1 //===-- CodeGen/AsmPrinter/WasmException.cpp - Wasm Exception Impl --------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file contains support for writing WebAssembly exception info into asm 11 // files. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #include "WasmException.h" 16 #include "llvm/IR/Mangler.h" 17 #include "llvm/MC/MCContext.h" 18 #include "llvm/MC/MCStreamer.h" 19 using namespace llvm; 20 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 26 // symbol only if so. 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); 32 } 33 } 34 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); 42 } 43 } 44 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; 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. 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)) 88 continue; 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; 96 } 97 } 98