CodeGen: support an extension to pass linker options on ELF

Introduce an extension to support passing linker options to the linker.
These would be ignored by older linkers, but newer linkers which support
this feature would be able to process the linker.

Emit a special discarded section `.linker-option`.  The content of this
section is a pair of strings (key, value).  The key is a type identifier for
the parameter.  This allows for an argument free parameter that will be
processed by the linker with the value being the parameter.  As an example,
`lib` identifies a library to be linked against, traditionally the `-l`
argument for Unix-based linkers with the parameter being the library name.

Thanks to James Henderson, Cary Coutant, Rafael Espinolda, Sean Silva
for the valuable discussion on the design of this feature.

llvm-svn: 323783

llvm/docs/Extensions.rst

@@ -221,6 +221,44 @@ which is equivalent to just
         .section .foo,"a",@progbits
         .section .bar,"ao",@progbits,.foo
 
+``.linker-options`` Section (linker options)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In order to support passing linker options from the frontend to the linker, a
+special section of type ``SHT_LLVM_LINKER_OPTIONS`` (usually named
+``.linker-options`` though the name is not significant as it is identified by
+the type).  The contents of this section is a simple pair-wise encoding of
+options for consideration by the linker.  The strings are encoded as standard
+null-terminated UTF-8 strings.  They are emitted inline to avoid having the
+linker to traverse the object file for retrieving the value.  The linker is
+permitted to not honour the option and instead provide a warning/error to the
+user that the requested option was not honoured.
+
+The section is marked as ``SHT_LLVM_LINKER_OPTIONS`` and has the ``SHF_EXCLUDE``
+flag to ensure that the section is treated as opaque by linkers which do not
+support the feature and will not be emitted into the final linked binary.
+
+This would be equivalent to the follow raw assembly:
+
+.. code-block:: gas
+
+  .section ".linker-options","e",@llvm_linker_options
+  .asciz "option 1"
+  .asciz "value 1"
+  .asciz "option 2"
+  .asciz "value 2"
+
+LLVM emits the following options:
+  - lib
+
+    The parameter identifies a library to be linked against.  The library will
+    be looked up in the default and any specified library search paths
+    (specified to this point).
+
+  - path
+
+    The paramter identifies an additional library search path to be considered
+    when looking up libraries after the inclusion of this option.
 
