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/CodeGen/AsmPrinter.h" 16 #include "llvm/CodeGen/MachineFunction.h" 17 #include "llvm/IR/Mangler.h" 18 #include "llvm/MC/MCContext.h" 19 #include "llvm/MC/MCStreamer.h" 20 using namespace llvm; 21 22 void WasmException::endModule() { 23 // These are symbols used to throw/catch C++ exceptions and C longjmps. These 24 // symbols have to be emitted somewhere once in the module. Check if each of 25 // the symbols has already been created, i.e., we have at least one 'throw' or 26 // 'catch' instruction with the symbol in the module, and emit the symbol only 27 // if so. 28 // 29 // But in dynamic linking, it is in general not possible to come up with a 30 // module instantiating order in which tag-defining modules are loaded before 31 // the importing modules. So we make them undefined symbols here, define tags 32 // in the JS side, and feed them to each importing module. 33 if (!Asm->isPositionIndependent()) { 34 for (const char *SymName : {"__cpp_exception", "__c_longjmp"}) { 35 SmallString<60> NameStr; 36 Mangler::getNameWithPrefix(NameStr, SymName, Asm->getDataLayout()); 37 if (Asm->OutContext.lookupSymbol(NameStr)) { 38 MCSymbol *ExceptionSym = Asm->GetExternalSymbolSymbol(SymName); 39 Asm->OutStreamer->emitLabel(ExceptionSym); 40 } 41 } 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 SmallVectorImpl<CallSiteRange> &CallSiteRanges, 81 const SmallVectorImpl<const LandingPadInfo *> &LandingPads, 82 const SmallVectorImpl<unsigned> &FirstActions) { 83 MachineFunction &MF = *Asm->MF; 84 for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) { 85 const LandingPadInfo *Info = LandingPads[I]; 86 MachineBasicBlock *LPad = Info->LandingPadBlock; 87 // We don't emit LSDA for single catch (...). 88 if (!MF.hasWasmLandingPadIndex(LPad)) 89 continue; 90 // Wasm EH must maintain the EH pads in the order assigned to them by the 91 // WasmEHPrepare pass. 92 unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad); 93 CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]}; 94 if (CallSites.size() < LPadIndex + 1) 95 CallSites.resize(LPadIndex + 1); 96 CallSites[LPadIndex] = Site; 97 } 98 } 99