MachO/InputSection.h

//===- InputSection.h -------------------------------------------*- C++ -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//

#ifndef LLD_MACHO_INPUT_SECTION_H
#define LLD_MACHO_INPUT_SECTION_H

#include "Config.h"
#include "Relocations.h"
#include "Symbols.h"

#include "lld/Common/LLVM.h"
#include "lld/Common/Memory.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/CachedHashString.h"
#include "llvm/ADT/TinyPtrVector.h"
#include "llvm/BinaryFormat/MachO.h"

namespace lld {
namespace macho {

class InputFile;
class OutputSection;

class InputSection {
public:
  enum Kind {
    ConcatKind,
    CStringLiteralKind,
    WordLiteralKind,
  };

  Kind kind() const { return shared->sectionKind; }
  virtual ~InputSection() = default;
  virtual uint64_t getSize() const { return data.size(); }
  virtual bool empty() const { return data.empty(); }
  InputFile *getFile() const { return shared->file; }
  StringRef getName() const { return shared->name; }
  StringRef getSegName() const { return shared->segname; }
  uint32_t getFlags() const { return shared->flags; }
  uint64_t getFileSize() const;
  // Translates \p off -- an offset relative to this InputSection -- into an
  // offset from the beginning of its parent OutputSection.
  virtual uint64_t getOffset(uint64_t off) const = 0;
  // The offset from the beginning of the file.
  uint64_t getVA(uint64_t off) const;
  // Whether the data at \p off in this InputSection is live.
  virtual bool isLive(uint64_t off) const = 0;
  virtual void markLive(uint64_t off) = 0;
  virtual InputSection *canonical() { return this; }

  OutputSection *parent = nullptr;

  uint32_t align = 1;
  uint32_t callSiteCount : 31;
  // is address assigned?
  uint32_t isFinal : 1;

  ArrayRef<uint8_t> data;
  std::vector<Reloc> relocs;
  // The symbols that belong to this InputSection, sorted by value. With
  // .subsections_via_symbols, there is typically only one element here.
  llvm::TinyPtrVector<Defined *> symbols;

protected:
  // The fields in this struct are immutable. Since we create a lot of
  // InputSections with identical values for them (due to
  // .subsections_via_symbols), factoring them out into a shared struct reduces
  // memory consumption and makes copying cheaper.
  struct Shared {
    InputFile *file;
    StringRef name;
    StringRef segname;
    uint32_t flags;
    Kind sectionKind;
    Shared(InputFile *file, StringRef name, StringRef segname, uint32_t flags,
           Kind kind)
        : file(file), name(name), segname(segname), flags(flags),
          sectionKind(kind) {}
  };

  InputSection(Kind kind, StringRef segname, StringRef name, InputFile *file,
               ArrayRef<uint8_t> data, uint32_t align, uint32_t flags)
      : align(align), callSiteCount(0), isFinal(false), data(data),
        shared(make<Shared>(file, name, segname, flags, kind)) {}

  InputSection(const InputSection &rhs)
      : align(rhs.align), callSiteCount(0), isFinal(false), data(rhs.data),
        shared(rhs.shared) {}

  const Shared *const shared;
};

// ConcatInputSections are combined into (Concat)OutputSections through simple
// concatenation, in contrast with literal sections which may have their
// contents merged before output.
class ConcatInputSection final : public InputSection {
public:
  ConcatInputSection(StringRef segname, StringRef name, InputFile *file,
                     ArrayRef<uint8_t> data, uint32_t align = 1,
                     uint32_t flags = 0)
      : InputSection(ConcatKind, segname, name, file, data, align, flags) {}

  ConcatInputSection(StringRef segname, StringRef name)
      : ConcatInputSection(segname, name, /*file=*/nullptr,
                           /*data=*/{},
                           /*align=*/1, /*flags=*/0) {}

  uint64_t getOffset(uint64_t off) const override { return outSecOff + off; }
  uint64_t getVA() const { return InputSection::getVA(0); }
  // ConcatInputSections are entirely live or dead, so the offset is irrelevant.
  bool isLive(uint64_t off) const override { return live; }
  void markLive(uint64_t off) override { live = true; }
  bool isCoalescedWeak() const { return wasCoalesced && symbols.empty(); }
  bool shouldOmitFromOutput() const { return !live || isCoalescedWeak(); }
  bool isHashableForICF() const;
  void hashForICF();
  void writeTo(uint8_t *buf);

  void foldIdentical(ConcatInputSection *redundant);
  ConcatInputSection *canonical() override {
    return replacement ? replacement : this;
  }

  static bool classof(const InputSection *isec) {
    return isec->kind() == ConcatKind;
  }

