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 #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"
27 STATISTIC(NumFunctions, "Number of functions removed");
28 STATISTIC(NumVariables, "Number of global variables removed");
31 struct GlobalDCE : public ModulePass {
32 // run - Do the GlobalDCE pass on the specified module, optionally updating
33 // the specified callgraph to reflect the changes.
35 bool runOnModule(Module &M);
38 std::set<GlobalValue*> AliveGlobals;
40 /// MarkGlobalIsNeeded - the specific global value as needed, and
41 /// recursively mark anything that it uses as also needed.
42 void GlobalIsNeeded(GlobalValue *GV);
43 void MarkUsedGlobalsAsNeeded(Constant *C);
45 bool SafeToDestroyConstant(Constant* C);
46 bool RemoveUnusedGlobalValue(GlobalValue &GV);
48 RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
51 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
53 bool GlobalDCE::runOnModule(Module &M) {
55 // Loop over the module, adding globals which are obviously necessary.
56 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
57 Changed |= RemoveUnusedGlobalValue(*I);
58 // Functions with external linkage are needed if they have a body
59 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
64 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I) {
65 Changed |= RemoveUnusedGlobalValue(*I);
66 // Externally visible & appending globals are needed, if they have an
68 if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
74 // Now that all globals which are needed are in the AliveGlobals set, we loop
75 // through the program, deleting those which are not alive.
78 // The first pass is to drop initializers of global variables which are dead.
79 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
80 for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I)
81 if (!AliveGlobals.count(I)) {
82 DeadGlobalVars.push_back(I); // Keep track of dead globals
87 // The second pass drops the bodies of functions which are dead...
88 std::vector<Function*> DeadFunctions;
89 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
90 if (!AliveGlobals.count(I)) {
91 DeadFunctions.push_back(I); // Keep track of dead globals
96 if (!DeadFunctions.empty()) {
97 // Now that all interreferences have been dropped, delete the actual objects
99 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
100 RemoveUnusedGlobalValue(*DeadFunctions[i]);
101 M.getFunctionList().erase(DeadFunctions[i]);
103 NumFunctions += DeadFunctions.size();
107 if (!DeadGlobalVars.empty()) {
108 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
109 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
110 M.getGlobalList().erase(DeadGlobalVars[i]);
112 NumVariables += DeadGlobalVars.size();
116 // Make sure that all memory is released
117 AliveGlobals.clear();
121 /// MarkGlobalIsNeeded - the specific global value as needed, and
122 /// recursively mark anything that it uses as also needed.
123 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
124 std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G);
126 // If the global is already in the set, no need to reprocess it.
127 if (I != AliveGlobals.end() && *I == G) return;
129 // Otherwise insert it now, so we do not infinitely recurse
130 AliveGlobals.insert(I, G);
132 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
133 // If this is a global variable, we must make sure to add any global values
134 // referenced by the initializer to the alive set.
135 if (GV->hasInitializer())
136 MarkUsedGlobalsAsNeeded(GV->getInitializer());
138 // Otherwise this must be a function object. We have to scan the body of
139 // the function looking for constants and global values which are used as
140 // operands. Any operands of these types must be processed to ensure that
141 // any globals used will be marked as needed.
142 Function *F = cast<Function>(G);
143 // For all basic blocks...
144 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
145 // For all instructions...
146 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
147 // For all operands...
148 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
149 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
151 else if (Constant *C = dyn_cast<Constant>(*U))
152 MarkUsedGlobalsAsNeeded(C);
156 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
157 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
160 // Loop over all of the operands of the constant, adding any globals they
161 // use to the list of needed globals.
162 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
163 MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
167 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
168 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
169 // If found, check to see if the constant pointer ref is safe to destroy, and if
170 // so, nuke it. This will reduce the reference count on the global value, which
171 // might make it deader.
173 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
174 if (GV.use_empty()) return false;
175 GV.removeDeadConstantUsers();
176 return GV.use_empty();
179 // SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
180 // by constants itself. Note that constants cannot be cyclic, so this test is
181 // pretty easy to implement recursively.
183 bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
184 for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
185 if (Constant *User = dyn_cast<Constant>(*I)) {
186 if (!SafeToDestroyConstant(User)) return false;