Transforms/IPO.h"
20 #include "llvm/Constants.h"
21 #include "llvm/Module.h"
22 #include "llvm/Pass.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/Support/Compiler.h"
28 STATISTIC(NumFunctions, "Number of functions removed");
29 STATISTIC(NumVariables, "Number of global variables removed");
32 struct VISIBILITY_HIDDEN GlobalDCE : public ModulePass {
33 static char ID; // Pass identification, replacement for typeid
34 GlobalDCE() : ModulePass((intptr_t)&ID) {}
36 // run - Do the GlobalDCE pass on the specified module, optionally updating
37 // the specified callgraph to reflect the changes.
39 bool runOnModule(Module &M);
42 std::set<GlobalValue*> AliveGlobals;
44 /// MarkGlobalIsNeeded - the specific global value as needed, and
45 /// recursively mark anything that it uses as also needed.
46 void GlobalIsNeeded(GlobalValue *GV);
47 void MarkUsedGlobalsAsNeeded(Constant *C);
49 bool SafeToDestroyConstant(Constant* C);
50 bool RemoveUnusedGlobalValue(GlobalValue &GV);
52 char GlobalDCE::ID = 0;
53 RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
56 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
58 bool GlobalDCE::runOnModule(Module &M) {
60 // Loop over the module, adding globals which are obviously necessary.
61 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
62 Changed |= RemoveUnusedGlobalValue(*I);
63 // Functions with external linkage are needed if they have a body
64 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
69 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
71 Changed |= RemoveUnusedGlobalValue(*I);
72 // Externally visible & appending globals are needed, if they have an
74 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
80 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
82 // Aliases are always needed even if they are not used.
83 MarkUsedGlobalsAsNeeded(I->getAliasee());
86 // Now that all globals which are needed are in the AliveGlobals set, we loop
87 // through the program, deleting those which are not alive.
90 // The first pass is to drop initializers of global variables which are dead.
91 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
92 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I)
93 if (!AliveGlobals.count(I)) {
94 DeadGlobalVars.push_back(I); // Keep track of dead globals
99 // The second pass drops the bodies of functions which are dead...
100 std::vector<Function*> DeadFunctions;
101 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
102 if (!AliveGlobals.count(I)) {
103 DeadFunctions.push_back(I); // Keep track of dead globals
104 if (!I->isDeclaration())
108 if (!DeadFunctions.empty()) {
109 // Now that all interreferences have been dropped, delete the actual objects
111 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
112 RemoveUnusedGlobalValue(*DeadFunctions[i]);
113 M.getFunctionList().erase(DeadFunctions[i]);
115 NumFunctions += DeadFunctions.size();
119 if (!DeadGlobalVars.empty()) {
120 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
121 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
122 M.getGlobalList().erase(DeadGlobalVars[i]);
124 NumVariables += DeadGlobalVars.size();
128 // Make sure that all memory is released
129 AliveGlobals.clear();
133 /// MarkGlobalIsNeeded - the specific global value as needed, and
134 /// recursively mark anything that it uses as also needed.
135 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
136 std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G);
138 // If the global is already in the set, no need to reprocess it.
139 if (I != AliveGlobals.end() && *I == G) return;
141 // Otherwise insert it now, so we do not infinitely recurse
142 AliveGlobals.insert(I, G);
144 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
145 // If this is a global variable, we must make sure to add any global values
146 // referenced by the initializer to the alive set.
147 if (GV->hasInitializer())
148 MarkUsedGlobalsAsNeeded(GV->getInitializer());
149 } else if (!isa<GlobalAlias>(G)) {
150 // Otherwise this must be a function object. We have to scan the body of
151 // the function looking for constants and global values which are used as
152 // operands. Any operands of these types must be processed to ensure that
153 // any globals used will be marked as needed.
154 Function *F = cast<Function>(G);
155 // For all basic blocks...
156 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
157 // For all instructions...
158 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
159 // For all operands...
160 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
161 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
163 else if (Constant *C = dyn_cast<Constant>(*U))
164 MarkUsedGlobalsAsNeeded(C);
168 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
169 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
172 // Loop over all of the operands of the constant, adding any globals they
173 // use to the list of needed globals.
174 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
175 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
179 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
180 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
181 // If found, check to see if the constant pointer ref is safe to destroy, and if
182 // so, nuke it. This will reduce the reference count on the global value, which
183 // might make it deader.
185 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
186 if (GV.use_empty()) return false;
187 GV.removeDeadConstantUsers();
188 return GV.use_empty();
191 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
192 // by constants itself. Note that constants cannot be cyclic, so this test is
193 // pretty easy to implement recursively.
195 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
196 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
197 if (Constant *User = dyn_cast<Constant>(*I)) {
198 if (!SafeToDestroyConstant(User)) return false;