ARMELFObjectWriter.cpp

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//===-- ARMELFObjectWriter.cpp - ARM ELF Writer ---------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
 
#include "MCTargetDesc/ARMFixupKinds.h"
#include "MCTargetDesc/ARMMCTargetDesc.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCELFObjectWriter.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCObjectFileInfo.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCValue.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <cstdint>
 
using namespace llvm;
 
namespace {
 
  class ARMELFObjectWriter : public MCELFObjectTargetWriter {
    enum { DefaultEABIVersion = 0x05000000U };
 
    unsigned GetRelocTypeInner(const MCValue &Target, const MCFixup &Fixup,
                               bool IsPCRel, MCContext &Ctx) const;
 
  public:
    ARMELFObjectWriter(uint8_t OSABI);
 
    ~ARMELFObjectWriter() override = default;
 
    unsigned getRelocType(MCContext &Ctx, const MCValue &Target,
                          const MCFixup &Fixup, bool IsPCRel) const override;
 
    bool needsRelocateWithSymbol(const MCSymbol &Sym,
                                 unsigned Type) const override;
 
    void addTargetSectionFlags(MCContext &Ctx, MCSectionELF &Sec) override;
  };
 
} // end anonymous namespace
 
ARMELFObjectWriter::ARMELFObjectWriter(uint8_t OSABI)
  : MCELFObjectTargetWriter(/*Is64Bit*/ false, OSABI,
                            ELF::EM_ARM,
                            /*HasRelocationAddend*/ false) {}
 
bool ARMELFObjectWriter::needsRelocateWithSymbol(const MCSymbol &Sym,
                                                 unsigned Type) const {
  // FIXME: This is extremely conservative. This really needs to use an
  // explicit list with a clear explanation for why each realocation needs to
  // point to the symbol, not to the section.
  switch (Type) {
  default:
    return true;
 
  case ELF::R_ARM_PREL31:
  case ELF::R_ARM_ABS32:
    return false;
  }
}
 
// Need to examine the Fixup when determining whether to
// emit the relocation as an explicit symbol or as a section relative
// offset
unsigned ARMELFObjectWriter::getRelocType(MCContext &Ctx, const MCValue &Target,
                                          const MCFixup &Fixup,
                                          bool IsPCRel) const {
  return GetRelocTypeInner(Target, Fixup, IsPCRel, Ctx);
}
 
unsigned ARMELFObjectWriter::GetRelocTypeInner(const MCValue &Target,
                                               const MCFixup &Fixup,
                                               bool IsPCRel,
                                               MCContext &Ctx) const {
  unsigned Kind = Fixup.getTargetKind();
  if (Kind >= FirstLiteralRelocationKind)
    return Kind - FirstLiteralRelocationKind;
  MCSymbolRefExpr::VariantKind Modifier = Target.getAccessVariant();
 
