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//===-- WasmEHPrepare - Prepare excepton handling for WebAssembly --------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This transformation is designed for use by code generators which use
// WebAssembly exception handling scheme. This currently supports C++
// exceptions.
//
// WebAssembly exception handling uses Windows exception IR for the middle level
// representation. This pass does the following transformation for every
// catchpad block:
// (In C-style pseudocode)
//
// - Before:
// catchpad ...
// exn = wasm.get.exception();
// selector = wasm.get.selector();
// ...
//
// - After:
// catchpad ...
// exn = wasm.catch(WebAssembly::CPP_EXCEPTION);
// // Only add below in case it's not a single catch (...)
// wasm.landingpad.index(index);
// __wasm_lpad_context.lpad_index = index;
// __wasm_lpad_context.lsda = wasm.lsda();
// _Unwind_CallPersonality(exn);
// selector = __wasm_lpad_context.selector;
// ...
//
//
// * Background: Direct personality function call
// In WebAssembly EH, the VM is responsible for unwinding the stack once an
// exception is thrown. After the stack is unwound, the control flow is
// transfered to WebAssembly 'catch' instruction.
//
// Unwinding the stack is not done by libunwind but the VM, so the personality
// function in libcxxabi cannot be called from libunwind during the unwinding
// process. So after a catch instruction, we insert a call to a wrapper function
// in libunwind that in turn calls the real personality function.
//
// In Itanium EH, if the personality function decides there is no matching catch
// clause in a call frame and no cleanup action to perform, the unwinder doesn't
// stop there and continues unwinding. But in Wasm EH, the unwinder stops at
// every call frame with a catch intruction, after which the personality
// function is called from the compiler-generated user code here.
//
// In libunwind, we have this struct that serves as a communincation channel
// between the compiler-generated user code and the personality function in
// libcxxabi.
//
// struct _Unwind_LandingPadContext {
// uintptr_t lpad_index;
// uintptr_t lsda;
// uintptr_t selector;
// };
// struct _Unwind_LandingPadContext __wasm_lpad_context = ...;
//
// And this wrapper in libunwind calls the personality function.
//
// _Unwind_Reason_Code _Unwind_CallPersonality(void *exception_ptr) {
// struct _Unwind_Exception *exception_obj =
// (struct _Unwind_Exception *)exception_ptr;
// _Unwind_Reason_Code ret = __gxx_personality_v0(
// 1, _UA_CLEANUP_PHASE, exception_obj->exception_class, exception_obj,
// (struct _Unwind_Context *)__wasm_lpad_context);
// return ret;
// }
//
// We pass a landing pad index, and the address of LSDA for the current function
// to the wrapper function _Unwind_CallPersonality in libunwind, and we retrieve
// the selector after it returns.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/WasmEHFuncInfo.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicsWebAssembly.h"
#include "llvm/InitializePasses.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
#define DEBUG_TYPE "wasmehprepare"
namespace {
class WasmEHPrepare : public FunctionPass {
Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
// Field addresses of struct _Unwind_LandingPadContext
Value *LPadIndexField = nullptr; // lpad_index field
Value *LSDAField = nullptr; // lsda field
Value *SelectorField = nullptr; // selector
Function *ThrowF = nullptr; // wasm.throw() intrinsic
Function *LPadIndexF = nullptr; // wasm.landingpad.index() intrinsic
Function *LSDAF = nullptr; // wasm.lsda() intrinsic
Function *GetExnF = nullptr; // wasm.get.exception() intrinsic
Function *CatchF = nullptr; // wasm.catch() intrinsic
Function *GetSelectorF = nullptr; // wasm.get.ehselector() intrinsic
FunctionCallee CallPersonalityF =
nullptr; // _Unwind_CallPersonality() wrapper
bool prepareThrows(Function &F);
bool prepareEHPads(Function &F);
void prepareEHPad(BasicBlock *BB, bool NeedPersonality, unsigned Index = 0);
public:
static char ID; // Pass identification, replacement for typeid
WasmEHPrepare() : FunctionPass(ID) {}
bool doInitialization(Module &M) override;
bool runOnFunction(Function &F) override;
StringRef getPassName() const override {
return "WebAssembly Exception handling preparation";
}
};
} // end anonymous namespace
char WasmEHPrepare::ID = 0;
INITIALIZE_PASS_BEGIN(WasmEHPrepare, DEBUG_TYPE,
"Prepare WebAssembly exceptions", false, false)
INITIALIZE_PASS_END(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
false, false)
FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
bool WasmEHPrepare::doInitialization(Module &M) {
IRBuilder<> IRB(M.getContext());
LPadContextTy = StructType::get(IRB.getInt32Ty(), // lpad_index
IRB.getInt8PtrTy(), // lsda
IRB.getInt32Ty() // selector
);
return false;
}
// Erase the specified BBs if the BB does not have any remaining predecessors,
// and also all its dead children.
