llvm/lib/MC/MCNaCl.cpp

//===- lib/MC/MCNaCl.cpp - NaCl-specific MC implementation ----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
// This file was written by the Native Client authors.
//
//===----------------------------------------------------------------------===//

#include "llvm/MC/MCNaCl.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCNaClExpander.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/MC/MCStreamer.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Support/Alignment.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/TargetParser/Triple.h"

static const char NoteNamespace[] = "NaCl";

namespace llvm {

cl::opt<bool> FlagEnableAutoSandboxing("nacl-enable-autosandboxing",
                                cl::desc("Don't use the autosandboxing"
                                         " assembler for the NaCl SFI."),
                                cl::init(true));

void initializeNaClMCStreamer(MCStreamer &Streamer, MCContext &Ctx,
                              const Triple &TheTriple) {
  assert(TheTriple.isOSNaCl());
  const char *NoteName;
  const char *NoteArch;
  Align BundleAlign;
  switch (TheTriple.getArch()) {
    case Triple::arm:
      NoteName = ".note.NaCl.ABI.arm";
      NoteArch = "arm";
      BundleAlign = Align(16);
      break;
    case Triple::mipsel:
      NoteName = ".note.NaCl.ABI.mipsel";
      NoteArch = "mipsel";
      BundleAlign = Align(16);
      break;
    case Triple::x86:
      NoteName = ".note.NaCl.ABI.x86-32";
      NoteArch = "x86-32";
      BundleAlign = Align(32);
      break;
    case Triple::x86_64:
      NoteName = ".note.NaCl.ABI.x86-64";
      NoteArch = "x86-64";
      BundleAlign = Align(32);
      break;
    default:
      report_fatal_error("Unsupported architecture for NaCl");
  }

  std::string Error; //empty
  const Target *TheTarget =
    TargetRegistry::lookupTarget(TheTriple.getTriple(), Error);

  // Create the Target specific MCNaClExpander
  assert(TheTarget != nullptr);
  if (FlagEnableAutoSandboxing) {
    TheTarget->createMCNaClExpander(
      Streamer,
      std::unique_ptr<MCRegisterInfo>(
          TheTarget->createMCRegInfo(TheTriple.getTriple())),
      std::unique_ptr<MCInstrInfo>(TheTarget->createMCInstrInfo()));
  }

  // Set bundle-alignment as required by the NaCl ABI for the target.
  Streamer.emitBundleAlignMode(BundleAlign);

  // For gas, override the size of DWARF address values generated by .loc
  // directives.
  if (TheTriple.getArch() == Triple::x86_64 &&
      Streamer.hasRawTextSupport()) {
    Streamer.emitRawText("\t.dwarf_addr_size 4\n");
  }

  // Emit an ELF Note section in its own COMDAT group which identifies NaCl
  // object files to the gold linker, so it can use the NaCl layout.
  MCSectionELF* Note = Ctx.getELFSection(NoteName, ELF::SHT_NOTE,
                                         ELF::SHF_ALLOC | ELF::SHF_GROUP,
                                         0, NoteName, /*IsComdat=*/true);

  Streamer.pushSection();
  Streamer.switchSection(Note);
  Streamer.emitIntValue(strlen(NoteNamespace) + 1, 4);
  Streamer.emitIntValue(strlen(NoteArch) + 1, 4);
  Streamer.emitIntValue(ELF::NT_VERSION, 4);
  Streamer.emitBytes(NoteNamespace);
  Streamer.emitIntValue(0, 1); // NUL terminator
  Streamer.emitValueToAlignment(Align(4));
  Streamer.emitBytes(NoteArch);
  Streamer.emitIntValue(0, 1); // NUL terminator
  Streamer.emitValueToAlignment(Align(4));
  Streamer.popSection();
}

} // namespace llvm