1 //===-- CodeGen/AsmPrinter/WasmException.cpp - Wasm Exception Impl --------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file contains support for writing WebAssembly exception info into asm
13 //===----------------------------------------------------------------------===//
15 #include "WasmException.h"
16 #include "llvm/IR/Mangler.h"
17 #include "llvm/MC/MCContext.h"
18 #include "llvm/MC/MCStreamer.h"
21 void WasmException::endModule() {
22 // This is the symbol used in 'throw' and 'if_except' instruction to denote
23 // this is a C++ exception. This symbol has to be emitted somewhere once in
24 // the module. Check if the symbol has already been created, i.e., we have at
25 // least one 'throw' or 'if_except' instruction in the module, and emit the
27 SmallString<60> NameStr;
28 Mangler::getNameWithPrefix(NameStr, "__cpp_exception", Asm->getDataLayout());
29 if (Asm->OutContext.lookupSymbol(NameStr)) {
30 MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol("__cpp_exception");
31 Asm->OutStreamer->EmitLabel(ExceptionSym);
35 void WasmException::markFunctionEnd() {
36 // Get rid of any dead landing pads.
37 if (!Asm->MF->getLandingPads().empty()) {
38 auto *NonConstMF = const_cast<MachineFunction *>(Asm->MF);
39 // Wasm does not set BeginLabel and EndLabel information for landing pads,
40 // so we should set the second argument false.
41 NonConstMF->tidyLandingPads(nullptr, /* TidyIfNoBeginLabels */ false);
45 void 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;
53 if (!ShouldEmitExceptionTable)
55 MCSymbol *LSDALabel = emitExceptionTable();
56 assert(LSDALabel && ".GCC_exception_table has not been emitted!");
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
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);
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
78 void WasmException::computeCallSiteTable(
79 SmallVectorImpl<CallSiteEntry> &CallSites,
80 const SmallVectorImpl<const LandingPadInfo *> &LandingPads,
81 const SmallVectorImpl<unsigned> &FirstActions) {
82 MachineFunction &MF = *Asm->MF;
83 for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) {
84 const LandingPadInfo *Info = LandingPads[I];
85 MachineBasicBlock *LPad = Info->LandingPadBlock;
86 // We don't emit LSDA for single catch (...).
87 if (!MF.hasWasmLandingPadIndex(LPad))
89 // Wasm EH must maintain the EH pads in the order assigned to them by the
90 // WasmEHPrepare pass.
91 unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad);
92 CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]};
93 if (CallSites.size() < LPadIndex + 1)
94 CallSites.resize(LPadIndex + 1);
95 CallSites[LPadIndex] = Site;