  if (IsPCRel) {
    switch (Fixup.getTargetKind()) {
    default:
      Ctx.reportError(Fixup.getLoc(), "unsupported relocation on symbol");
      return ELF::R_ARM_NONE;
    case FK_Data_4:
      switch (Modifier) {
      default:
        Ctx.reportError(Fixup.getLoc(),
                        "invalid fixup for 4-byte pc-relative data relocation");
        return ELF::R_ARM_NONE;
      case MCSymbolRefExpr::VK_None: {
        if (const MCSymbolRefExpr *SymRef = Target.getSymA()) {
          // For GNU AS compatibility expressions such as
          // _GLOBAL_OFFSET_TABLE_ - label emit a R_ARM_BASE_PREL relocation.
          if (SymRef->getSymbol().getName() == "_GLOBAL_OFFSET_TABLE_")
            return ELF::R_ARM_BASE_PREL;
        }
        return ELF::R_ARM_REL32;
      }
      case MCSymbolRefExpr::VK_GOTTPOFF:
        return ELF::R_ARM_TLS_IE32;
      case MCSymbolRefExpr::VK_ARM_GOT_PREL:
        return ELF::R_ARM_GOT_PREL;
      case MCSymbolRefExpr::VK_ARM_PREL31:
        return ELF::R_ARM_PREL31;
      }
    case ARM::fixup_arm_blx:
    case ARM::fixup_arm_uncondbl:
      switch (Modifier) {
      case MCSymbolRefExpr::VK_PLT:
        return ELF::R_ARM_CALL;
      case MCSymbolRefExpr::VK_TLSCALL:
        return ELF::R_ARM_TLS_CALL;
      default:
        return ELF::R_ARM_CALL;
      }
    case ARM::fixup_arm_condbl:
    case ARM::fixup_arm_condbranch:
    case ARM::fixup_arm_uncondbranch:
      return ELF::R_ARM_JUMP24;
    case ARM::fixup_t2_condbranch:
      return ELF::R_ARM_THM_JUMP19;
    case ARM::fixup_t2_uncondbranch:
      return ELF::R_ARM_THM_JUMP24;
    case ARM::fixup_arm_movt_hi16:
      return ELF::R_ARM_MOVT_PREL;
    case ARM::fixup_arm_movw_lo16:
      return ELF::R_ARM_MOVW_PREL_NC;
    case ARM::fixup_t2_movt_hi16:
      return ELF::R_ARM_THM_MOVT_PREL;
    case ARM::fixup_t2_movw_lo16:
      return ELF::R_ARM_THM_MOVW_PREL_NC;
    case ARM::fixup_arm_thumb_br:
      return ELF::R_ARM_THM_JUMP11;
    case ARM::fixup_arm_thumb_bcc:
      return ELF::R_ARM_THM_JUMP8;
    case ARM::fixup_arm_thumb_bl:
    case ARM::fixup_arm_thumb_blx:
      switch (Modifier) {
      case MCSymbolRefExpr::VK_TLSCALL:
        return ELF::R_ARM_THM_TLS_CALL;
      default:
        return ELF::R_ARM_THM_CALL;
      }
    case ARM::fixup_bf_target:
      return ELF::R_ARM_THM_BF16;
    case ARM::fixup_bfc_target:
      return ELF::R_ARM_THM_BF12;
    case ARM::fixup_bfl_target:
      return ELF::R_ARM_THM_BF18;
    }
  }
  switch (Kind) {
  default:
    Ctx.reportError(Fixup.getLoc(), "unsupported relocation on symbol");
    return ELF::R_ARM_NONE;
  case FK_Data_1:
    switch (Modifier) {
    default:
      Ctx.reportError(Fixup.getLoc(),
                      "invalid fixup for 1-byte data relocation");
      return ELF::R_ARM_NONE;
    case MCSymbolRefExpr::VK_None:
      return ELF::R_ARM_ABS8;
    }
  case FK_Data_2:
    switch (Modifier) {
    default:
      Ctx.reportError(Fixup.getLoc(),
                      "invalid fixup for 2-byte data relocation");
      return ELF::R_ARM_NONE;
    case MCSymbolRefExpr::VK_None:
      return ELF::R_ARM_ABS16;
    }
  case FK_Data_4:
    switch (Modifier) {
    default:
      Ctx.reportError(Fixup.getLoc(),
                      "invalid fixup for 4-byte data relocation");
      return ELF::R_ARM_NONE;
    case MCSymbolRefExpr::VK_ARM_NONE:
      return ELF::R_ARM_NONE;
    case MCSymbolRefExpr::VK_GOT:
      return ELF::R_ARM_GOT_BREL;
    case MCSymbolRefExpr::VK_TLSGD:
      return ELF::R_ARM_TLS_GD32;
