Subzero: Add basic ELFObjectWriter (text section, symtab, strtab, headers)

src/IceELFObjectWriter.cpp

+//===- subzero/src/IceELFObjectWriter.cpp - ELF object file writer --------===//
+//
+//                        The Subzero Code Generator
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the writer for ELF relocatable object files.
+//
+//===----------------------------------------------------------------------===//
+
+#include "IceDefs.h"
+#include "IceELFObjectWriter.h"
+#include "IceELFSection.h"
+#include "IceELFStreamer.h"
+#include "IceGlobalContext.h"
+#include "IceGlobalInits.h"
+
+using namespace llvm::ELF;
+
+namespace Ice {
+
+namespace {
+
+struct {
+  bool IsELF64;
+  uint16_t ELFMachine;
+  uint32_t ELFFlags;
+} ELFTargetInfo[] = {
+#define X(tag, str, is_elf64, e_machine, e_flags)                              \
+  { is_elf64, e_machine, e_flags }                                             \
+  ,
+    TARGETARCH_TABLE
+#undef X
+};
+
+bool isELF64(TargetArch Arch) {
+  size_t Index = static_cast<size_t>(Arch);

[Jim Stichnoth, 2014/11/21 21:32:22]

Can you just directly use Arch as the index?

[jvoung (off chromium), 2014/11/24 21:35:46]

Done.

+  if (Index < TargetArch_NUM)
+    return ELFTargetInfo[Index].IsELF64;
+  llvm_unreachable("Invalid target arch for isELF64");
+  return false;
+}
+
+uint16_t getELFMachine(TargetArch Arch) {
+  size_t Index = static_cast<size_t>(Arch);
+  if (Index < TargetArch_NUM)
+    return ELFTargetInfo[Index].ELFMachine;
+  llvm_unreachable("Invalid target arch for getELFMachine");
+  return EM_NONE;
+}
+
+uint32_t getELFFlags(TargetArch Arch) {
+  size_t Index = static_cast<size_t>(Arch);
+  if (Index < TargetArch_NUM)
+    return ELFTargetInfo[Index].ELFFlags;
+  llvm_unreachable("Invalid target arch for getELFFlags");
+  return 0;
+}
+
+} // end of anonymous namespace
+
+ELFObjectWriter::ELFObjectWriter(GlobalContext &Ctx, ELFStreamer &Out)
+    : Ctx(Ctx), Str(Out), SectionNumbersAssigned(false) {
+  // Create the special bookkeeping sections now.
+  const IceString NullSectionName("");
+  NullSection = new (Ctx.allocate<ELFSection>())
+      ELFSection(NullSectionName, SHT_NULL, 0, 0, 0);
+
+  const IceString ShStrTabName(".shstrtab");
+  ShStrTab = new (Ctx.allocate<ELFStringTableSection>())
+      ELFStringTableSection(ShStrTabName, SHT_STRTAB, 0, 1, 0);
+  ShStrTab->add(ShStrTabName);
+
+  const IceString SymTabName(".symtab");
+  const Elf64_Xword SymTabAlign = isELF64(Ctx.getTargetArch()) ? 8 : 4;
+  const Elf64_Xword SymTabEntSize =
+      isELF64(Ctx.getTargetArch()) ? sizeof(Elf64_Sym) : sizeof(Elf32_Sym);

[Jim Stichnoth, 2014/11/21 21:32:22]

Probably want to confirm that SymTabAlign and sizeof() calculations are correct
and stable across targets, compilers, etc.

[jvoung (off chromium), 2014/11/24 21:35:46]

Hmm, llvm instances where they use sizeof(Elf_Sym) -- but that is for 
yet another definition in include/llvm/Object/ELFTypes.h, and not this Elfxx_Sym
coming from Support/ELF.h.

There are also instances of sizeof(Elfxx_Shdr) and sizeof(Elfxx_Ehdr).

I'm thinking of keeping this, but adding a static assert here, so the sizeof
helps document what it represents while the hardcoded constant is just for the
assert.

[Jim Stichnoth, 2014/12/01 22:41:06]

Cool, static_assert sounds good.

+  SymTab = createSection<ELFSymbolTableSection>(SymTabName, SHT_SYMTAB, 0,
+                                                SymTabAlign, SymTabEntSize);
+  // The first entry in the symbol table should be a NULL entry.
+  const IceString NullSymName("");
+  SymTab->createDefinedSym(NullSymName, STT_NOTYPE, STB_LOCAL, NullSection, 0,
+                           0);
+
+  const IceString StrTabName(".strtab");
+  StrTab =
+      createSection<ELFStringTableSection>(StrTabName, SHT_STRTAB, 0, 1, 0);
+}
+
+template <typename T>
+T *ELFObjectWriter::createSection(const IceString &Name, Elf64_Word sh_type,

[Jim Stichnoth, 2014/11/21 21:32:22]

Capitalize arg names

[jvoung (off chromium), 2014/11/24 21:35:46]

Done.

