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/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 }