  // Points to the surviving section after this one is folded by ICF
  ConcatInputSection *replacement = nullptr;
  // Equivalence-class ID for ICF
  uint64_t icfEqClass[2] = {0, 0};

  // With subsections_via_symbols, most symbols have their own InputSection,
  // and for weak symbols (e.g. from inline functions), only the
  // InputSection from one translation unit will make it to the output,
  // while all copies in other translation units are coalesced into the
  // first and not copied to the output.
  bool wasCoalesced = false;
  bool live = !config->deadStrip;
  // This variable has two usages. Initially, it represents the input order.
  // After assignAddresses is called, it represents the offset from the
  // beginning of the output section this section was assigned to.
  uint64_t outSecOff = 0;
};

// Helper functions to make it easy to sprinkle asserts.

inline bool shouldOmitFromOutput(InputSection *isec) {
  return isa<ConcatInputSection>(isec) &&
         cast<ConcatInputSection>(isec)->shouldOmitFromOutput();
}

inline bool isCoalescedWeak(InputSection *isec) {
  return isa<ConcatInputSection>(isec) &&
         cast<ConcatInputSection>(isec)->isCoalescedWeak();
}

// We allocate a lot of these and binary search on them, so they should be as
// compact as possible. Hence the use of 31 rather than 64 bits for the hash.
struct StringPiece {
  // Offset from the start of the containing input section.
  uint32_t inSecOff;
  uint32_t live : 1;
  // Only set if deduplicating literals
  uint32_t hash : 31;
  // Offset from the start of the containing output section.
  uint64_t outSecOff = 0;

  StringPiece(uint64_t off, uint32_t hash)
      : inSecOff(off), live(!config->deadStrip), hash(hash) {}
};

static_assert(sizeof(StringPiece) == 16, "StringPiece is too big!");

// CStringInputSections are composed of multiple null-terminated string
// literals, which we represent using StringPieces. These literals can be
// deduplicated and tail-merged, so translating offsets between the input and
// outputs sections is more complicated.
//
// NOTE: One significant difference between LLD and ld64 is that we merge all
// cstring literals, even those referenced directly by non-private symbols.
// ld64 is more conservative and does not do that. This was mostly done for
// implementation simplicity; if we find programs that need the more
// conservative behavior we can certainly implement that.
class CStringInputSection final : public InputSection {
public:
  CStringInputSection(StringRef segname, StringRef name, InputFile *file,
                      ArrayRef<uint8_t> data, uint32_t align, uint32_t flags)
      : InputSection(CStringLiteralKind, segname, name, file, data, align,
                     flags) {}
  uint64_t getOffset(uint64_t off) const override;
  bool isLive(uint64_t off) const override { return getStringPiece(off).live; }
  void markLive(uint64_t off) override { getStringPiece(off).live = true; }
  // Find the StringPiece that contains this offset.
  StringPiece &getStringPiece(uint64_t off);
  const StringPiece &getStringPiece(uint64_t off) const;
  // Split at each null byte.
  void splitIntoPieces();

  LLVM_ATTRIBUTE_ALWAYS_INLINE
  StringRef getStringRef(size_t i) const {
    size_t begin = pieces[i].inSecOff;
    size_t end =
        (pieces.size() - 1 == i) ? data.size() : pieces[i + 1].inSecOff;
    return toStringRef(data.slice(begin, end - begin));
  }

  // Returns i'th piece as a CachedHashStringRef. This function is very hot when
  // string merging is enabled, so we want to inline.
  LLVM_ATTRIBUTE_ALWAYS_INLINE
  llvm::CachedHashStringRef getCachedHashStringRef(size_t i) const {
    assert(config->dedupLiterals);
    return {getStringRef(i), pieces[i].hash};
  }

  static bool classof(const InputSection *isec) {
    return isec->kind() == CStringLiteralKind;
  }

  std::vector<StringPiece> pieces;
};

class WordLiteralInputSection final : public InputSection {
public:
  WordLiteralInputSection(StringRef segname, StringRef name, InputFile *file,
                          ArrayRef<uint8_t> data, uint32_t align,
                          uint32_t flags);
  uint64_t getOffset(uint64_t off) const override;
  bool isLive(uint64_t off) const override {
    return live[off >> power2LiteralSize];
  }
  void markLive(uint64_t off) override { live[off >> power2LiteralSize] = 1; }

  static bool classof(const InputSection *isec) {
    return isec->kind() == WordLiteralKind;
  }

private:
  unsigned power2LiteralSize;
  // The liveness of data[off] is tracked by live[off >> power2LiteralSize].
  llvm::BitVector live;
};

inline uint8_t sectionType(uint32_t flags) {
  return flags & llvm::MachO::SECTION_TYPE;
}

inline bool isZeroFill(uint32_t flags) {
  return llvm::MachO::isVirtualSection(sectionType(flags));
}

inline bool isThreadLocalVariables(uint32_t flags) {
  return sectionType(flags) == llvm::MachO::S_THREAD_LOCAL_VARIABLES;
}

