llvm/lib/CodeGen/AsmPrinter/WasmException.cpp

   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 }