1 //===-- LowerGlobalDtors.cpp - Lower @llvm.global_dtors -------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 ///
9 /// \file
10 /// Lower @llvm.global_dtors.
11 ///
12 /// Implement @llvm.global_dtors by creating wrapper functions that are
13 /// registered in @llvm.global_ctors and which contain a call to
14 /// `__cxa_atexit` to register their destructor functions.
15 ///
16 //===----------------------------------------------------------------------===//
17 
18 #include "llvm/Transforms/Utils/LowerGlobalDtors.h"
19 
20 #include "llvm/IR/Constants.h"
21 #include "llvm/IR/Instructions.h"
22 #include "llvm/IR/Intrinsics.h"
23 #include "llvm/InitializePasses.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Transforms/Utils.h"
26 #include "llvm/Transforms/Utils/ModuleUtils.h"
27 #include <map>
28 
29 using namespace llvm;
30 
31 #define DEBUG_TYPE "lower-global-dtors"
32 
33 namespace {
34 class LowerGlobalDtorsLegacyPass final : public ModulePass {
35   StringRef getPassName() const override {
36     return "Lower @llvm.global_dtors via `__cxa_atexit`";
37   }
38 
39   void getAnalysisUsage(AnalysisUsage &AU) const override {
40     AU.setPreservesCFG();
41     ModulePass::getAnalysisUsage(AU);
42   }
43 
44   bool runOnModule(Module &M) override;
45 
46 public:
47   static char ID;
48   LowerGlobalDtorsLegacyPass() : ModulePass(ID) {
49     initializeLowerGlobalDtorsLegacyPassPass(*PassRegistry::getPassRegistry());
50   }
51 };
52 } // End anonymous namespace
53 
54 char LowerGlobalDtorsLegacyPass::ID = 0;
55 INITIALIZE_PASS(LowerGlobalDtorsLegacyPass, DEBUG_TYPE,
56                 "Lower @llvm.global_dtors via `__cxa_atexit`", false, false)
57 
58 ModulePass *llvm::createLowerGlobalDtorsLegacyPass() {
59   return new LowerGlobalDtorsLegacyPass();
60 }
61 
62 static bool runImpl(Module &M);
63 bool LowerGlobalDtorsLegacyPass::runOnModule(Module &M) { return runImpl(M); }
64 
65 PreservedAnalyses LowerGlobalDtorsPass::run(Module &M,
66                                             ModuleAnalysisManager &AM) {
67   bool Changed = runImpl(M);
68   if (!Changed)
69     return PreservedAnalyses::all();
70 
71   PreservedAnalyses PA;
72   PA.preserveSet<CFGAnalyses>();
73   return PA;
74 }
75 
76 static bool runImpl(Module &M) {
77   GlobalVariable *GV = M.getGlobalVariable("llvm.global_dtors");
78   if (!GV || !GV->hasInitializer())
79     return false;
80 
81   const ConstantArray *InitList = dyn_cast<ConstantArray>(GV->getInitializer());
82   if (!InitList)
83     return false;
84 
85   // Validate @llvm.global_dtor's type.
86   auto *ETy = dyn_cast<StructType>(InitList->getType()->getElementType());
87   if (!ETy || ETy->getNumElements() != 3 ||
88       !ETy->getTypeAtIndex(0U)->isIntegerTy() ||
89       !ETy->getTypeAtIndex(1U)->isPointerTy() ||
90       !ETy->getTypeAtIndex(2U)->isPointerTy())
91     return false; // Not (int, ptr, ptr).
92 
93   // Collect the contents of @llvm.global_dtors, ordered by priority. Within a
94   // priority, sequences of destructors with the same associated object are
95   // recorded so that we can register them as a group.
96   std::map<
97       uint16_t,
98       std::vector<std::pair<Constant *, std::vector<Constant *>>>
99   > DtorFuncs;
100   for (Value *O : InitList->operands()) {
101     auto *CS = dyn_cast<ConstantStruct>(O);
102     if (!CS)
103       continue; // Malformed.
104 
105     auto *Priority = dyn_cast<ConstantInt>(CS->getOperand(0));
106     if (!Priority)
107       continue; // Malformed.
108     uint16_t PriorityValue = Priority->getLimitedValue(UINT16_MAX);
109 
110     Constant *DtorFunc = CS->getOperand(1);
111     if (DtorFunc->isNullValue())
112       break; // Found a null terminator, skip the rest.