// These sections contain the data for initializing thread-local variables.
inline bool isThreadLocalData(uint32_t flags) {
  return sectionType(flags) == llvm::MachO::S_THREAD_LOCAL_REGULAR ||
         sectionType(flags) == llvm::MachO::S_THREAD_LOCAL_ZEROFILL;
}

inline bool isDebugSection(uint32_t flags) {
  return (flags & llvm::MachO::SECTION_ATTRIBUTES_USR) ==
         llvm::MachO::S_ATTR_DEBUG;
}

inline bool isWordLiteralSection(uint32_t flags) {
  return sectionType(flags) == llvm::MachO::S_4BYTE_LITERALS ||
         sectionType(flags) == llvm::MachO::S_8BYTE_LITERALS ||
         sectionType(flags) == llvm::MachO::S_16BYTE_LITERALS;
}

bool isCodeSection(const InputSection *);

bool isCfStringSection(const InputSection *);

extern std::vector<ConcatInputSection *> inputSections;

namespace section_names {

constexpr const char authGot[] = "__auth_got";
constexpr const char authPtr[] = "__auth_ptr";
constexpr const char binding[] = "__binding";
constexpr const char bitcodeBundle[] = "__bundle";
constexpr const char cString[] = "__cstring";
constexpr const char cfString[] = "__cfstring";
constexpr const char codeSignature[] = "__code_signature";
constexpr const char common[] = "__common";
constexpr const char compactUnwind[] = "__compact_unwind";
constexpr const char data[] = "__data";
constexpr const char debugAbbrev[] = "__debug_abbrev";
constexpr const char debugInfo[] = "__debug_info";
constexpr const char debugStr[] = "__debug_str";
constexpr const char ehFrame[] = "__eh_frame";
constexpr const char gccExceptTab[] = "__gcc_except_tab";
constexpr const char export_[] = "__export";
constexpr const char dataInCode[] = "__data_in_code";
constexpr const char functionStarts[] = "__func_starts";
constexpr const char got[] = "__got";
constexpr const char header[] = "__mach_header";
constexpr const char indirectSymbolTable[] = "__ind_sym_tab";
constexpr const char const_[] = "__const";
constexpr const char lazySymbolPtr[] = "__la_symbol_ptr";
constexpr const char lazyBinding[] = "__lazy_binding";
constexpr const char literals[] = "__literals";
constexpr const char moduleInitFunc[] = "__mod_init_func";
constexpr const char moduleTermFunc[] = "__mod_term_func";
constexpr const char nonLazySymbolPtr[] = "__nl_symbol_ptr";
constexpr const char objcCatList[] = "__objc_catlist";
constexpr const char objcClassList[] = "__objc_classlist";
constexpr const char objcConst[] = "__objc_const";
constexpr const char objcImageInfo[] = "__objc_imageinfo";
constexpr const char objcNonLazyCatList[] = "__objc_nlcatlist";
constexpr const char objcNonLazyClassList[] = "__objc_nlclslist";
constexpr const char objcProtoList[] = "__objc_protolist";
constexpr const char pageZero[] = "__pagezero";
constexpr const char pointers[] = "__pointers";
constexpr const char rebase[] = "__rebase";
constexpr const char staticInit[] = "__StaticInit";
constexpr const char stringTable[] = "__string_table";
constexpr const char stubHelper[] = "__stub_helper";
constexpr const char stubs[] = "__stubs";
constexpr const char swift[] = "__swift";
constexpr const char symbolTable[] = "__symbol_table";
constexpr const char textCoalNt[] = "__textcoal_nt";
constexpr const char text[] = "__text";
constexpr const char threadPtrs[] = "__thread_ptrs";
constexpr const char threadVars[] = "__thread_vars";
constexpr const char unwindInfo[] = "__unwind_info";
constexpr const char weakBinding[] = "__weak_binding";
constexpr const char zeroFill[] = "__zerofill";

} // namespace section_names

} // namespace macho

std::string toString(const macho::InputSection *);

} // namespace lld

#endif