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//===-- CodeGen/AsmPrinter/WasmException.cpp - Wasm Exception Impl --------===//
//
// 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 file contains support for writing WebAssembly exception info into asm
// files.
//
//===----------------------------------------------------------------------===//
#include "WasmException.h"
#include "llvm/IR/Mangler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCStreamer.h"
using namespace llvm;
void WasmException::endModule() {
// These are symbols used to throw/catch C++ exceptions and C longjmps. These
// symbols have to be emitted somewhere once in the module. Check if each of
// the symbols has already been created, i.e., we have at least one 'throw' or
// 'catch' instruction with the symbol in the module, and emit the symbol only
// if so.
//
// But in dynamic linking, it is in general not possible to come up with a
// module instantiating order in which tag-defining modules are loaded before
// the importing modules. So we make them undefined symbols here, define tags
// in the JS side, and feed them to each importing module.
if (!Asm->isPositionIndependent()) {
for (const char *SymName : {"__cpp_exception", "__c_longjmp"}) {
SmallString<60> NameStr;
Mangler::getNameWithPrefix(NameStr, SymName, Asm->getDataLayout());
if (Asm->OutContext.lookupSymbol(NameStr)) {
MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(SymName);
Asm->OutStreamer->emitLabel(ExceptionSym);
}
}
}
}
void WasmException::markFunctionEnd() {
// Get rid of any dead landing pads.
if (!Asm->MF->getLandingPads().empty()) {
auto *NonConstMF = const_cast<MachineFunction *>(Asm->MF);
// Wasm does not set BeginLabel and EndLabel information for landing pads,
// so we should set the second argument false.
NonConstMF->tidyLandingPads(nullptr, /* TidyIfNoBeginLabels */ false);
}
}
void WasmException::endFunction(const MachineFunction *MF) {
bool ShouldEmitExceptionTable = false;
for (const LandingPadInfo &Info : MF->getLandingPads()) {
if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) {
ShouldEmitExceptionTable = true;
break;
}
}
if (!ShouldEmitExceptionTable)
return;
MCSymbol *LSDALabel = emitExceptionTable();
assert(LSDALabel && ".GCC_exception_table has not been emitted!");
// Wasm requires every data section symbol to have a .size set. So we emit an
// end marker and set the size as the difference between the start end the end
// marker.
MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end");
Asm->OutStreamer->emitLabel(LSDAEndLabel);
MCContext &OutContext = Asm->OutStreamer->getContext();
const MCExpr *SizeExp = MCBinaryExpr::createSub(
MCSymbolRefExpr::create(LSDAEndLabel, OutContext),
MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext);
Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp);
}
// Compute the call-site table for wasm EH. Even though we use the same function
// name to share the common routines, a call site entry in the table corresponds
// to not a call site for possibly-throwing functions but a landing pad. In wasm
// EH the VM is responsible for stack unwinding. After an exception occurs and
// the stack is unwound, the control flow is transferred to wasm 'catch'
// instruction by the VM, after which the personality function is called from
// the compiler-generated code. Refer to WasmEHPrepare pass for more
// information.
void WasmException::computeCallSiteTable(
SmallVectorImpl<CallSiteEntry> &CallSites,
SmallVectorImpl<CallSiteRange> &CallSiteRanges,
const SmallVectorImpl<const LandingPadInfo *> &LandingPads,
const SmallVectorImpl<unsigned> &FirstActions) {
MachineFunction &MF = *Asm->MF;
for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) {
const LandingPadInfo *Info = LandingPads[I];
MachineBasicBlock *LPad = Info->LandingPadBlock;
// We don't emit LSDA for single catch (...).
if (!MF.hasWasmLandingPadIndex(LPad))
continue;
// Wasm EH must maintain the EH pads in the order assigned to them by the
// WasmEHPrepare pass.
unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad);
CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]};
if (CallSites.size() < LPadIndex + 1)
CallSites.resize(LPadIndex + 1);
CallSites[LPadIndex] = Site;
}
}
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