 Target Specific Behaviour
 =========================

llvm/include/llvm/BinaryFormat/ELF.h

@@ -714,36 +714,37 @@ enum {
 
 // Section types.
 enum : unsigned {
-  SHT_NULL = 0,                    // No associated section (inactive entry).
-  SHT_PROGBITS = 1,                // Program-defined contents.
-  SHT_SYMTAB = 2,                  // Symbol table.
-  SHT_STRTAB = 3,                  // String table.
-  SHT_RELA = 4,                    // Relocation entries; explicit addends.
-  SHT_HASH = 5,                    // Symbol hash table.
-  SHT_DYNAMIC = 6,                 // Information for dynamic linking.
-  SHT_NOTE = 7,                    // Information about the file.
-  SHT_NOBITS = 8,                  // Data occupies no space in the file.
-  SHT_REL = 9,                     // Relocation entries; no explicit addends.
-  SHT_SHLIB = 10,                  // Reserved.
-  SHT_DYNSYM = 11,                 // Symbol table.
-  SHT_INIT_ARRAY = 14,             // Pointers to initialization functions.
-  SHT_FINI_ARRAY = 15,             // Pointers to termination functions.
-  SHT_PREINIT_ARRAY = 16,          // Pointers to pre-init functions.
-  SHT_GROUP = 17,                  // Section group.
-  SHT_SYMTAB_SHNDX = 18,           // Indices for SHN_XINDEX entries.
-  SHT_LOOS = 0x60000000,           // Lowest operating system-specific type.
+  SHT_NULL = 0,                         // No associated section (inactive entry).
+  SHT_PROGBITS = 1,                     // Program-defined contents.
+  SHT_SYMTAB = 2,                       // Symbol table.
+  SHT_STRTAB = 3,                       // String table.
+  SHT_RELA = 4,                         // Relocation entries; explicit addends.
+  SHT_HASH = 5,                         // Symbol hash table.
+  SHT_DYNAMIC = 6,                      // Information for dynamic linking.
+  SHT_NOTE = 7,                         // Information about the file.
+  SHT_NOBITS = 8,                       // Data occupies no space in the file.
+  SHT_REL = 9,                          // Relocation entries; no explicit addends.
+  SHT_SHLIB = 10,                       // Reserved.
+  SHT_DYNSYM = 11,                      // Symbol table.
+  SHT_INIT_ARRAY = 14,                  // Pointers to initialization functions.
+  SHT_FINI_ARRAY = 15,                  // Pointers to termination functions.
+  SHT_PREINIT_ARRAY = 16,               // Pointers to pre-init functions.
+  SHT_GROUP = 17,                       // Section group.
+  SHT_SYMTAB_SHNDX = 18,                // Indices for SHN_XINDEX entries.
+  SHT_LOOS = 0x60000000,                // Lowest operating system-specific type.
   // Android packed relocation section types.
   // https://android.googlesource.com/platform/bionic/+/6f12bfece5dcc01325e0abba56a46b1bcf991c69/tools/relocation_packer/src/elf_file.cc#37
   SHT_ANDROID_REL = 0x60000001,
   SHT_ANDROID_RELA = 0x60000002,
-  SHT_LLVM_ODRTAB = 0x6fff4c00,    // LLVM ODR table.
-  SHT_GNU_ATTRIBUTES = 0x6ffffff5, // Object attributes.
-  SHT_GNU_HASH = 0x6ffffff6,       // GNU-style hash table.
-  SHT_GNU_verdef = 0x6ffffffd,     // GNU version definitions.
-  SHT_GNU_verneed = 0x6ffffffe,    // GNU version references.
-  SHT_GNU_versym = 0x6fffffff,     // GNU symbol versions table.
-  SHT_HIOS = 0x6fffffff,           // Highest operating system-specific type.
-  SHT_LOPROC = 0x70000000,         // Lowest processor arch-specific type.
+  SHT_LLVM_ODRTAB = 0x6fff4c00,         // LLVM ODR table.
+  SHT_LLVM_LINKER_OPTIONS = 0x6fff4c01, // LLVM Linker Options.
+  SHT_GNU_ATTRIBUTES = 0x6ffffff5,      // Object attributes.
+  SHT_GNU_HASH = 0x6ffffff6,            // GNU-style hash table.
+  SHT_GNU_verdef = 0x6ffffffd,          // GNU version definitions.
+  SHT_GNU_verneed = 0x6ffffffe,         // GNU version references.
+  SHT_GNU_versym = 0x6fffffff,          // GNU symbol versions table.
+  SHT_HIOS = 0x6fffffff,                // Highest operating system-specific type.
+  SHT_LOPROC = 0x70000000,              // Lowest processor arch-specific type.
   // Fixme: All this is duplicated in MCSectionELF. Why??
   // Exception Index table
   SHT_ARM_EXIDX = 0x70000001U,
@@ -753,18 +754,18 @@ enum : unsigned {
   SHT_ARM_ATTRIBUTES = 0x70000003U,
   SHT_ARM_DEBUGOVERLAY = 0x70000004U,
   SHT_ARM_OVERLAYSECTION = 0x70000005U,
-  SHT_HEX_ORDERED = 0x70000000,   // Link editor is to sort the entries in
-                                  // this section based on their sizes
-  SHT_X86_64_UNWIND = 0x70000001, // Unwind information
-
-  SHT_MIPS_REGINFO = 0x70000006,  // Register usage information
-  SHT_MIPS_OPTIONS = 0x7000000d,  // General options
-  SHT_MIPS_DWARF = 0x7000001e,    // DWARF debugging section.
-  SHT_MIPS_ABIFLAGS = 0x7000002a, // ABI information.
-
-  SHT_HIPROC = 0x7fffffff, // Highest processor arch-specific type.
-  SHT_LOUSER = 0x80000000, // Lowest type reserved for applications.
-  SHT_HIUSER = 0xffffffff  // Highest type reserved for applications.
+  SHT_HEX_ORDERED = 0x70000000,         // Link editor is to sort the entries in
+                                        // this section based on their sizes
+  SHT_X86_64_UNWIND = 0x70000001,       // Unwind information
+
+  SHT_MIPS_REGINFO = 0x70000006,        // Register usage information
+  SHT_MIPS_OPTIONS = 0x7000000d,        // General options
+  SHT_MIPS_DWARF = 0x7000001e,          // DWARF debugging section.
+  SHT_MIPS_ABIFLAGS = 0x7000002a,       // ABI information.
+
+  SHT_HIPROC = 0x7fffffff,              // Highest processor arch-specific type.
+  SHT_LOUSER = 0x80000000,              // Lowest type reserved for applications.
+  SHT_HIUSER = 0xffffffff               // Highest type reserved for applications.
 };
 
 // Section flags.