+                                  Elf64_Xword sh_flags,
+                                  Elf64_Xword sh_addralign,
+                                  Elf64_Xword sh_entsize) {
+  assert(!SectionNumbersAssigned);
+  T *NewSection = new (Ctx.allocate<T>())
+      T(Name, sh_type, sh_flags, sh_addralign, sh_entsize);
+  ShStrTab->add(Name);
+  return NewSection;
+}
+
+template <typename UserSectionList>
+void ELFObjectWriter::assignRelSectionNumInPairs(SizeT &CurSectionNumber,
+                                                 UserSectionList &UserSections,
+                                                 RelSectionList &RelSections,
+                                                 SectionList &AllSections) {
+  auto UserIt = UserSections.begin();

[Jim Stichnoth, 2014/11/21 21:32:22]

Can UserIt/UserE be subsumed into a range-based for loop?

Also, it might be better not to use auto for RelIt/RelE since it requires some
searching for the reader to figure out the actual type.

[jvoung (off chromium), 2014/11/24 21:35:45]

Done.

+  auto UserE = UserSections.end();
+  auto RelIt = RelSections.begin();
+  auto RelE = RelSections.end();
+  while (UserIt != UserE) {
+    ELFSection *UserSection = *UserIt;
+    UserSection->setNumber(CurSectionNumber++);
+    UserSection->setNameStrIndex(ShStrTab->getIndex(UserSection->getName()));
+    AllSections.push_back(UserSection);
+    if (RelIt != RelE) {
+      ELFRelocationSectionBase *RelSection = *RelIt;
+      if (RelSection->getRelatedSection() == UserSection) {
+        RelSection->setInfoNum(UserSection->getNumber());
+        RelSection->setNumber(CurSectionNumber++);
+        RelSection->setNameStrIndex(ShStrTab->getIndex(RelSection->getName()));
+        AllSections.push_back(RelSection);
+        ++RelIt;
+      }
+    }
+    ++UserIt;
+  }
+  // Should finish with UserIt at the same time as RelIt.
+  assert(RelIt == RelE);
+  return;
+}
+
+void ELFObjectWriter::assignRelLinkNum(SizeT SymTabNumber,
+                                       RelSectionList &RelSections) {
+  for (auto S : RelSections) {

[Jim Stichnoth, 2014/11/21 21:32:22]

same comment on auto

[jvoung (off chromium), 2014/11/24 21:35:46]

Done.

+    S->setLinkNum(SymTabNumber);
+  }
+}
+
+void ELFObjectWriter::assignSectionNumbersInfo(SectionList &AllSections) {
+  // Go through each section, assigning them section numbers and
+  // and fill in the size for sections that aren't incrementally updated.
+  assert(!SectionNumbersAssigned);
+  SizeT CurSectionNumber = 0;
+  NullSection->setNumber(CurSectionNumber++);
+  // The rest of the fields are initialized to 0, and stay that way.
+  AllSections.push_back(NullSection);
+
+  assignRelSectionNumInPairs<TextSectionList>(CurSectionNumber, TextSections,
+                                              RelTextSections, AllSections);
+  assignRelSectionNumInPairs<DataSectionList>(CurSectionNumber, DataSections,
+                                              RelDataSections, AllSections);
+  assignRelSectionNumInPairs<DataSectionList>(CurSectionNumber, RoDataSections,
+                                              RelRoDataSections, AllSections);
+
+  ShStrTab->setNumber(CurSectionNumber++);
+  ShStrTab->setNameStrIndex(ShStrTab->getIndex(ShStrTab->getName()));
+  AllSections.push_back(ShStrTab);
+
+  SymTab->setNumber(CurSectionNumber++);
+  SymTab->setNameStrIndex(ShStrTab->getIndex(SymTab->getName()));
+  AllSections.push_back(SymTab);
+
+  StrTab->setNumber(CurSectionNumber++);
+  StrTab->setNameStrIndex(ShStrTab->getIndex(StrTab->getName()));
+  AllSections.push_back(StrTab);
+
+  SymTab->setLinkNum(StrTab->getNumber());
+  SymTab->setInfoNum(SymTab->getNumLocals());
+
+  assignRelLinkNum(SymTab->getNumber(), RelTextSections);
+  assignRelLinkNum(SymTab->getNumber(), RelDataSections);
+  assignRelLinkNum(SymTab->getNumber(), RelRoDataSections);
+  SectionNumbersAssigned = true;
+}
+
+Elf64_Off ELFObjectWriter::alignFileOffset(Elf64_Xword Align) {

[Jim Stichnoth, 2014/11/21 21:32:22]

The % operations could be more efficient if Align were guaranteed to be a power
of 2.  Probably not worth it if only function entries are aligned, but maybe
more so if it's used for bundle alignment?

