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 // These are symbols used to throw/catch C++ exceptions and C longjmps. These 22 // symbols have to be emitted somewhere once in the module. Check if each of 23 // the symbols has already been created, i.e., we have at least one 'throw' or 24 // 'catch' instruction with the symbol in the module, and emit the symbol only 25 // if so. 26 for (const char *SymName : {"__cpp_exception", "__c_longjmp"}) { 27 SmallString<60> NameStr; 28 Mangler::getNameWithPrefix(NameStr, SymName, Asm->getDataLayout()); 29 if (Asm->OutContext.lookupSymbol(NameStr)) { 30 MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(SymName); 31 Asm->OutStreamer->emitLabel(ExceptionSym); 32 } 33 } 34 } 35 36 void WasmException::markFunctionEnd() { 37 // Get rid of any dead landing pads. 38 if (!Asm->MF->getLandingPads().empty()) { 39 auto *NonConstMF = const_cast<MachineFunction *>(Asm->MF); 40 // Wasm does not set BeginLabel and EndLabel information for landing pads, 41 // so we should set the second argument false. 42 NonConstMF->tidyLandingPads(nullptr, /* TidyIfNoBeginLabels */ false); 43 } 44 } 45 46 void WasmException::endFunction(const MachineFunction *MF) { 47 bool ShouldEmitExceptionTable = false; 48 for (const LandingPadInfo &Info : MF->getLandingPads()) { 49 if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) { 50 ShouldEmitExceptionTable = true; 51 break; 52 } 53 } 54 if (!ShouldEmitExceptionTable) 55 return; 56 MCSymbol *LSDALabel = emitExceptionTable(); 57 assert(LSDALabel && ".GCC_exception_table has not been emitted!"); 58 59 // Wasm requires every data section symbol to have a .size set. So we emit an 60 // end marker and set the size as the difference between the start end the end 61 // marker. 62 MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end"); 63 Asm->OutStreamer->emitLabel(LSDAEndLabel); 64 MCContext &OutContext = Asm->OutStreamer->getContext(); 65 const MCExpr *SizeExp = MCBinaryExpr::createSub( 66 MCSymbolRefExpr::create(LSDAEndLabel, OutContext), 67 MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext); 68 Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp); 69 } 70 71 // Compute the call-site table for wasm EH. Even though we use the same function 72 // name to share the common routines, a call site entry in the table corresponds 73 // to not a call site for possibly-throwing functions but a landing pad. In wasm 74 // EH the VM is responsible for stack unwinding. After an exception occurs and 75 // the stack is unwound, the control flow is transferred to wasm 'catch' 76 // instruction by the VM, after which the personality function is called from 77 // the compiler-generated code. Refer to WasmEHPrepare pass for more 78 // information. 79 void WasmException::computeCallSiteTable( 80 SmallVectorImpl<CallSiteEntry> &CallSites, 81 SmallVectorImpl<CallSiteRange> &CallSiteRanges, 82 const SmallVectorImpl<const LandingPadInfo *> &LandingPads, 83 const SmallVectorImpl<unsigned> &FirstActions) { 84 MachineFunction &MF = *Asm->MF; 85 for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) { 86 const LandingPadInfo *Info = LandingPads[I]; 87 MachineBasicBlock *LPad = Info->LandingPadBlock; 88 // We don't emit LSDA for single catch (...). 89 if (!MF.hasWasmLandingPadIndex(LPad)) 90 continue; 91 // Wasm EH must maintain the EH pads in the order assigned to them by the 92 // WasmEHPrepare pass. 93 unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad); 94 CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]}; 95 if (CallSites.size() < LPadIndex + 1) 96 CallSites.resize(LPadIndex + 1); 97 CallSites[LPadIndex] = Site; 98 } 99 } 100