113 
114     Constant *Associated = CS->getOperand(2);
115     Associated = cast<Constant>(Associated->stripPointerCasts());
116 
117     auto &AtThisPriority = DtorFuncs[PriorityValue];
118     if (AtThisPriority.empty() || AtThisPriority.back().first != Associated) {
119         std::vector<Constant *> NewList;
120         NewList.push_back(DtorFunc);
121         AtThisPriority.push_back(std::make_pair(Associated, NewList));
122     } else {
123         AtThisPriority.back().second.push_back(DtorFunc);
124     }
125   }
126   if (DtorFuncs.empty())
127     return false;
128 
129   // extern "C" int __cxa_atexit(void (*f)(void *), void *p, void *d);
130   LLVMContext &C = M.getContext();
131   PointerType *VoidStar = Type::getInt8PtrTy(C);
132   Type *AtExitFuncArgs[] = {VoidStar};
133   FunctionType *AtExitFuncTy =
134       FunctionType::get(Type::getVoidTy(C), AtExitFuncArgs,
135                         /*isVarArg=*/false);
136 
137   FunctionCallee AtExit = M.getOrInsertFunction(
138       "__cxa_atexit",
139       FunctionType::get(Type::getInt32Ty(C),
140                         {PointerType::get(AtExitFuncTy, 0), VoidStar, VoidStar},
141                         /*isVarArg=*/false));
142 
143   // Declare __dso_local.
144   Constant *DsoHandle = M.getNamedValue("__dso_handle");
145   if (!DsoHandle) {
146     Type *DsoHandleTy = Type::getInt8Ty(C);
147     GlobalVariable *Handle = new GlobalVariable(
148         M, DsoHandleTy, /*isConstant=*/true,
149         GlobalVariable::ExternalWeakLinkage, nullptr, "__dso_handle");
150     Handle->setVisibility(GlobalVariable::HiddenVisibility);
151     DsoHandle = Handle;
152   }
153 
154   // For each unique priority level and associated symbol, generate a function
155   // to call all the destructors at that level, and a function to register the
156   // first function with __cxa_atexit.
157   for (auto &PriorityAndMore : DtorFuncs) {
158     uint16_t Priority = PriorityAndMore.first;
159     uint64_t Id = 0;
160     auto &AtThisPriority = PriorityAndMore.second;
161     for (auto &AssociatedAndMore : AtThisPriority) {
162       Constant *Associated = AssociatedAndMore.first;
163       auto ThisId = Id++;
164 
165       Function *CallDtors = Function::Create(
166           AtExitFuncTy, Function::PrivateLinkage,
167           "call_dtors" +
168               (Priority != UINT16_MAX ? (Twine(".") + Twine(Priority))
169                                       : Twine()) +
170               (AtThisPriority.size() > 1 ? Twine("$") + Twine(ThisId)
171                                          : Twine()) +
172               (!Associated->isNullValue() ? (Twine(".") + Associated->getName())
173                                           : Twine()),
174           &M);
175       BasicBlock *BB = BasicBlock::Create(C, "body", CallDtors);
176       FunctionType *VoidVoid = FunctionType::get(Type::getVoidTy(C),
177                                                  /*isVarArg=*/false);
178 
179       for (auto Dtor : reverse(AssociatedAndMore.second))
180         CallInst::Create(VoidVoid, Dtor, "", BB);
181       ReturnInst::Create(C, BB);
182 
183       Function *RegisterCallDtors = Function::Create(
184           VoidVoid, Function::PrivateLinkage,
185           "register_call_dtors" +
186               (Priority != UINT16_MAX ? (Twine(".") + Twine(Priority))
187                                       : Twine()) +
188               (AtThisPriority.size() > 1 ? Twine("$") + Twine(ThisId)
189                                          : Twine()) +
190               (!Associated->isNullValue() ? (Twine(".") + Associated->getName())
191                                           : Twine()),
192           &M);
193       BasicBlock *EntryBB = BasicBlock::Create(C, "entry", RegisterCallDtors);
194       BasicBlock *FailBB = BasicBlock::Create(C, "fail", RegisterCallDtors);
195       BasicBlock *RetBB = BasicBlock::Create(C, "return", RegisterCallDtors);
196 
197       Value *Null = ConstantPointerNull::get(VoidStar);
198       Value *Args[] = {CallDtors, Null, DsoHandle};
199       Value *Res = CallInst::Create(AtExit, Args, "call", EntryBB);
200       Value *Cmp = new ICmpInst(*EntryBB, ICmpInst::ICMP_NE, Res,
201                                 Constant::getNullValue(Res->getType()));
202       BranchInst::Create(FailBB, RetBB, Cmp, EntryBB);
203 
204       // If `__cxa_atexit` hits out-of-memory, trap, so that we don't misbehave.
205       // This should be very rare, because if the process is running out of
206       // memory before main has even started, something is wrong.
207       CallInst::Create(Intrinsic::getDeclaration(&M, Intrinsic::trap), "",
208                        FailBB);
209       new UnreachableInst(C, FailBB);
210 
211       ReturnInst::Create(C, RetBB);
212 
213       // Now register the registration function with @llvm.global_ctors.
214       appendToGlobalCtors(M, RegisterCallDtors, Priority, Associated);
215     }
216   }
217 
218   // Now that we've lowered everything, remove @llvm.global_dtors.
219   GV->eraseFromParent();
220 
221   return true;
222 }
223