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/CodeGen/AsmPrinter.h"
16#include "llvm/CodeGen/MachineFunction.h"
17#include "llvm/IR/Mangler.h"
18#include "llvm/MC/MCContext.h"
19#include "llvm/MC/MCStreamer.h"
20using namespace llvm;
21
22void WasmException::endModule() {
23  // These are symbols used to throw/catch C++ exceptions and C longjmps. These
24  // symbols have to be emitted somewhere once in the module. Check if each of
25  // the symbols has already been created, i.e., we have at least one 'throw' or
26  // 'catch' instruction with the symbol in the module, and emit the symbol only
27  // if so.
28  //
29  // But in dynamic linking, it is in general not possible to come up with a
30  // module instantiating order in which tag-defining modules are loaded before
31  // the importing modules. So we make them undefined symbols here, define tags
32  // in the JS side, and feed them to each importing module.
33  if (!Asm->isPositionIndependent()) {
34    for (const char *SymName : {"__cpp_exception", "__c_longjmp"}) {
35      SmallString<60> NameStr;
36      Mangler::getNameWithPrefix(NameStr, SymName, Asm->getDataLayout());
37      if (Asm->OutContext.lookupSymbol(NameStr)) {
38        MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(SymName);
39        Asm->OutStreamer->emitLabel(ExceptionSym);
40      }
41    }
42  }
43}
44
45void WasmException::endFunction(const MachineFunction *MF) {
46  bool ShouldEmitExceptionTable = false;
47  for (const LandingPadInfo &Info : MF->getLandingPads()) {
48    if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) {
49      ShouldEmitExceptionTable = true;
50      break;
51    }
52  }
53  if (!ShouldEmitExceptionTable)
54    return;
55  MCSymbol *LSDALabel = emitExceptionTable();
56  assert(LSDALabel && ".GCC_exception_table has not been emitted!");
57
58  // Wasm requires every data section symbol to have a .size set. So we emit an
59  // end marker and set the size as the difference between the start end the end
60  // marker.
61  MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end");
62  Asm->OutStreamer->emitLabel(LSDAEndLabel);
63  MCContext &OutContext = Asm->OutStreamer->getContext();
64  const MCExpr *SizeExp = MCBinaryExpr::createSub(
65      MCSymbolRefExpr::create(LSDAEndLabel, OutContext),
66      MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext);
67  Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp);
68}
69
70// Compute the call-site table for wasm EH. Even though we use the same function
71// name to share the common routines, a call site entry in the table corresponds
72// to not a call site for possibly-throwing functions but a landing pad. In wasm
73// EH the VM is responsible for stack unwinding. After an exception occurs and
74// the stack is unwound, the control flow is transferred to wasm 'catch'
75// instruction by the VM, after which the personality function is called from
76// the compiler-generated code. Refer to WasmEHPrepare pass for more
77// information.
78void WasmException::computeCallSiteTable(
79    SmallVectorImpl<CallSiteEntry> &CallSites,
80    SmallVectorImpl<CallSiteRange> &CallSiteRanges,
81    const SmallVectorImpl<const LandingPadInfo *> &LandingPads,
82    const SmallVectorImpl<unsigned> &FirstActions) {
83  MachineFunction &MF = *Asm->MF;
84  for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) {
85    const LandingPadInfo *Info = LandingPads[I];
86    MachineBasicBlock *LPad = Info->LandingPadBlock;
87    // We don't emit LSDA for single catch (...).
88    if (!MF.hasWasmLandingPadIndex(LPad))
89      continue;
90    // Wasm EH must maintain the EH pads in the order assigned to them by the
91    // WasmEHPrepare pass.
92    unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad);
93    CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]};
94    if (CallSites.size() < LPadIndex + 1)
95      CallSites.resize(LPadIndex + 1);
96    CallSites[LPadIndex] = Site;
97  }
98}
99