1//===-- CodeGen/AsmPrinter/WasmException.cpp - Wasm Exception Impl --------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file contains support for writing WebAssembly exception info into asm
10// files.
11//
12//===----------------------------------------------------------------------===//
13
14#include "WasmException.h"
15#include "llvm/IR/Mangler.h"
16#include "llvm/MC/MCContext.h"
17#include "llvm/MC/MCStreamer.h"
18using namespace llvm;
19
20void WasmException::endModule() {
21  // This is the symbol used in 'throw' and 'br_on_exn' instruction to denote
22  // this is a C++ exception. This symbol has to be emitted somewhere once in
23  // the module.  Check if the symbol has already been created, i.e., we have at
24  // least one 'throw' or 'br_on_exn' instruction in the module, and emit the
25  // symbol only if so.
26  SmallString<60> NameStr;
27  Mangler::getNameWithPrefix(NameStr, "__cpp_exception", Asm->getDataLayout());
28  if (Asm->OutContext.lookupSymbol(NameStr)) {
29    MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol("__cpp_exception");
30    Asm->OutStreamer->emitLabel(ExceptionSym);
31  }
32}
33
34void WasmException::markFunctionEnd() {
35  // Get rid of any dead landing pads.
36  if (!Asm->MF->getLandingPads().empty()) {
37    auto *NonConstMF = const_cast<MachineFunction *>(Asm->MF);
38    // Wasm does not set BeginLabel and EndLabel information for landing pads,
39    // so we should set the second argument false.
40    NonConstMF->tidyLandingPads(nullptr, /* TidyIfNoBeginLabels */ false);
41  }
42}
43
44void WasmException::endFunction(const MachineFunction *MF) {
45  bool ShouldEmitExceptionTable = false;
46  for (const LandingPadInfo &Info : MF->getLandingPads()) {
47    if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) {
48      ShouldEmitExceptionTable = true;
49      break;
50    }
51  }
52  if (!ShouldEmitExceptionTable)
53    return;
54  MCSymbol *LSDALabel = emitExceptionTable();
55  assert(LSDALabel && ".GCC_exception_table has not been emitted!");
56
57  // Wasm requires every data section symbol to have a .size set. So we emit an
58  // end marker and set the size as the difference between the start end the end
59  // marker.
60  MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end");
61  Asm->OutStreamer->emitLabel(LSDAEndLabel);
62  MCContext &OutContext = Asm->OutStreamer->getContext();
63  const MCExpr *SizeExp = MCBinaryExpr::createSub(
64      MCSymbolRefExpr::create(LSDAEndLabel, OutContext),
65      MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext);
66  Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp);
67}
68
69// Compute the call-site table for wasm EH. Even though we use the same function
70// name to share the common routines, a call site entry in the table corresponds
71// to not a call site for possibly-throwing functions but a landing pad. In wasm
72// EH the VM is responsible for stack unwinding. After an exception occurs and
73// the stack is unwound, the control flow is transferred to wasm 'catch'
74// instruction by the VM, after which the personality function is called from
75// the compiler-generated code. Refer to WasmEHPrepare pass for more
76// information.
77void WasmException::computeCallSiteTable(
78    SmallVectorImpl<CallSiteEntry> &CallSites,
79    const SmallVectorImpl<const LandingPadInfo *> &LandingPads,
80    const SmallVectorImpl<unsigned> &FirstActions) {
81  MachineFunction &MF = *Asm->MF;
82  for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) {
83    const LandingPadInfo *Info = LandingPads[I];
84    MachineBasicBlock *LPad = Info->LandingPadBlock;
85    // We don't emit LSDA for single catch (...).
86    if (!MF.hasWasmLandingPadIndex(LPad))
87      continue;
88    // Wasm EH must maintain the EH pads in the order assigned to them by the
89    // WasmEHPrepare pass.
90    unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad);
91    CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]};
92    if (CallSites.size() < LPadIndex + 1)
93      CallSites.resize(LPadIndex + 1);
94    CallSites[LPadIndex] = Site;
95  }
96}
97