blob: 53424556682d942583539bb8e7c3baeb95ffe082 [file] [log] [blame]
//===-- WasmEHPrepare - Prepare excepton handling for WebAssembly --------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See 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.landingpad_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/ADT/BreadthFirstIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Triple.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/CodeGen/WasmEHFuncInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/IntrinsicsWebAssembly.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.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 prepareEHPads(Function &F);
bool prepareThrows(Function &F);
bool IsEHPadFunctionsSetUp = false;
void setupEHPadFunctions(Function &F);
void prepareEHPad(BasicBlock *BB, bool NeedPersonality, bool NeedLSDA = false,
unsigned Index = 0);
static char ID; // Pass identification, replacement for typeid
WasmEHPrepare() : FunctionPass(ID) {}
void getAnalysisUsage(AnalysisUsage &AU) const override;
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;
"Prepare WebAssembly exceptions", false, false)
INITIALIZE_PASS_END(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
false, false)
FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
void WasmEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
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, DomTreeUpdater *DTU) {
SmallVector<BasicBlock *, 8> WL(BBs.begin(), BBs.end());
while (!WL.empty()) {
auto *BB = WL.pop_back_val();
if (!pred_empty(BB))
WL.append(succ_begin(BB), succ_end(BB));
DeleteDeadBlock(BB, DTU);
bool WasmEHPrepare::runOnFunction(Function &F) {
IsEHPadFunctionsSetUp = false;
bool Changed = false;
Changed |= prepareThrows(F);
Changed |= prepareEHPads(F);
return Changed;
bool WasmEHPrepare::prepareThrows(Function &F) {
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
DomTreeUpdater DTU(&DT, /*PostDominatorTree*/ nullptr,
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)
Changed = true;
auto *BB = ThrowI->getParent();
SmallVector<BasicBlock *, 4> Succs(successors(BB));
auto &InstList = BB->getInstList();
InstList.erase(std::next(BasicBlock::iterator(ThrowI)), InstList.end());
eraseDeadBBsAndChildren(Succs, &DTU);
return Changed;
bool WasmEHPrepare::prepareEHPads(Function &F) {
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
bool Changed = false;
// There are two things to decide: whether we need a personality function call
// and whether we need a `wasm.lsda()` call and its store.
// For the personality function call, catchpads with `catch (...)` and
// cleanuppads don't need it, because exceptions are always caught. Others all
// need it.
// For `wasm.lsda()` and its store, in order to minimize the number of them,
// we need a way to figure out whether we have encountered `wasm.lsda()` call
// in any of EH pads that dominates the current EH pad. To figure that out, we
// now visit EH pads in BFS order in the dominator tree so that we visit
// parent BBs first before visiting its child BBs in the domtree.
// We keep a set named `ExecutedLSDA`, which basically means "Do we have
// `wasm.lsda() either in the current EH pad or any of its parent EH pads in
// the dominator tree?". This is to prevent scanning the domtree up to the
// root every time we examine an EH pad, in the worst case: each EH pad only
// needs to check its immediate parent EH pad.
// - If any of its parent EH pads in the domtree has `wasm.lsda`, this means
// we don't need `wasm.lsda()` in the current EH pad. We also insert the
// current EH pad in `ExecutedLSDA` set.
// - If none of its parent EH pad has `wasm.lsda()`,
// - If the current EH pad is a `catch (...)` or a cleanuppad, done.
// - If the current EH pad is neither a `catch (...)` nor a cleanuppad,
// add `wasm.lsda()` and the store in the current EH pad, and add the
// current EH pad to `ExecutedLSDA` set.
// TODO Can we not store LSDA address in user function but make libcxxabi
// compute it?
DenseSet<Value *> ExecutedLSDA;
unsigned Index = 0;
for (auto DomNode : breadth_first(&DT)) {
auto *BB = DomNode->getBlock();
auto *Pad = BB->getFirstNonPHI();
if (!Pad || (!isa<CatchPadInst>(Pad) && !isa<CleanupPadInst>(Pad)))
Changed = true;
Value *ParentPad = nullptr;
if (CatchPadInst *CPI = dyn_cast<CatchPadInst>(Pad)) {
ParentPad = CPI->getCatchSwitch()->getParentPad();
if (ExecutedLSDA.count(ParentPad)) {
// We insert its associated catchswitch too, because
// FuncletPadInst::getParentPad() returns a CatchSwitchInst if the child
// FuncletPadInst is a CleanupPadInst.
} else { // CleanupPadInst
ParentPad = cast<CleanupPadInst>(Pad)->getParentPad();
if (ExecutedLSDA.count(ParentPad))
if (CatchPadInst *CPI = dyn_cast<CatchPadInst>(Pad)) {
if (CPI->getNumArgOperands() == 1 &&
// In case of a single catch (...), we need neither personality call nor
// wasm.lsda() call
prepareEHPad(BB, false);
else {
if (ExecutedLSDA.count(CPI))
// catch (type), but one of parents already has wasm.lsda() call
prepareEHPad(BB, true, false, Index++);
else {
// catch (type), and none of parents has wasm.lsda() call. We have to
// add the call in this EH pad, and record this EH pad in
// ExecutedLSDA.
prepareEHPad(BB, true, true, Index++);
} else if (isa<CleanupPadInst>(Pad)) {
// Cleanup pads need neither personality call nor wasm.lsda() call
prepareEHPad(BB, false);
return Changed;
void WasmEHPrepare::setupEHPadFunctions(Function &F) {
Module &M = *F.getParent();
IRBuilder<> IRB(F.getContext());
assert(F.hasPersonalityFn() && "Personality function not found");
// __wasm_lpad_context global variable
LPadContextGV = cast<GlobalVariable>(
M.getOrInsertGlobal("__wasm_lpad_context", LPadContextTy));
LPadIndexField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 0,
LSDAField =
IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 1, "lsda_gep");
SelectorField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 2,
// 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()))
// Prepare an EH pad for Wasm EH handling. If NeedPersonality is false, Index is
// ignored.
void WasmEHPrepare::prepareEHPad(BasicBlock *BB, bool NeedPersonality,
bool NeedLSDA, unsigned Index) {
if (!IsEHPadFunctionsSetUp) {
IsEHPadFunctionsSetUp = true;
assert(BB->isEHPad() && "BB is not an EHPad!");
IRBuilder<> IRB(BB->getContext());
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 w/o __clang_call_terminate call 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()");
// 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");
// 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!");
// 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);
if (NeedLSDA)
// 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));
// Pseudocode: int selector = __wasm.landingpad_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");
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())
const Instruction *Pad = BB.getFirstNonPHI();
if (const auto *CatchPad = dyn_cast<CatchPadInst>(Pad)) {
const auto *UnwindBB = CatchPad->getCatchSwitch()->getUnwindDest();
if (!UnwindBB)
const Instruction *UnwindPad = UnwindBB->getFirstNonPHI();
if (const auto *CatchSwitch = dyn_cast<CatchSwitchInst>(UnwindPad))
// Currently there should be only one handler per a catchswitch.
EHInfo.setEHPadUnwindDest(&BB, *CatchSwitch->handlers().begin());
else // cleanuppad
EHInfo.setEHPadUnwindDest(&BB, UnwindBB);