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 #include "llvm/Transforms/IPO.h"
19 #include "llvm/ADT/SmallPtrSet.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/IR/Constants.h"
22 #include "llvm/IR/Instructions.h"
23 #include "llvm/IR/Module.h"
24 #include "llvm/Transforms/Utils/CtorUtils.h"
25 #include "llvm/Pass.h"
28 #define DEBUG_TYPE "globaldce"
30 STATISTIC(NumAliases , "Number of global aliases removed");
31 STATISTIC(NumFunctions, "Number of functions removed");
32 STATISTIC(NumVariables, "Number of global variables removed");
35 struct GlobalDCE : public ModulePass {
36 static char ID; // Pass identification, replacement for typeid
37 GlobalDCE() : ModulePass(ID) {
38 initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
41 // run - Do the GlobalDCE pass on the specified module, optionally updating
42 // the specified callgraph to reflect the changes.
44 bool runOnModule(Module &M) override;
47 SmallPtrSet<GlobalValue*, 32> AliveGlobals;
48 SmallPtrSet<Constant *, 8> SeenConstants;
50 /// GlobalIsNeeded - mark the specific global value as needed, and
51 /// recursively mark anything that it uses as also needed.
52 void GlobalIsNeeded(GlobalValue *GV);
53 void MarkUsedGlobalsAsNeeded(Constant *C);
55 bool RemoveUnusedGlobalValue(GlobalValue &GV);
58 /// Returns true if F contains only a single "ret" instruction.
59 bool isEmptyFunction(void *Context, Function *F) {
60 BasicBlock &Entry = F->getEntryBlock();
61 if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
63 ReturnInst &RI = cast<ReturnInst>(Entry.front());
64 return RI.getReturnValue() == NULL;
68 char GlobalDCE::ID = 0;
69 INITIALIZE_PASS(GlobalDCE, "globaldce",
70 "Dead Global Elimination", false, false)
72 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
74 bool GlobalDCE::runOnModule(Module &M) {
77 // Remove empty functions from the global ctors list.
78 Changed |= optimizeGlobalCtorsList(M, isEmptyFunction, nullptr);
80 // Loop over the module, adding globals which are obviously necessary.
81 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
82 Changed |= RemoveUnusedGlobalValue(*I);
83 // Functions with external linkage are needed if they have a body
84 if (!I->isDiscardableIfUnused() &&
85 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
89 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
91 Changed |= RemoveUnusedGlobalValue(*I);
92 // Externally visible & appending globals are needed, if they have an
94 if (!I->isDiscardableIfUnused() &&
95 !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
99 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
101 Changed |= RemoveUnusedGlobalValue(*I);
102 // Externally visible aliases are needed.
103 if (!I->isDiscardableIfUnused())
107 // Now that all globals which are needed are in the AliveGlobals set, we loop
108 // through the program, deleting those which are not alive.
111 // The first pass is to drop initializers of global variables which are dead.
112 std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
113 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
115 if (!AliveGlobals.count(I)) {
116 DeadGlobalVars.push_back(I); // Keep track of dead globals
117 I->setInitializer(nullptr);
120 // The second pass drops the bodies of functions which are dead...
121 std::vector<Function*> DeadFunctions;
122 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
123 if (!AliveGlobals.count(I)) {
124 DeadFunctions.push_back(I); // Keep track of dead globals
125 if (!I->isDeclaration())
129 // The third pass drops targets of aliases which are dead...
130 std::vector<GlobalAlias*> DeadAliases;
131 for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
133 if (!AliveGlobals.count(I)) {
134 DeadAliases.push_back(I);
135 I->setAliasee(nullptr);
138 if (!DeadFunctions.empty()) {
139 // Now that all interferences have been dropped, delete the actual objects
141 for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
142 RemoveUnusedGlobalValue(*DeadFunctions[i]);
143 M.getFunctionList().erase(DeadFunctions[i]);
145 NumFunctions += DeadFunctions.size();
149 if (!DeadGlobalVars.empty()) {
150 for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
151 RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
152 M.getGlobalList().erase(DeadGlobalVars[i]);
154 NumVariables += DeadGlobalVars.size();
158 // Now delete any dead aliases.
159 if (!DeadAliases.empty()) {
160 for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
161 RemoveUnusedGlobalValue(*DeadAliases[i]);
162 M.getAliasList().erase(DeadAliases[i]);
164 NumAliases += DeadAliases.size();
168 // Make sure that all memory is released
169 AliveGlobals.clear();
170 SeenConstants.clear();
175 /// GlobalIsNeeded - the specific global value as needed, and
176 /// recursively mark anything that it uses as also needed.
177 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
178 // If the global is already in the set, no need to reprocess it.
179 if (!AliveGlobals.insert(G))
182 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
183 // If this is a global variable, we must make sure to add any global values
184 // referenced by the initializer to the alive set.
185 if (GV->hasInitializer())
186 MarkUsedGlobalsAsNeeded(GV->getInitializer());
187 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
188 // The target of a global alias is needed.
189 MarkUsedGlobalsAsNeeded(GA->getAliasee());
191 // Otherwise this must be a function object. We have to scan the body of
192 // the function looking for constants and global values which are used as
193 // operands. Any operands of these types must be processed to ensure that
194 // any globals used will be marked as needed.
195 Function *F = cast<Function>(G);
197 if (F->hasPrefixData())
198 MarkUsedGlobalsAsNeeded(F->getPrefixData());
200 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
201 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
202 for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
203 if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
205 else if (Constant *C = dyn_cast<Constant>(*U))
206 MarkUsedGlobalsAsNeeded(C);
210 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
211 if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
212 return GlobalIsNeeded(GV);
214 // Loop over all of the operands of the constant, adding any globals they
215 // use to the list of needed globals.
216 for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
217 // If we've already processed this constant there's no need to do it again.
218 Constant *Op = dyn_cast<Constant>(*I);
219 if (Op && SeenConstants.insert(Op))
220 MarkUsedGlobalsAsNeeded(Op);
224 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
225 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
226 // If found, check to see if the constant pointer ref is safe to destroy, and if
227 // so, nuke it. This will reduce the reference count on the global value, which
228 // might make it deader.
230 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
231 if (GV.use_empty()) return false;
232 GV.removeDeadConstantUsers();
233 return GV.use_empty();