1 //===-- WebAssemblyLowerGlobalDtors.cpp - Lower @llvm.global_dtors --------===//
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 /// \file
11 /// \brief Lower @llvm.global_dtors.
12 ///
13 /// WebAssembly doesn't have a builtin way to invoke static destructors.
14 /// Implement @llvm.global_dtors by creating wrapper functions that are
15 /// registered in @llvm.global_ctors and which contain a call to
16 /// `__cxa_atexit` to register their destructor functions.
17 ///
18 //===----------------------------------------------------------------------===//
19 
20 #include "WebAssembly.h"
21 #include "llvm/IR/Constants.h"
22 #include "llvm/IR/Instructions.h"
23 #include "llvm/IR/Intrinsics.h"
24 #include "llvm/IR/Module.h"
25 #include "llvm/Transforms/Utils/ModuleUtils.h"
26 #include "llvm/Pass.h"
27 #include "llvm/ADT/MapVector.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/raw_ostream.h"
30 using namespace llvm;
31 
32 #define DEBUG_TYPE "wasm-lower-global-dtors"
33 
34 namespace {
35 class LowerGlobalDtors final : public ModulePass {
36   StringRef getPassName() const override {
37     return "WebAssembly Lower @llvm.global_dtors";
38   }
39 
40   void getAnalysisUsage(AnalysisUsage &AU) const override {
41     AU.setPreservesCFG();
42     ModulePass::getAnalysisUsage(AU);
43   }
44 
45   bool runOnModule(Module &M) override;
46 
47 public:
48   static char ID;
49   LowerGlobalDtors() : ModulePass(ID) {}
50 };
51 } // End anonymous namespace
52 
53 char LowerGlobalDtors::ID = 0;
54 ModulePass *llvm::createWebAssemblyLowerGlobalDtors() {
55   return new LowerGlobalDtors();
56 }
57 
58 bool LowerGlobalDtors::runOnModule(Module &M) {
59   GlobalVariable *GV = M.getGlobalVariable("llvm.global_dtors");
60   if (!GV)
61     return false;
62 
63   const ConstantArray *InitList = dyn_cast<ConstantArray>(GV->getInitializer());
64   if (!InitList)
65     return false;
66 
67   // Sanity-check @llvm.global_dtor's type.
68   StructType *ETy = dyn_cast<StructType>(InitList->getType()->getElementType());
69   if (!ETy || ETy->getNumElements() != 3 ||
70       !ETy->getTypeAtIndex(0U)->isIntegerTy() ||
71       !ETy->getTypeAtIndex(1U)->isPointerTy() ||
72       !ETy->getTypeAtIndex(2U)->isPointerTy())
73     return false; // Not (int, ptr, ptr).
74 
75   // Collect the contents of @llvm.global_dtors, collated by priority and
76   // associated symbol.
77   std::map<uint16_t, MapVector<Constant *, std::vector<Constant *> > > DtorFuncs;
78   for (Value *O : InitList->operands()) {
79     ConstantStruct *CS = dyn_cast<ConstantStruct>(O);
80     if (!CS) continue; // Malformed.
81 
82     ConstantInt *Priority = dyn_cast<ConstantInt>(CS->getOperand(0));
83     if (!Priority) continue; // Malformed.
84     uint16_t PriorityValue = Priority->getLimitedValue(UINT16_MAX);
85 
86     Constant *DtorFunc = CS->getOperand(1);
87     if (DtorFunc->isNullValue())
88       break;  // Found a null terminator, skip the rest.
89 
90     Constant *Associated = CS->getOperand(2);
91     Associated = cast<Constant>(Associated->stripPointerCastsNoFollowAliases());
92 
93     DtorFuncs[PriorityValue][Associated].push_back(DtorFunc);
94   }
95   if (DtorFuncs.empty())
96     return false;
97 
98   // extern "C" int __cxa_atexit(void (*f)(void *), void *p, void *d);
99   LLVMContext &C = M.getContext();
100   PointerType *VoidStar = Type::getInt8PtrTy(C);
101   Type *AtExitFuncArgs[] = { VoidStar };
102   FunctionType *AtExitFuncTy = FunctionType::get(
103           Type::getVoidTy(C),
104           AtExitFuncArgs,
105           /*isVarArg=*/false);
106 
107   Type *AtExitArgs[] = {
108     PointerType::get(AtExitFuncTy, 0),
109     VoidStar,
110     VoidStar
111   };
112   FunctionType *AtExitTy = FunctionType::get(
113           Type::getInt32Ty(C),
114           AtExitArgs,
115           /*isVarArg=*/false);
116   Constant *AtExit = M.getOrInsertFunction("__cxa_atexit", AtExitTy);
117 
118   // Declare __dso_local.
