1 //===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
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.
15 //
16 //===----------------------------------------------------------------------===//
17 
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/Transforms/Utils/GlobalStatus.h"
26 #include "llvm/Pass.h"
27 #include <unordered_map>
28 using namespace llvm;
29 
30 #define DEBUG_TYPE "globaldce"
31 
32 STATISTIC(NumAliases  , "Number of global aliases removed");
33 STATISTIC(NumFunctions, "Number of functions removed");
34 STATISTIC(NumVariables, "Number of global variables removed");
35 
36 namespace {
37   struct GlobalDCE : public ModulePass {
38     static char ID; // Pass identification, replacement for typeid
GlobalDCE__anon26ae98040111::GlobalDCE39     GlobalDCE() : ModulePass(ID) {
40       initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
41     }
42 
43     // run - Do the GlobalDCE pass on the specified module, optionally updating
44     // the specified callgraph to reflect the changes.
45     //
46     bool runOnModule(Module &M) override;
47 
48   private:
49     SmallPtrSet<GlobalValue*, 32> AliveGlobals;
50     SmallPtrSet<Constant *, 8> SeenConstants;
51     std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
52 
53     /// GlobalIsNeeded - mark the specific global value as needed, and
54     /// recursively mark anything that it uses as also needed.
55     void GlobalIsNeeded(GlobalValue *GV);
56     void MarkUsedGlobalsAsNeeded(Constant *C);
57 
58     bool RemoveUnusedGlobalValue(GlobalValue &GV);
59   };
60 }
61 
62 /// Returns true if F contains only a single "ret" instruction.
isEmptyFunction(Function * F)63 static bool isEmptyFunction(Function *F) {
64   BasicBlock &Entry = F->getEntryBlock();
65   if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
66     return false;
67   ReturnInst &RI = cast<ReturnInst>(Entry.front());
68   return RI.getReturnValue() == nullptr;
69 }
70 
71 char GlobalDCE::ID = 0;
72 INITIALIZE_PASS(GlobalDCE, "globaldce",
73                 "Dead Global Elimination", false, false)
74 
createGlobalDCEPass()75 ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
76 
runOnModule(Module & M)77 bool GlobalDCE::runOnModule(Module &M) {
78   bool Changed = false;
79 
80   // Remove empty functions from the global ctors list.
81   Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
82 
83   // Collect the set of members for each comdat.
84   for (Function &F : M)
85     if (Comdat *C = F.getComdat())
86       ComdatMembers.insert(std::make_pair(C, &F));
87   for (GlobalVariable &GV : M.globals())
88     if (Comdat *C = GV.getComdat())
89       ComdatMembers.insert(std::make_pair(C, &GV));
90   for (GlobalAlias &GA : M.aliases())
91     if (Comdat *C = GA.getComdat())
92       ComdatMembers.insert(std::make_pair(C, &GA));
93 
94   // Loop over the module, adding globals which are obviously necessary.
95   for (Function &F : M) {
96     Changed |= RemoveUnusedGlobalValue(F);
97     // Functions with external linkage are needed if they have a body
98     if (!F.isDeclaration() && !F.hasAvailableExternallyLinkage())
99       if (!F.isDiscardableIfUnused())
100         GlobalIsNeeded(&F);
101   }
102 
103   for (GlobalVariable &GV : M.globals()) {
104     Changed |= RemoveUnusedGlobalValue(GV);
105     // Externally visible & appending globals are needed, if they have an
106     // initializer.
107     if (!GV.isDeclaration() && !GV.hasAvailableExternallyLinkage())
108       if (!GV.isDiscardableIfUnused())
109         GlobalIsNeeded(&GV);
110   }
111 
112   for (GlobalAlias &GA : M.aliases()) {
113     Changed |= RemoveUnusedGlobalValue(GA);
114     // Externally visible aliases are needed.
115     if (!GA.isDiscardableIfUnused())
116       GlobalIsNeeded(&GA);
117   }
118 
119   // Now that all globals which are needed are in the AliveGlobals set, we loop
120   // through the program, deleting those which are not alive.
121   //
122 
123   // The first pass is to drop initializers of global variables which are dead.
124   std::vector<GlobalVariable *> DeadGlobalVars; // Keep track of dead globals
125   for (GlobalVariable &GV : M.globals())
126     if (!AliveGlobals.count(&GV)) {
127       DeadGlobalVars.push_back(&GV);         // Keep track of dead globals
128       if (GV.hasInitializer()) {
129         Constant *Init = GV.getInitializer();
130         GV.setInitializer(nullptr);
131         if (isSafeToDestroyConstant(Init))
132           Init->destroyConstant();
133       }
134     }
135 
136   // The second pass drops the bodies of functions which are dead...
