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//===-- LowerGlobalDtors.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.
///
/// 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 "llvm/Transforms/Utils/LowerGlobalDtors.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
#include <map>
using namespace llvm;
#define DEBUG_TYPE "lower-global-dtors"
namespace {
class LowerGlobalDtorsLegacyPass final : public ModulePass {
StringRef getPassName() const override {
return "Lower @llvm.global_dtors via `__cxa_atexit`";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
ModulePass::getAnalysisUsage(AU);
}
bool runOnModule(Module &M) override;
public:
static char ID;
LowerGlobalDtorsLegacyPass() : ModulePass(ID) {
initializeLowerGlobalDtorsLegacyPassPass(*PassRegistry::getPassRegistry());
}
};
} // End anonymous namespace
char LowerGlobalDtorsLegacyPass::ID = 0;
INITIALIZE_PASS(LowerGlobalDtorsLegacyPass, DEBUG_TYPE,
"Lower @llvm.global_dtors via `__cxa_atexit`", false, false)
ModulePass *llvm::createLowerGlobalDtorsLegacyPass() {
return new LowerGlobalDtorsLegacyPass();
}
static bool runImpl(Module &M);
bool LowerGlobalDtorsLegacyPass::runOnModule(Module &M) { return runImpl(M); }
PreservedAnalyses LowerGlobalDtorsPass::run(Module &M,
ModuleAnalysisManager &AM) {
bool Changed = runImpl(M);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
return PA;
}
static bool runImpl(Module &M) {
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;
// Validate @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, ordered by priority. Within a
// priority, sequences of destructors with the same associated object are
// recorded so that we can register them as a group.
std::map<
uint16_t,
std::vector<std::pair<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->stripPointerCasts());
auto &AtThisPriority = DtorFuncs[PriorityValue];
if (AtThisPriority.empty() || AtThisPriority.back().first != Associated) {
std::vector<Constant *> NewList;
NewList.push_back(DtorFunc);
AtThisPriority.push_back(std::make_pair(Associated, NewList));
} else {
AtThisPriority.back().second.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.
Type *DsoHandleTy = Type::getInt8Ty(C);
Constant *DsoHandle = M.getOrInsertGlobal("__dso_handle", DsoHandleTy, [&] {
auto *GV = new GlobalVariable(M, DsoHandleTy, /*isConstant=*/true,
GlobalVariable::ExternalWeakLinkage, nullptr,
"__dso_handle");
GV->setVisibility(GlobalVariable::HiddenVisibility);
return GV;
});
// 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;
uint64_t Id = 0;
auto &AtThisPriority = PriorityAndMore.second;
for (auto &AssociatedAndMore : AtThisPriority) {
Constant *Associated = AssociatedAndMore.first;
auto ThisId = Id++;
Function *CallDtors = Function::Create(
AtExitFuncTy, Function::PrivateLinkage,
"call_dtors" +
(Priority != UINT16_MAX ? (Twine(".") + Twine(Priority))
: Twine()) +
(AtThisPriority.size() > 1 ? Twine("$") + Twine(ThisId)
: Twine()) +
(!Associated->isNullValue() ? (Twine(".") + Associated->getName())
: Twine()),
&M);
BasicBlock *BB = BasicBlock::Create(C, "body", CallDtors);
FunctionType *VoidVoid = FunctionType::get(Type::getVoidTy(C),
/*isVarArg=*/false);
for (auto *Dtor : reverse(AssociatedAndMore.second))
CallInst::Create(VoidVoid, Dtor, "", BB);
ReturnInst::Create(C, BB);
Function *RegisterCallDtors = Function::Create(
VoidVoid, Function::PrivateLinkage,
"register_call_dtors" +
(Priority != UINT16_MAX ? (Twine(".") + Twine(Priority))
: Twine()) +
(AtThisPriority.size() > 1 ? Twine("$") + Twine(ThisId)
: 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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