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[WebAssembly] Implement @llvm.global_ctors and @llvm.global_dtors
Summary: - lowers @llvm.global_dtors by adding @llvm.global_ctors functions which register the destructors with `__cxa_atexit`. - impements @llvm.global_ctors with wasm start functions and linker metadata See [here](WebAssembly/tool-conventions#25) for more background. Subscribers: jfb, dschuff, mgorny, jgravelle-google, aheejin, sunfish Differential Revision: https://reviews.llvm.org/D41211 llvm-svn: 320774
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llvm/lib/Target/WebAssembly/WebAssemblyLowerGlobalDtors.cpp
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| //===-- WebAssemblyLowerGlobalDtors.cpp - Lower @llvm.global_dtors --------===// | ||
| // | ||
| // The LLVM Compiler Infrastructure | ||
| // | ||
| // This file is distributed under the University of Illinois Open Source | ||
| // License. See LICENSE.TXT for details. | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
| /// | ||
| /// \file | ||
| /// \brief Lower @llvm.global_dtors. | ||
| /// | ||
| /// WebAssembly doesn't have a builtin way to invoke static destructors. | ||
| /// Implement @llvm.global_dtors by creating wrapper functions that are | ||
| /// registered in @llvm.global_ctors and which contain a call to | ||
| /// `__cxa_atexit` to register their destructor functions. | ||
| /// | ||
| //===----------------------------------------------------------------------===// | ||
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| #include "WebAssembly.h" | ||
| #include "llvm/IR/Constants.h" | ||
| #include "llvm/IR/Instructions.h" | ||
| #include "llvm/IR/Intrinsics.h" | ||
| #include "llvm/IR/Module.h" | ||
| #include "llvm/Transforms/Utils/ModuleUtils.h" | ||
| #include "llvm/Pass.h" | ||
| #include "llvm/ADT/MapVector.h" | ||
| #include "llvm/Support/Debug.h" | ||
| #include "llvm/Support/raw_ostream.h" | ||
| using namespace llvm; | ||
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| #define DEBUG_TYPE "wasm-lower-global-dtors" | ||
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| namespace { | ||
| class LowerGlobalDtors final : public ModulePass { | ||
| StringRef getPassName() const override { | ||
| return "WebAssembly Lower @llvm.global_dtors"; | ||
| } | ||
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| void getAnalysisUsage(AnalysisUsage &AU) const override { | ||
| AU.setPreservesCFG(); | ||
| ModulePass::getAnalysisUsage(AU); | ||
| } | ||
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| bool runOnModule(Module &M) override; | ||
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| public: | ||
| static char ID; | ||
| LowerGlobalDtors() : ModulePass(ID) {} | ||
| }; | ||
| } // End anonymous namespace | ||
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| char LowerGlobalDtors::ID = 0; | ||
| ModulePass *llvm::createWebAssemblyLowerGlobalDtors() { | ||
| return new LowerGlobalDtors(); | ||
| } | ||
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| bool LowerGlobalDtors::runOnModule(Module &M) { | ||
| GlobalVariable *GV = M.getGlobalVariable("llvm.global_dtors"); | ||
| if (!GV) | ||
| return false; | ||
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| const ConstantArray *InitList = dyn_cast<ConstantArray>(GV->getInitializer()); | ||
| if (!InitList) | ||
| return false; | ||
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| // Sanity-check @llvm.global_dtor's type. | ||
| StructType *ETy = dyn_cast<StructType>(InitList->getType()->getElementType()); | ||
| if (!ETy || ETy->getNumElements() != 3 || | ||
| !ETy->getTypeAtIndex(0U)->isIntegerTy() || | ||
| !ETy->getTypeAtIndex(1U)->isPointerTy() || | ||
| !ETy->getTypeAtIndex(2U)->isPointerTy()) | ||
| return false; // Not (int, ptr, ptr). | ||
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| // Collect the contents of @llvm.global_dtors, collated by priority and | ||
| // associated symbol. | ||
| std::map<uint16_t, MapVector<Constant *, std::vector<Constant *> > > DtorFuncs; | ||
| for (Value *O : InitList->operands()) { | ||
| ConstantStruct *CS = dyn_cast<ConstantStruct>(O); | ||
| if (!CS) continue; // Malformed. | ||
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| ConstantInt *Priority = dyn_cast<ConstantInt>(CS->getOperand(0)); | ||
| if (!Priority) continue; // Malformed. | ||
| uint16_t PriorityValue = Priority->getLimitedValue(UINT16_MAX); | ||
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| Constant *DtorFunc = CS->getOperand(1); | ||
| if (DtorFunc->isNullValue()) | ||
| break; // Found a null terminator, skip the rest. | ||
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| Constant *Associated = CS->getOperand(2); | ||