137   std::vector<Function *> DeadFunctions;
138   for (Function &F : M)
139     if (!AliveGlobals.count(&F)) {
140       DeadFunctions.push_back(&F);         // Keep track of dead globals
141       if (!F.isDeclaration())
142         F.deleteBody();
143     }
144 
145   // The third pass drops targets of aliases which are dead...
146   std::vector<GlobalAlias*> DeadAliases;
147   for (GlobalAlias &GA : M.aliases())
148     if (!AliveGlobals.count(&GA)) {
149       DeadAliases.push_back(&GA);
150       GA.setAliasee(nullptr);
151     }
152 
153   if (!DeadFunctions.empty()) {
154     // Now that all interferences have been dropped, delete the actual objects
155     // themselves.
156     for (Function *F : DeadFunctions) {
157       RemoveUnusedGlobalValue(*F);
158       M.getFunctionList().erase(F);
159     }
160     NumFunctions += DeadFunctions.size();
161     Changed = true;
162   }
163 
164   if (!DeadGlobalVars.empty()) {
165     for (GlobalVariable *GV : DeadGlobalVars) {
166       RemoveUnusedGlobalValue(*GV);
167       M.getGlobalList().erase(GV);
168     }
169     NumVariables += DeadGlobalVars.size();
170     Changed = true;
171   }
172 
173   // Now delete any dead aliases.
174   if (!DeadAliases.empty()) {
175     for (GlobalAlias *GA : DeadAliases) {
176       RemoveUnusedGlobalValue(*GA);
177       M.getAliasList().erase(GA);
178     }
179     NumAliases += DeadAliases.size();
180     Changed = true;
181   }
182 
183   // Make sure that all memory is released
184   AliveGlobals.clear();
185   SeenConstants.clear();
186   ComdatMembers.clear();
187 
188   return Changed;
189 }
190 
191 /// GlobalIsNeeded - the specific global value as needed, and
192 /// recursively mark anything that it uses as also needed.
GlobalIsNeeded(GlobalValue * G)193 void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
194   // If the global is already in the set, no need to reprocess it.
195   if (!AliveGlobals.insert(G).second)
196     return;
197 
198   if (Comdat *C = G->getComdat()) {
199     for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
200       GlobalIsNeeded(CM.second);
201   }
202 
203   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
204     // If this is a global variable, we must make sure to add any global values
205     // referenced by the initializer to the alive set.
206     if (GV->hasInitializer())
207       MarkUsedGlobalsAsNeeded(GV->getInitializer());
208   } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
209     // The target of a global alias is needed.
210     MarkUsedGlobalsAsNeeded(GA->getAliasee());
211   } else {
212     // Otherwise this must be a function object.  We have to scan the body of
213     // the function looking for constants and global values which are used as
214     // operands.  Any operands of these types must be processed to ensure that
215     // any globals used will be marked as needed.
216     Function *F = cast<Function>(G);
217 
218     for (Use &U : F->operands())
219       MarkUsedGlobalsAsNeeded(cast<Constant>(U.get()));
220 
221     for (BasicBlock &BB : *F)
222       for (Instruction &I : BB)
223         for (Use &U : I.operands())
224           if (GlobalValue *GV = dyn_cast<GlobalValue>(U))
225             GlobalIsNeeded(GV);
226           else if (Constant *C = dyn_cast<Constant>(U))
227             MarkUsedGlobalsAsNeeded(C);
228   }
229 }
230 
MarkUsedGlobalsAsNeeded(Constant * C)231 void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
232   if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
233     return GlobalIsNeeded(GV);
234 
235   // Loop over all of the operands of the constant, adding any globals they
236   // use to the list of needed globals.
237   for (Use &U : C->operands()) {
238     // If we've already processed this constant there's no need to do it again.
239     Constant *Op = dyn_cast<Constant>(U);
240     if (Op && SeenConstants.insert(Op).second)
241       MarkUsedGlobalsAsNeeded(Op);
242   }
243 }
244 
245 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
246 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
247 // If found, check to see if the constant pointer ref is safe to destroy, and if
248 // so, nuke it.  This will reduce the reference count on the global value, which
249 // might make it deader.
250 //
RemoveUnusedGlobalValue(GlobalValue & GV)251 bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
252   if (GV.use_empty())
253     return false;
254   GV.removeDeadConstantUsers();
255   return GV.use_empty();
256 }
257