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//===-- WebAssemblyLowerGlobalDtors.cpp - Lower @llvm.global_dtors --------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// 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.
///
//===----------------------------------------------------------------------===//
#include "WebAssembly.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
using namespace llvm;
#define DEBUG_TYPE "wasm-lower-global-dtors"
namespace {
class LowerGlobalDtors final : public ModulePass {
StringRef getPassName() const override {
return "WebAssembly Lower @llvm.global_dtors";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
ModulePass::getAnalysisUsage(AU);
}
bool runOnModule(Module &M) override;
public:
static char ID;
LowerGlobalDtors() : ModulePass(ID) {}
};
} // End anonymous namespace
char LowerGlobalDtors::ID = 0;
INITIALIZE_PASS(LowerGlobalDtors, DEBUG_TYPE,
"Lower @llvm.global_dtors for WebAssembly", false, false)
ModulePass *llvm::createWebAssemblyLowerGlobalDtors() {
return new LowerGlobalDtors();
}
bool LowerGlobalDtors::runOnModule(Module &M) {
LLVM_DEBUG(dbgs() << "********** Lower Global Destructors **********\n");
GlobalVariable *GV = M.getGlobalVariable("llvm.global_dtors");
if (!GV || !GV->hasInitializer())
return false;
const ConstantArray *InitList = dyn_cast<ConstantArray>(GV->getInitializer());
if (!InitList)
return false;
// Sanity-check @llvm.global_dtor's type.
auto *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).
// 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()) {
auto *CS = dyn_cast<ConstantStruct>(O);
if (!CS)
continue; // Malformed.
auto *Priority = dyn_cast<ConstantInt>(CS->getOperand(0));
if (!Priority)
continue; // Malformed.
uint16_t PriorityValue = Priority->getLimitedValue(UINT16_MAX);
Constant *DtorFunc = CS->getOperand(1);
if (DtorFunc->isNullValue())
break; // Found a null terminator, skip the rest.
Constant *Associated = CS->getOperand(2);
Associated = cast<Constant>(Associated->stripPointerCastsNoFollowAliases());
DtorFuncs[PriorityValue][Associated].push_back(DtorFunc);
}
if (DtorFuncs.empty())
return false;
// 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);
FunctionCallee AtExit = M.getOrInsertFunction(
"__cxa_atexit",
FunctionType::get(Type::getInt32Ty(C),
{PointerType::get(AtExitFuncTy, 0), VoidStar, VoidStar},
/*isVarArg=*/false));
// 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;
}
// 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;
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);
FunctionType *VoidVoid = FunctionType::get(Type::getVoidTy(C),
/*isVarArg=*/false);
for (auto Dtor : AssociatedAndMore.second)
CallInst::Create(VoidVoid, Dtor, "", BB);
ReturnInst::Create(C, BB);
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);
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);
// 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);
ReturnInst::Create(C, RetBB);
// Now register the registration function with @llvm.global_ctors.
appendToGlobalCtors(M, RegisterCallDtors, Priority, Associated);
}
}
// Now that we've lowered everything, remove @llvm.global_dtors.
GV->eraseFromParent();
return true;
}
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