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -93,6 +93,24 @@ static void GetObjCImageInfo(Module &M, unsigned &Version, unsigned &Flags,
 
 void TargetLoweringObjectFileELF::emitModuleMetadata(
     MCStreamer &Streamer, Module &M, const TargetMachine &TM) const {
+  auto &C = getContext();
+
+  if (NamedMDNode *LinkerOptions = M.getNamedMetadata("llvm.linker.options")) {
+    auto *S = C.getELFSection(".linker-options", ELF::SHT_LLVM_LINKER_OPTIONS,
+                              ELF::SHF_EXCLUDE);
+
+    Streamer.SwitchSection(S);
+
+    for (const auto &Operand : LinkerOptions->operands()) {
+      if (cast<MDNode>(Operand)->getNumOperands() != 2)
+        report_fatal_error("invalid llvm.linker.options");
+      for (const auto &Option : cast<MDNode>(Operand)->operands()) {
+        Streamer.EmitBytes(cast<MDString>(Option)->getString());
+        Streamer.EmitIntValue(0, 1);
+      }
+    }
+  }
+
   unsigned Version = 0;
   unsigned Flags = 0;
   StringRef Section;
@@ -101,7 +119,6 @@ void TargetLoweringObjectFileELF::emitModuleMetadata(
   if (Section.empty())
     return;
 
-  auto &C = getContext();
   auto *S = C.getELFSection(Section, ELF::SHT_PROGBITS, ELF::SHF_ALLOC);
   Streamer.SwitchSection(S);
   Streamer.EmitLabel(C.getOrCreateSymbol(StringRef("OBJC_IMAGE_INFO")));

llvm/lib/MC/MCParser/ELFAsmParser.cpp

@@ -608,6 +608,8 @@ bool ELFAsmParser::ParseSectionArguments(bool IsPush, SMLoc loc) {
       Type = ELF::SHT_X86_64_UNWIND;
     else if (TypeName == "llvm_odrtab")
       Type = ELF::SHT_LLVM_ODRTAB;
+    else if (TypeName == "llvm_linker_options")
+      Type = ELF::SHT_LLVM_LINKER_OPTIONS;
     else if (TypeName.getAsInteger(0, Type))
       return TokError("unknown section type");
   }

llvm/lib/MC/MCSectionELF.cpp

@@ -148,6 +148,8 @@ void MCSectionELF::PrintSwitchToSection(const MCAsmInfo &MAI, const Triple &T,
     OS << "0x7000001e";
   else if (Type == ELF::SHT_LLVM_ODRTAB)
     OS << "llvm_odrtab";
+  else if (Type == ELF::SHT_LLVM_LINKER_OPTIONS)
+    OS << "llvm_linker_options";
   else
     report_fatal_error("unsupported type 0x" + Twine::utohexstr(Type) +
                        " for section " + getSectionName());

llvm/lib/Object/ELF.cpp

@@ -205,6 +205,7 @@ StringRef llvm::object::getELFSectionTypeName(uint32_t Machine, unsigned Type) {
     STRINGIFY_ENUM_CASE(ELF, SHT_ANDROID_REL);
     STRINGIFY_ENUM_CASE(ELF, SHT_ANDROID_RELA);
     STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_ODRTAB);
+    STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_LINKER_OPTIONS);
     STRINGIFY_ENUM_CASE(ELF, SHT_GNU_ATTRIBUTES);
     STRINGIFY_ENUM_CASE(ELF, SHT_GNU_HASH);
     STRINGIFY_ENUM_CASE(ELF, SHT_GNU_verdef);

llvm/lib/ObjectYAML/ELFYAML.cpp

@@ -408,6 +408,7 @@ void ScalarEnumerationTraits<ELFYAML::ELF_SHT>::enumeration(
   ECase(SHT_ANDROID_REL);
   ECase(SHT_ANDROID_RELA);
   ECase(SHT_LLVM_ODRTAB);
+  ECase(SHT_LLVM_LINKER_OPTIONS);
   ECase(SHT_GNU_ATTRIBUTES);
   ECase(SHT_GNU_HASH);
   ECase(SHT_GNU_verdef);

