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 // 27 // But in dynamic linking, it is in general not possible to come up with a 28 // module instantiating order in which tag-defining modules are loaded before 29 // the importing modules. So we make them undefined symbols here, define tags 30 // in the JS side, and feed them to each importing module. 31 if (!Asm->isPositionIndependent()) { 32 for (const char *SymName : {"__cpp_exception", "__c_longjmp"}) { 33 SmallString<60> NameStr; 34 Mangler::getNameWithPrefix(NameStr, SymName, Asm->getDataLayout()); 35 if (Asm->OutContext.lookupSymbol(NameStr)) { 36 MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(SymName); 37 Asm->OutStreamer->emitLabel(ExceptionSym); 38 } 39 } 40 } 41 } 42 43 void WasmException::markFunctionEnd() { 44 // Get rid of any dead landing pads. 45 if (!Asm->MF->getLandingPads().empty()) { 46 auto *NonConstMF = const_cast<MachineFunction *>(Asm->MF); 47 // Wasm does not set BeginLabel and EndLabel information for landing pads, 48 // so we should set the second argument false. 49 NonConstMF->tidyLandingPads(nullptr, /* TidyIfNoBeginLabels */ false); 50 } 51 } 52 53 void WasmException::endFunction(const MachineFunction *MF) { 54 bool ShouldEmitExceptionTable = false; 55 for (const LandingPadInfo &Info : MF->getLandingPads()) { 56 if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) { 57 ShouldEmitExceptionTable = true; 58 break; 59 } 60 } 61 if (!ShouldEmitExceptionTable) 62 return; 63 MCSymbol *LSDALabel = emitExceptionTable(); 64 assert(LSDALabel && ".GCC_exception_table has not been emitted!"); 65 66 // Wasm requires every data section symbol to have a .size set. So we emit an 67 // end marker and set the size as the difference between the start end the end 68 // marker. 69 MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end"); 70 Asm->OutStreamer->emitLabel(LSDAEndLabel); 71 MCContext &OutContext = Asm->OutStreamer->getContext(); 72 const MCExpr *SizeExp = MCBinaryExpr::createSub( 73 MCSymbolRefExpr::create(LSDAEndLabel, OutContext), 74 MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext); 75 Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp); 76 } 77 78 // Compute the call-site table for wasm EH. Even though we use the same function 79 // name to share the common routines, a call site entry in the table corresponds 80 // to not a call site for possibly-throwing functions but a landing pad. In wasm 81 // EH the VM is responsible for stack unwinding. After an exception occurs and 82 // the stack is unwound, the control flow is transferred to wasm 'catch' 83 // instruction by the VM, after which the personality function is called from 84 // the compiler-generated code. Refer to WasmEHPrepare pass for more 85 // information. 86 void WasmException::computeCallSiteTable( 87 SmallVectorImpl<CallSiteEntry> &CallSites, 88 SmallVectorImpl<CallSiteRange> &CallSiteRanges, 89 const SmallVectorImpl<const LandingPadInfo *> &LandingPads, 90 const SmallVectorImpl<unsigned> &FirstActions) { 91 MachineFunction &MF = *Asm->MF; 92 for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) { 93 const LandingPadInfo *Info = LandingPads[I]; 94 MachineBasicBlock *LPad = Info->LandingPadBlock; 95 // We don't emit LSDA for single catch (...). 96 if (!MF.hasWasmLandingPadIndex(LPad)) 97 continue; 98 // Wasm EH must maintain the EH pads in the order assigned to them by the 99 // WasmEHPrepare pass. 100 unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad); 101 CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]}; 102 if (CallSites.size() < LPadIndex + 1) 103 CallSites.resize(LPadIndex + 1); 104 CallSites[LPadIndex] = Site; 105 } 106 } 107