[jvoung (off chromium), 2014/11/24 21:35:46]

Good point -- the align isn't statically known to be a power of two, so the %
isn't so efficient.

I made this more like AssemblerX86::Align() which asserts that it's a power of
2.

For bundle alignment, I think that AssemblerX86::Align(), will end up being the
thing that is used (at least that knows the nop encodings).

+  Elf64_Off OffsetInFile = Str.tell();
+  Elf64_Xword AlignDiff = Align - (OffsetInFile % Align);
+  if (AlignDiff != Align) {
+    Str.writeZeroPadding(AlignDiff);
+    OffsetInFile += AlignDiff;
+    assert(OffsetInFile % Align == 0);
+  }
+  return OffsetInFile;
+}
+
+// Need the function's declaration.

[Jim Stichnoth, 2014/11/21 21:32:22]

Is this a TODO, or, ... ?

[jvoung (off chromium), 2014/11/24 21:35:46]

Done -- actually had a TODO in the .h file already, so removed this.

+void ELFObjectWriter::writeFunctionCode(const IceString &FuncName,
+                                        bool IsInternal,
+                                        const llvm::StringRef Data) {
+  assert(!SectionNumbersAssigned);
+  // TODO(jvoung): handle ffunction-sections.
+  IceString SectionName = ".text";
+  ELFTextSection *Section = nullptr;
+  if (TextSections.size() == 0) {
+    const Elf64_Xword ShFlags = SHF_ALLOC | SHF_EXECINSTR;
+    // TODO(jvoung): Should be bundle size. Grab it from that target?
+    const Elf64_Xword ShAlign = 32;
+    Section = createSection<ELFTextSection>(SectionName, SHT_PROGBITS, ShFlags,
+                                            ShAlign, 0);
+    Elf64_Off OffsetInFile = alignFileOffset(Section->getSectionAlign());
+    Section->setFileOffset(OffsetInFile);
+    TextSections.push_back(Section);
+  } else {
+    Section = TextSections[0];
+  }
+  RelocOffsetT OffsetInSection = Section->getCurrentSize();
+  // Function symbols are set to 0 size in the symbol table,
+  // in contrast to data symbols which have a proper size.
+  SizeT SymbolSize = 0;
+  Section->appendData(Str, Data);
+  uint8_t SymbolType;
+  uint8_t SymbolBinding;
+  if (IsInternal) {
+    SymbolType = STT_NOTYPE;
+    SymbolBinding = STB_LOCAL;
+  } else {
+    SymbolType = STT_FUNC;
+    SymbolBinding = STB_GLOBAL;
+  }
+  SymTab->createDefinedSym(FuncName, SymbolType, SymbolBinding, Section,
+                           OffsetInSection, SymbolSize);
+  StrTab->add(FuncName);
+}
+
+void ELFObjectWriter::writeDataInitializer(const IceString &VarName,
+                                           const llvm::StringRef Data) {

[Jim Stichnoth, 2014/11/21 21:32:22]

(void)Data;
to silence unused parameter warning in the meantime

[jvoung (off chromium), 2014/11/24 21:35:46]

Done.