llvm/test/Feature/elf-linker-options.ll

@@ -0,0 +1,16 @@
+; RUN: llc -mtriple x86_64-elf -filetype asm -o - %s | FileCheck %s
+
+!llvm.linker.options = !{!0, !1}
+
+!0 = !{!"option 0", !"value 0"}
+!1 = !{!"option 1", !"value 1"}
+
+; CHECK: .section ".linker-options","e",@llvm_linker_options
+; CHECK-NEXT: .ascii  "option 0"
+; CHECK-NEXT: .byte   0
+; CHECK-NEXT: .ascii  "value 0"
+; CHECK-NEXT: .byte   0
+; CHECK-NEXT: .ascii  "option 1"
+; CHECK-NEXT: .byte   0
+; CHECK-NEXT: .ascii  "value 1"
+; CHECK-NEXT: .byte   0

llvm/test/MC/ELF/section.s

@@ -279,3 +279,17 @@ bar:
 // CHECK-NEXT:     Flags [
 // CHECK-NEXT:       SHF_EXCLUDE
 // CHECK-NEXT:     ]
+
+// Test SHT_LLVM_LINKER_OPTIONS
+
+.section ".linker-options","e",@llvm_linker_options
+// ASM: .section ".linker-options","e",@llvm_linker_options
+
+// CHECK: Section {
+// CHECK:   Name: .linker-options
+// CHECK-NEXT:   Type: SHT_LLVM_LINKER_OPTIONS
+// CHECK-NEXT:   Flags [
+// CHECK-NEXT:     SHF_EXCLUDE
+// CHECK-NEXT:   ]
+// CHECK: }
+

llvm/test/tools/llvm-readobj/elf-linker-options.ll

@@ -0,0 +1,11 @@
+; RUN: llc -mtriple x86_64-elf -filetype obj -o - %s | llvm-readobj -elf-linker-options - | FileCheck %s
+
+!llvm.linker.options = !{!0, !1}
+
+!0 = !{!"option 0", !"value 0"}
+!1 = !{!"option 1", !"value 1"}
+
+; CHECK: LinkerOptions [
+; CHECK:  option 0: value 0
+; CHECK:  option 1: value 1
+; CHECK: ]

llvm/tools/llvm-readobj/ELFDumper.cpp

@@ -164,6 +164,8 @@ class ELFDumper : public ObjDumper {
 
   void printNotes() override;
 
+  void printELFLinkerOptions() override;
+
 private:
   std::unique_ptr<DumpStyle<ELFT>> ELFDumperStyle;
 
@@ -316,6 +318,7 @@ template <typename ELFT> class DumpStyle {
   virtual void printProgramHeaders(const ELFFile<ELFT> *Obj) = 0;
   virtual void printHashHistogram(const ELFFile<ELFT> *Obj) = 0;
   virtual void printNotes(const ELFFile<ELFT> *Obj) = 0;
+  virtual void printELFLinkerOptions(const ELFFile<ELFT> *Obj) = 0;
   virtual void printMipsGOT(const MipsGOTParser<ELFT> &Parser) = 0;
   virtual void printMipsPLT(const MipsGOTParser<ELFT> &Parser) = 0;
   const ELFDumper<ELFT> *dumper() const { return Dumper; }
@@ -345,6 +348,7 @@ template <typename ELFT> class GNUStyle : public DumpStyle<ELFT> {
   void printProgramHeaders(const ELFO *Obj) override;
   void printHashHistogram(const ELFFile<ELFT> *Obj) override;
   void printNotes(const ELFFile<ELFT> *Obj) override;
+  void printELFLinkerOptions(const ELFFile<ELFT> *Obj) override;
   void printMipsGOT(const MipsGOTParser<ELFT> &Parser) override;
   void printMipsPLT(const MipsGOTParser<ELFT> &Parser) override;
 
@@ -405,6 +409,7 @@ template <typename ELFT> class LLVMStyle : public DumpStyle<ELFT> {
   void printProgramHeaders(const ELFO *Obj) override;
   void printHashHistogram(const ELFFile<ELFT> *Obj) override;
   void printNotes(const ELFFile<ELFT> *Obj) override;
+  void printELFLinkerOptions(const ELFFile<ELFT> *Obj) override;
   void printMipsGOT(const MipsGOTParser<ELFT> &Parser) override;
   void printMipsPLT(const MipsGOTParser<ELFT> &Parser) override;
 