| Associated = cast<Constant>(Associated->stripPointerCastsNoFollowAliases()); | ||
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| DtorFuncs[PriorityValue][Associated].push_back(DtorFunc); | ||
| } | ||
| if (DtorFuncs.empty()) | ||
| return false; | ||
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| // extern "C" int __cxa_atexit(void (*f)(void *), void *p, void *d); | ||
| LLVMContext &C = M.getContext(); | ||
| PointerType *VoidStar = Type::getInt8PtrTy(C); | ||
| Type *AtExitFuncArgs[] = { VoidStar }; | ||
| FunctionType *AtExitFuncTy = FunctionType::get( | ||
| Type::getVoidTy(C), | ||
| AtExitFuncArgs, | ||
| /*isVarArg=*/false); | ||
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| Type *AtExitArgs[] = { | ||
| PointerType::get(AtExitFuncTy, 0), | ||
| VoidStar, | ||
| VoidStar | ||
| }; | ||
| FunctionType *AtExitTy = FunctionType::get( | ||
| Type::getInt32Ty(C), | ||
| AtExitArgs, | ||
| /*isVarArg=*/false); | ||
| Constant *AtExit = M.getOrInsertFunction("__cxa_atexit", AtExitTy); | ||
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| // Declare __dso_local. | ||
| Constant *DsoHandle = M.getNamedValue("__dso_handle"); | ||
| if (!DsoHandle) { | ||
| Type *DsoHandleTy = Type::getInt8Ty(C); | ||
| GlobalVariable *Handle = | ||
| new GlobalVariable(M, DsoHandleTy, /*isConstant=*/true, | ||
| GlobalVariable::ExternalWeakLinkage, | ||
| nullptr, "__dso_handle"); | ||
| Handle->setVisibility(GlobalVariable::HiddenVisibility); | ||
| DsoHandle = Handle; | ||
| } | ||
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| // For each unique priority level and associated symbol, generate a function | ||
| // to call all the destructors at that level, and a function to register the | ||
| // first function with __cxa_atexit. | ||
| for (auto &PriorityAndMore : DtorFuncs) { | ||
| uint16_t Priority = PriorityAndMore.first; | ||
| for (auto &AssociatedAndMore : PriorityAndMore.second) { | ||
| Constant *Associated = AssociatedAndMore.first; | ||
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| Function *CallDtors = Function::Create( | ||
| AtExitFuncTy, Function::PrivateLinkage, | ||
| "call_dtors" + | ||
| (Priority != UINT16_MAX ? | ||
| (Twine(".") + Twine(Priority)) : Twine()) + | ||
| (!Associated->isNullValue() ? | ||
| (Twine(".") + Associated->getName()) : Twine()), | ||
| &M); | ||
| BasicBlock *BB = BasicBlock::Create(C, "body", CallDtors); | ||
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| for (auto Dtor : AssociatedAndMore.second) | ||
| CallInst::Create(Dtor, "", BB); | ||
| ReturnInst::Create(C, BB); | ||
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| FunctionType *VoidVoid = FunctionType::get(Type::getVoidTy(C), | ||
| /*isVarArg=*/false); | ||
| Function *RegisterCallDtors = Function::Create( | ||
| VoidVoid, Function::PrivateLinkage, | ||
| "register_call_dtors" + | ||
| (Priority != UINT16_MAX ? | ||
| (Twine(".") + Twine(Priority)) : Twine()) + | ||
| (!Associated->isNullValue() ? | ||
| (Twine(".") + Associated->getName()) : Twine()), | ||
| &M); | ||
| BasicBlock *EntryBB = BasicBlock::Create(C, "entry", RegisterCallDtors); | ||
| BasicBlock *FailBB = BasicBlock::Create(C, "fail", RegisterCallDtors); | ||
| BasicBlock *RetBB = BasicBlock::Create(C, "return", RegisterCallDtors); | ||
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| Value *Null = ConstantPointerNull::get(VoidStar); | ||
| Value *Args[] = { CallDtors, Null, DsoHandle }; | ||
| Value *Res = CallInst::Create(AtExit, Args, "call", EntryBB); | ||
| Value *Cmp = new ICmpInst(*EntryBB, ICmpInst::ICMP_NE, Res, | ||
| Constant::getNullValue(Res->getType())); | ||
| BranchInst::Create(FailBB, RetBB, Cmp, EntryBB); | ||
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| // If `__cxa_atexit` hits out-of-memory, trap, so that we don't misbehave. | ||
| // This should be very rare, because if the process is running out of memory | ||
| // before main has even started, something is wrong. | ||
| CallInst::Create(Intrinsic::getDeclaration(&M, Intrinsic::trap), | ||
| "", FailBB); | ||
| new UnreachableInst(C, FailBB); | ||
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| ReturnInst::Create(C, RetBB); | ||
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| // Now register the registration function with @llvm.global_ctors. | ||
| appendToGlobalCtors(M, RegisterCallDtors, Priority, Associated); | ||
| } | ||
| } | ||
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| // Now that we've lowered everything, remove @llvm.global_dtors. | ||
| GV->eraseFromParent(); | ||
|
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| return true; | ||
| } |
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