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" 18 using namespace llvm; 19 20 void 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 34 void 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 44 void 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. 77 void 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