template <typename Container>
static void eraseDeadBBsAndChildren(const Container &BBs) {
SmallVector<BasicBlock *, 8> WL(BBs.begin(), BBs.end());
while (!WL.empty()) {
auto *BB = WL.pop_back_val();
if (!pred_empty(BB))
continue;
WL.append(succ_begin(BB), succ_end(BB));
DeleteDeadBlock(BB);
}
}
bool WasmEHPrepare::runOnFunction(Function &F) {
bool Changed = false;
Changed |= prepareThrows(F);
Changed |= prepareEHPads(F);
return Changed;
}
bool WasmEHPrepare::prepareThrows(Function &F) {
Module &M = *F.getParent();
IRBuilder<> IRB(F.getContext());
bool Changed = false;
// wasm.throw() intinsic, which will be lowered to wasm 'throw' instruction.
ThrowF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_throw);
// Insert an unreachable instruction after a call to @llvm.wasm.throw and
// delete all following instructions within the BB, and delete all the dead
// children of the BB as well.
for (User *U : ThrowF->users()) {
// A call to @llvm.wasm.throw() is only generated from __cxa_throw()
// builtin call within libcxxabi, and cannot be an InvokeInst.
auto *ThrowI = cast<CallInst>(U);
if (ThrowI->getFunction() != &F)
continue;
Changed = true;
auto *BB = ThrowI->getParent();
SmallVector<BasicBlock *, 4> Succs(successors(BB));
BB->erase(std::next(BasicBlock::iterator(ThrowI)), BB->end());
IRB.SetInsertPoint(BB);
IRB.CreateUnreachable();
eraseDeadBBsAndChildren(Succs);
}
return Changed;
}
bool WasmEHPrepare::prepareEHPads(Function &F) {
Module &M = *F.getParent();
IRBuilder<> IRB(F.getContext());
SmallVector<BasicBlock *, 16> CatchPads;
SmallVector<BasicBlock *, 16> CleanupPads;
for (BasicBlock &BB : F) {
if (!BB.isEHPad())
continue;
auto *Pad = BB.getFirstNonPHI();
if (isa<CatchPadInst>(Pad))
CatchPads.push_back(&BB);
else if (isa<CleanupPadInst>(Pad))
CleanupPads.push_back(&BB);
}
if (CatchPads.empty() && CleanupPads.empty())
return false;
assert(F.hasPersonalityFn() && "Personality function not found");
// __wasm_lpad_context global variable.
// This variable should be thread local. If the target does not support TLS,
// we depend on CoalesceFeaturesAndStripAtomics to downgrade it to
// non-thread-local ones, in which case we don't allow this object to be
// linked with other objects using shared memory.
LPadContextGV = cast<GlobalVariable>(
M.getOrInsertGlobal("__wasm_lpad_context", LPadContextTy));
LPadContextGV->setThreadLocalMode(GlobalValue::GeneralDynamicTLSModel);
LPadIndexField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 0,
"lpad_index_gep");
LSDAField =
IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 1, "lsda_gep");
SelectorField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 2,
"selector_gep");
// wasm.landingpad.index() intrinsic, which is to specify landingpad index
LPadIndexF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_landingpad_index);
// wasm.lsda() intrinsic. Returns the address of LSDA table for the current
// function.
LSDAF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_lsda);
// wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
// are generated in clang.
GetExnF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_exception);
GetSelectorF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_ehselector);
// wasm.catch() will be lowered down to wasm 'catch' instruction in
// instruction selection.