    case MCSymbolRefExpr::VK_TPOFF:
      return ELF::R_ARM_TLS_LE32;
    case MCSymbolRefExpr::VK_GOTTPOFF:
      return ELF::R_ARM_TLS_IE32;
    case MCSymbolRefExpr::VK_None:
      return ELF::R_ARM_ABS32;
    case MCSymbolRefExpr::VK_GOTOFF:
      return ELF::R_ARM_GOTOFF32;
    case MCSymbolRefExpr::VK_ARM_GOT_PREL:
      return ELF::R_ARM_GOT_PREL;
    case MCSymbolRefExpr::VK_ARM_TARGET1:
      return ELF::R_ARM_TARGET1;
    case MCSymbolRefExpr::VK_ARM_TARGET2:
      return ELF::R_ARM_TARGET2;
    case MCSymbolRefExpr::VK_ARM_PREL31:
      return ELF::R_ARM_PREL31;
    case MCSymbolRefExpr::VK_ARM_SBREL:
      return ELF::R_ARM_SBREL32;
    case MCSymbolRefExpr::VK_ARM_TLSLDO:
      return ELF::R_ARM_TLS_LDO32;
    case MCSymbolRefExpr::VK_TLSCALL:
      return ELF::R_ARM_TLS_CALL;
    case MCSymbolRefExpr::VK_TLSDESC:
      return ELF::R_ARM_TLS_GOTDESC;
    case MCSymbolRefExpr::VK_TLSLDM:
      return ELF::R_ARM_TLS_LDM32;
    case MCSymbolRefExpr::VK_ARM_TLSDESCSEQ:
      return ELF::R_ARM_TLS_DESCSEQ;
    }
  case ARM::fixup_arm_condbranch:
  case ARM::fixup_arm_uncondbranch:
    return ELF::R_ARM_JUMP24;
  case ARM::fixup_arm_movt_hi16:
    switch (Modifier) {
    default:
      Ctx.reportError(Fixup.getLoc(), "invalid fixup for ARM MOVT instruction");
      return ELF::R_ARM_NONE;
    case MCSymbolRefExpr::VK_None:
      return ELF::R_ARM_MOVT_ABS;
    case MCSymbolRefExpr::VK_ARM_SBREL:
      return ELF::R_ARM_MOVT_BREL;
    }
  case ARM::fixup_arm_movw_lo16:
    switch (Modifier) {
    default:
      Ctx.reportError(Fixup.getLoc(), "invalid fixup for ARM MOVW instruction");
      return ELF::R_ARM_NONE;
    case MCSymbolRefExpr::VK_None:
      return ELF::R_ARM_MOVW_ABS_NC;
    case MCSymbolRefExpr::VK_ARM_SBREL:
      return ELF::R_ARM_MOVW_BREL_NC;
    }
  case ARM::fixup_t2_movt_hi16:
    switch (Modifier) {
    default:
      Ctx.reportError(Fixup.getLoc(),
                      "invalid fixup for Thumb MOVT instruction");
      return ELF::R_ARM_NONE;
    case MCSymbolRefExpr::VK_None:
      return ELF::R_ARM_THM_MOVT_ABS;
    case MCSymbolRefExpr::VK_ARM_SBREL:
      return ELF::R_ARM_THM_MOVT_BREL;
    }
  case ARM::fixup_t2_movw_lo16:
    switch (Modifier) {
    default:
      Ctx.reportError(Fixup.getLoc(),
                      "invalid fixup for Thumb MOVW instruction");
      return ELF::R_ARM_NONE;
    case MCSymbolRefExpr::VK_None:
      return ELF::R_ARM_THM_MOVW_ABS_NC;
    case MCSymbolRefExpr::VK_ARM_SBREL:
      return ELF::R_ARM_THM_MOVW_BREL_NC;
    }
  }
}
 
void ARMELFObjectWriter::addTargetSectionFlags(MCContext &Ctx,
                                               MCSectionELF &Sec) {
  // The mix of execute-only and non-execute-only at link time is
  // non-execute-only. To avoid the empty implicitly created .text
  // section from making the whole .text section non-execute-only, we
  // mark it execute-only if it is empty and there is at least one
  // execute-only section in the object.
  MCSectionELF *TextSection =
      static_cast<MCSectionELF *>(Ctx.getObjectFileInfo()->getTextSection());
  if (Sec.getKind().isExecuteOnly() && !TextSection->hasInstructions()) {
    for (auto &F : TextSection->getFragmentList())
      if (auto *DF = dyn_cast<MCDataFragment>(&F))
        if (!DF->getContents().empty())
          return;
    TextSection->setFlags(TextSection->getFlags() | ELF::SHF_ARM_PURECODE);
  }
}
 
std::unique_ptr<MCObjectTargetWriter>
llvm::createARMELFObjectWriter(uint8_t OSABI) {
  return std::make_unique<ARMELFObjectWriter>(OSABI);
}