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(OutName&: NameStr, GVName: SymName, DL: Asm->getDataLayout());
37 if (Asm->OutContext.lookupSymbol(Name: NameStr)) {
38 MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(Sym: SymName);
39 Asm->OutStreamer->emitLabel(Symbol: 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(LPad: 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(Name: "GCC_except_table_end");
62 Asm->OutStreamer->emitLabel(Symbol: LSDAEndLabel);
63 MCContext &OutContext = Asm->OutStreamer->getContext();
64 const MCExpr *SizeExp = MCBinaryExpr::createSub(
65 LHS: MCSymbolRefExpr::create(Symbol: LSDAEndLabel, Ctx&: OutContext),
66 RHS: MCSymbolRefExpr::create(Symbol: LSDALabel, Ctx&: OutContext), Ctx&: OutContext);
67 Asm->OutStreamer->emitELFSize(Symbol: LSDALabel, Value: 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 = {.BeginLabel: nullptr, .EndLabel: nullptr, .LPad: Info, .Action: FirstActions[I]};
94 if (CallSites.size() < LPadIndex + 1)
95 CallSites.resize(N: LPadIndex + 1);
96 CallSites[LPadIndex] = Site;
97 }
98}
99