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::markFunctionEnd() { 46 // Get rid of any dead landing pads. 47 if (!Asm->MF->getLandingPads().empty()) { 48 auto *NonConstMF = const_cast<MachineFunction *>(Asm->MF); 49 // Wasm does not set BeginLabel and EndLabel information for landing pads, 50 // so we should set the second argument false. 51 NonConstMF->tidyLandingPads(nullptr, /* TidyIfNoBeginLabels */ false); 52 } 53 } 54 55 void WasmException::endFunction(const MachineFunction *MF) { 56 bool ShouldEmitExceptionTable = false; 57 for (const LandingPadInfo &Info : MF->getLandingPads()) { 58 if (MF->hasWasmLandingPadIndex(Info.LandingPadBlock)) { 59 ShouldEmitExceptionTable = true; 60 break; 61 } 62 } 63 if (!ShouldEmitExceptionTable) 64 return; 65 MCSymbol *LSDALabel = emitExceptionTable(); 66 assert(LSDALabel && ".GCC_exception_table has not been emitted!"); 67 68 // Wasm requires every data section symbol to have a .size set. So we emit an 69 // end marker and set the size as the difference between the start end the end 70 // marker. 71 MCSymbol *LSDAEndLabel = Asm->createTempSymbol("GCC_except_table_end"); 72 Asm->OutStreamer->emitLabel(LSDAEndLabel); 73 MCContext &OutContext = Asm->OutStreamer->getContext(); 74 const MCExpr *SizeExp = MCBinaryExpr::createSub( 75 MCSymbolRefExpr::create(LSDAEndLabel, OutContext), 76 MCSymbolRefExpr::create(LSDALabel, OutContext), OutContext); 77 Asm->OutStreamer->emitELFSize(LSDALabel, SizeExp); 78 } 79 80 // Compute the call-site table for wasm EH. Even though we use the same function 81 // name to share the common routines, a call site entry in the table corresponds 82 // to not a call site for possibly-throwing functions but a landing pad. In wasm 83 // EH the VM is responsible for stack unwinding. After an exception occurs and 84 // the stack is unwound, the control flow is transferred to wasm 'catch' 85 // instruction by the VM, after which the personality function is called from 86 // the compiler-generated code. Refer to WasmEHPrepare pass for more 87 // information. 88 void WasmException::computeCallSiteTable( 89 SmallVectorImpl<CallSiteEntry> &CallSites, 90 SmallVectorImpl<CallSiteRange> &CallSiteRanges, 91 const SmallVectorImpl<const LandingPadInfo *> &LandingPads, 92 const SmallVectorImpl<unsigned> &FirstActions) { 93 MachineFunction &MF = *Asm->MF; 94 for (unsigned I = 0, N = LandingPads.size(); I < N; ++I) { 95 const LandingPadInfo *Info = LandingPads[I]; 96 MachineBasicBlock *LPad = Info->LandingPadBlock; 97 // We don't emit LSDA for single catch (...). 98 if (!MF.hasWasmLandingPadIndex(LPad)) 99 continue; 100 // Wasm EH must maintain the EH pads in the order assigned to them by the 101 // WasmEHPrepare pass. 102 unsigned LPadIndex = MF.getWasmLandingPadIndex(LPad); 103 CallSiteEntry Site = {nullptr, nullptr, Info, FirstActions[I]}; 104 if (CallSites.size() < LPadIndex + 1) 105 CallSites.resize(LPadIndex + 1); 106 CallSites[LPadIndex] = Site; 107 } 108 } 109