@@ -1501,6 +1506,10 @@ template <class ELFT> void ELFDumper<ELFT>::printNotes() {
   ELFDumperStyle->printNotes(Obj);
 }
 
+template <class ELFT> void ELFDumper<ELFT>::printELFLinkerOptions() {
+  ELFDumperStyle->printELFLinkerOptions(Obj);
+}
+
 #define LLVM_READOBJ_TYPE_CASE(name) \
   case DT_##name: return #name
 
@@ -2694,6 +2703,8 @@ std::string getSectionTypeString(unsigned Arch, unsigned Type) {
     return "SYMTAB SECTION INDICES";
   case SHT_LLVM_ODRTAB:
     return "LLVM_ODRTAB";
+  case SHT_LLVM_LINKER_OPTIONS:
+    return "LLVM_LINKER_OPTIONS";
   // FIXME: Parse processor specific GNU attributes
   case SHT_GNU_ATTRIBUTES:
     return "ATTRIBUTES";
@@ -3554,6 +3565,11 @@ void GNUStyle<ELFT>::printNotes(const ELFFile<ELFT> *Obj) {
   }
 }
 
+template <class ELFT>
+void GNUStyle<ELFT>::printELFLinkerOptions(const ELFFile<ELFT> *Obj) {
+  OS << "printELFLinkerOptions not implemented!\n";
+}
+
 template <class ELFT>
 void GNUStyle<ELFT>::printMipsGOT(const MipsGOTParser<ELFT> &Parser) {
   size_t Bias = ELFT::Is64Bits ? 8 : 0;
@@ -4062,6 +4078,27 @@ void LLVMStyle<ELFT>::printNotes(const ELFFile<ELFT> *Obj) {
   W.startLine() << "printNotes not implemented!\n";
 }
 
+template <class ELFT>
+void LLVMStyle<ELFT>::printELFLinkerOptions(const ELFFile<ELFT> *Obj) {
+  ListScope L(W, "LinkerOptions");
+
+  for (const Elf_Shdr &Shdr : unwrapOrError(Obj->sections())) {
+    if (Shdr.sh_type != ELF::SHT_LLVM_LINKER_OPTIONS)
+      continue;
+
+    ArrayRef<uint8_t> Contents = unwrapOrError(Obj->getSectionContents(&Shdr));
+    for (const uint8_t *P = Contents.begin(), *E = Contents.end(); P < E; ) {
+      StringRef Key = StringRef(reinterpret_cast<const char *>(P));
+      StringRef Value =
+          StringRef(reinterpret_cast<const char *>(P) + Key.size() + 1);
+
+      W.printString(Key, Value);
+
+      P = P + Key.size() + Value.size() + 2;
+    }
+  }
+}
+
 template <class ELFT>
 void LLVMStyle<ELFT>::printMipsGOT(const MipsGOTParser<ELFT> &Parser) {
   auto PrintEntry = [&](const Elf_Addr *E) {

llvm/tools/llvm-readobj/ObjDumper.h

@@ -48,6 +48,7 @@ class ObjDumper {
   virtual void printGroupSections() {}
   virtual void printHashHistogram() {}
   virtual void printNotes() {}
+  virtual void printELFLinkerOptions() {}
 
   // Only implemented for ARM ELF at this time.
   virtual void printAttributes() { }

llvm/tools/llvm-readobj/llvm-readobj.cpp

@@ -229,6 +229,11 @@ namespace opts {
   COFFLoadConfig("coff-load-config",
                  cl::desc("Display the PE/COFF load config"));
 
+  // -elf-linker-options
+  cl::opt<bool>
+  ELFLinkerOptions("elf-linker-options",
+                   cl::desc("Display the ELF .linker-options section"));
+
   // -macho-data-in-code
   cl::opt<bool>
   MachODataInCode("macho-data-in-code",
@@ -419,6 +424,8 @@ static void dumpObject(const ObjectFile *Obj, ScopedPrinter &Writer) {
   if (opts::VersionInfo)
     Dumper->printVersionInfo();
   if (Obj->isELF()) {
+    if (opts::ELFLinkerOptions)
+      Dumper->printELFLinkerOptions();
     if (Obj->getArch() == llvm::Triple::arm)
       if (opts::ARMAttributes)
         Dumper->printAttributes();