+  assert(!SectionNumbersAssigned);
+  llvm_unreachable("TODO");
+  StrTab->add(VarName);
+}
+
+void ELFObjectWriter::writeInitialELFHeader() {
+  assert(!SectionNumbersAssigned);
+  const size_t DummySHOffset = 0;
+  const SizeT DummySHStrIndex = 0;
+  const SizeT DummyNumSections = 0;
+  if (isELF64(Ctx.getTargetArch())) {
+    writeELFHeaderInternal<true>(DummySHOffset, DummySHStrIndex,
+                                 DummyNumSections);
+  } else {
+    writeELFHeaderInternal<false>(DummySHOffset, DummySHStrIndex,
+                                  DummyNumSections);
+  }
+}
+
+template <bool IsELF64>
+void ELFObjectWriter::writeELFHeaderInternal(size_t SectionHeaderOffset,
+                                             SizeT SectHeaderStrIndex,
+                                             SizeT NumSections) {
+  // Write the e_ident: magic number, class, etc.
+  // The e_ident is byte order and ELF class independent.
+  Str.writeBytes(llvm::StringRef(ElfMagic, strlen(ElfMagic)));
+  Str.write8(IsELF64 ? ELFCLASS64 : ELFCLASS32);
+  Str.write8(ELFDATA2LSB);
+  Str.write8(EV_CURRENT);
+  Str.write8(ELFOSABI_NONE);
+  const uint8_t ELF_ABIVersion = 0;
+  Str.write8(ELF_ABIVersion);
+  Str.writeZeroPadding(EI_NIDENT - EI_PAD);
+
+  // TODO(jvoung): Handle and test > 64K sections.  See the generic ABI doc:
+  // https://refspecs.linuxbase.org/elf/gabi4+/ch4.eheader.html
+  // e_shnum should be 0 and then actual number of sections is
+  // stored in the sh_size member of the 0th section.
+  assert(NumSections < SHN_LORESERVE);
+  assert(SectHeaderStrIndex < SHN_LORESERVE);
+
+  // Write the rest of the file header, which does depend on byte order
+  // and ELF class.
+  Str.writeLE16(ET_REL);                             // e_type
+  Str.writeLE16(getELFMachine(Ctx.getTargetArch())); // e_machine
+  Str.writeELFWord<IsELF64>(1);                      // e_version
+  // Since this is for a relocatable object, there is no entry point,
+  // and no program headers.
+  Str.writeAddrOrOffset<IsELF64>(0);                                // e_entry
+  Str.writeAddrOrOffset<IsELF64>(0);                                // e_phoff
+  Str.writeAddrOrOffset<IsELF64>(SectionHeaderOffset);              // e_shoff
+  Str.writeELFWord<IsELF64>(getELFFlags(Ctx.getTargetArch()));      // e_flags
+  Str.writeLE16(IsELF64 ? sizeof(Elf64_Ehdr) : sizeof(Elf32_Ehdr)); // e_ehsize
+  Str.writeLE16(0); // e_phentsize
+  Str.writeLE16(0); // e_phnum
+  Str.writeLE16(IsELF64 ? sizeof(Elf64_Shdr)
+                        : sizeof(Elf32_Shdr));                // e_shentsize
+  Str.writeLE16(static_cast<Elf64_Half>(NumSections));        // e_shnum
+  Str.writeLE16(static_cast<Elf64_Half>(SectHeaderStrIndex)); // e_shstrndx
+}
+
+void ELFObjectWriter::writeNonUserSections() {
+  bool IsELF64 = isELF64(Ctx.getTargetArch());
+
+  // Write out the shstrtab now that all sections are known.
+  ShStrTab->doLayout();
+  ShStrTab->setSize(ShStrTab->getSectionDataSize());
+  Elf64_Off ShStrTabOffset = alignFileOffset(ShStrTab->getSectionAlign());
+  ShStrTab->setFileOffset(ShStrTabOffset);
+  Str.writeBytes(ShStrTab->getSectionData());
+
+  SectionList AllSections;
+  assignSectionNumbersInfo(AllSections);
+
+  // Finalize the regular StrTab and fix up references in the SymTab.
+  StrTab->doLayout();
+  StrTab->setSize(StrTab->getSectionDataSize());
+
+  SymTab->updateIndices(StrTab);
+
+  Elf64_Off SymTabOffset = alignFileOffset(SymTab->getSectionAlign());
+  SymTab->setFileOffset(SymTabOffset);
+  SymTab->setSize(SymTab->getSectionDataSize());
+  SymTab->writeData(Str, IsELF64);
+
+  Elf64_Off StrTabOffset = alignFileOffset(StrTab->getSectionAlign());
+  StrTab->setFileOffset(StrTabOffset);
+  Str.writeBytes(StrTab->getSectionData());
+
+  // TODO: Write out the relocation sections.
+  // May also be able to seek around the file and resolve function calls
+  // that are for functions within the same section.
+
+  // Write out the section headers.
+  const size_t ShdrAlign = IsELF64 ? alignof(Elf64_Shdr) : alignof(Elf32_Shdr);
+  Elf64_Off ShOffset = alignFileOffset(ShdrAlign);
+  for (const auto S : AllSections) {
+    if (IsELF64)
+      S->writeHeader<true>(Str);
+    else
+      S->writeHeader<false>(Str);
+  }
+
+  // Finally write the updated ELF header w/ the correct number of sections.
+  Str.seek(0);
+  if (IsELF64) {
+    writeELFHeaderInternal<true>(ShOffset, ShStrTab->getNumber(),
+                                 AllSections.size());
+  } else {
+    writeELFHeaderInternal<false>(ShOffset, ShStrTab->getNumber(),
+                                  AllSections.size());
+  }
+}
+
+} // end of namespace Ice