lib/Target/WebAssembly/WebAssemblyLowerGlobalDtors.cpp

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