119   Constant *DsoHandle = M.getNamedValue("__dso_handle");
120   if (!DsoHandle) {
121     Type *DsoHandleTy = Type::getInt8Ty(C);
122     GlobalVariable *Handle =
123         new GlobalVariable(M, DsoHandleTy, /*isConstant=*/true,
124                            GlobalVariable::ExternalWeakLinkage,
125                            nullptr, "__dso_handle");
126     Handle->setVisibility(GlobalVariable::HiddenVisibility);
127     DsoHandle = Handle;
128   }
129 
130   // For each unique priority level and associated symbol, generate a function
131   // to call all the destructors at that level, and a function to register the
132   // first function with __cxa_atexit.
133   for (auto &PriorityAndMore : DtorFuncs) {
134     uint16_t Priority = PriorityAndMore.first;
135     for (auto &AssociatedAndMore : PriorityAndMore.second) {
136       Constant *Associated = AssociatedAndMore.first;
137 
138       Function *CallDtors = Function::Create(
139               AtExitFuncTy, Function::PrivateLinkage,
140               "call_dtors" +
141               (Priority != UINT16_MAX ?
142                  (Twine(".") + Twine(Priority)) : Twine()) +
143               (!Associated->isNullValue() ?
144                  (Twine(".") + Associated->getName()) : Twine()),
145               &M);
146       BasicBlock *BB = BasicBlock::Create(C, "body", CallDtors);
147 
148       for (auto Dtor : AssociatedAndMore.second)
149         CallInst::Create(Dtor, "", BB);
150       ReturnInst::Create(C, BB);
151 
152       FunctionType *VoidVoid = FunctionType::get(Type::getVoidTy(C),
153                                                  /*isVarArg=*/false);
154       Function *RegisterCallDtors = Function::Create(
155               VoidVoid, Function::PrivateLinkage,
156               "register_call_dtors" +
157               (Priority != UINT16_MAX ?
158                  (Twine(".") + Twine(Priority)) : Twine()) +
159               (!Associated->isNullValue() ?
160                  (Twine(".") + Associated->getName()) : Twine()),
161               &M);
162       BasicBlock *EntryBB = BasicBlock::Create(C, "entry", RegisterCallDtors);
163       BasicBlock *FailBB = BasicBlock::Create(C, "fail", RegisterCallDtors);
164       BasicBlock *RetBB = BasicBlock::Create(C, "return", RegisterCallDtors);
165 
166       Value *Null = ConstantPointerNull::get(VoidStar);
167       Value *Args[] = { CallDtors, Null, DsoHandle };
168       Value *Res = CallInst::Create(AtExit, Args, "call", EntryBB);
169       Value *Cmp = new ICmpInst(*EntryBB, ICmpInst::ICMP_NE, Res,
170                                 Constant::getNullValue(Res->getType()));
171       BranchInst::Create(FailBB, RetBB, Cmp, EntryBB);
172 
173       // If `__cxa_atexit` hits out-of-memory, trap, so that we don't misbehave.
174       // This should be very rare, because if the process is running out of memory
175       // before main has even started, something is wrong.
176       CallInst::Create(Intrinsic::getDeclaration(&M, Intrinsic::trap),
177                        "", FailBB);
178       new UnreachableInst(C, FailBB);
179 
180       ReturnInst::Create(C, RetBB);
181 
182       // Now register the registration function with @llvm.global_ctors.
183       appendToGlobalCtors(M, RegisterCallDtors, Priority, Associated);
184     }
185   }
186 
187   // Now that we've lowered everything, remove @llvm.global_dtors.
188   GV->eraseFromParent();
189 
190   return true;
191 }
192