CatchF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_catch);
// _Unwind_CallPersonality() wrapper function, which calls the personality
CallPersonalityF = M.getOrInsertFunction(
"_Unwind_CallPersonality", IRB.getInt32Ty(), IRB.getInt8PtrTy());
if (Function *F = dyn_cast<Function>(CallPersonalityF.getCallee()))
F->setDoesNotThrow();
unsigned Index = 0;
for (auto *BB : CatchPads) {
auto *CPI = cast<CatchPadInst>(BB->getFirstNonPHI());
// In case of a single catch (...), we don't need to emit a personalify
// function call
if (CPI->arg_size() == 1 &&
cast<Constant>(CPI->getArgOperand(0))->isNullValue())
prepareEHPad(BB, false);
else
prepareEHPad(BB, true, Index++);
}
// Cleanup pads don't need a personality function call.
for (auto *BB : CleanupPads)
prepareEHPad(BB, false);
return true;
}
// Prepare an EH pad for Wasm EH handling. If NeedPersonality is false, Index is
// ignored.
void WasmEHPrepare::prepareEHPad(BasicBlock *BB, bool NeedPersonality,
unsigned Index) {
assert(BB->isEHPad() && "BB is not an EHPad!");
IRBuilder<> IRB(BB->getContext());
IRB.SetInsertPoint(&*BB->getFirstInsertionPt());
auto *FPI = cast<FuncletPadInst>(BB->getFirstNonPHI());
Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
for (auto &U : FPI->uses()) {
if (auto *CI = dyn_cast<CallInst>(U.getUser())) {
if (CI->getCalledOperand() == GetExnF)
GetExnCI = CI;
if (CI->getCalledOperand() == GetSelectorF)
GetSelectorCI = CI;
}
}
// Cleanup pads do not have any of wasm.get.exception() or
// wasm.get.ehselector() calls. We need to do nothing.
if (!GetExnCI) {
assert(!GetSelectorCI &&
"wasm.get.ehselector() cannot exist w/o wasm.get.exception()");
return;
}
// Replace wasm.get.exception intrinsic with wasm.catch intrinsic, which will
// be lowered to wasm 'catch' instruction. We do this mainly because
// instruction selection cannot handle wasm.get.exception intrinsic's token
// argument.
Instruction *CatchCI =
IRB.CreateCall(CatchF, {IRB.getInt32(WebAssembly::CPP_EXCEPTION)}, "exn");
GetExnCI->replaceAllUsesWith(CatchCI);
GetExnCI->eraseFromParent();
// In case it is a catchpad with single catch (...) or a cleanuppad, we don't
// need to call personality function because we don't need a selector.
if (!NeedPersonality) {
if (GetSelectorCI) {
assert(GetSelectorCI->use_empty() &&
"wasm.get.ehselector() still has uses!");
GetSelectorCI->eraseFromParent();
}
return;
}
IRB.SetInsertPoint(CatchCI->getNextNode());
// This is to create a map of <landingpad EH label, landingpad index> in
// SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
// Pseudocode: wasm.landingpad.index(Index);
IRB.CreateCall(LPadIndexF, {FPI, IRB.getInt32(Index)});
// Pseudocode: __wasm_lpad_context.lpad_index = index;
IRB.CreateStore(IRB.getInt32(Index), LPadIndexField);
auto *CPI = cast<CatchPadInst>(FPI);
// TODO Sometimes storing the LSDA address every time is not necessary, in
// case it is already set in a dominating EH pad and there is no function call
// between from that EH pad to here. Consider optimizing those cases.
// Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
IRB.CreateStore(IRB.CreateCall(LSDAF), LSDAField);
// Pseudocode: _Unwind_CallPersonality(exn);
CallInst *PersCI = IRB.CreateCall(CallPersonalityF, CatchCI,
OperandBundleDef("funclet", CPI));
PersCI->setDoesNotThrow();
// Pseudocode: int selector = __wasm_lpad_context.selector;
Instruction *Selector =
IRB.CreateLoad(IRB.getInt32Ty(), SelectorField, "selector");
// Replace the return value from wasm.get.ehselector() with the selector value
// loaded from __wasm_lpad_context.selector.
assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
GetSelectorCI->replaceAllUsesWith(Selector);
GetSelectorCI->eraseFromParent();
}
void llvm::calculateWasmEHInfo(const Function *F, WasmEHFuncInfo &EHInfo) {
// If an exception is not caught by a catchpad (i.e., it is a foreign
// exception), it will unwind to its parent catchswitch's unwind destination.
// We don't record an unwind destination for cleanuppads because every
// exception should be caught by it.
for (const auto &BB : *F) {
if (!BB.isEHPad())
continue;
const Instruction *Pad = BB.getFirstNonPHI();
if (const auto *CatchPad = dyn_cast<CatchPadInst>(Pad)) {
const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
if (!UnwindBB)
continue;
const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
// Currently there should be only one handler per a catchswitch.
EHInfo.setUnwindDest(&BB, *CatchSwitch->handlers().begin());
else // cleanuppad
EHInfo.setUnwindDest(&BB, UnwindBB);
}
}
}