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