[yaml2obj/obj2yaml] - Add support for SHT_LLVM_DEPENDENT_LIBRARIES sections.

This section contains strings specifying libraries to be added to the link by the linker.
The strings are encoded as standard null-terminated UTF-8 strings.

This patch adds a way to describe and dump SHT_LLVM_DEPENDENT_LIBRARIES sections.

I introduced a new YAMLFlowString type here. That used to teach obj2yaml to dump
them like:

```
Libraries: [ foo, bar ]
```

instead of the following (if StringRef would be used):

```
Libraries:
  - foo
  - bar
```

Differential revision: https://reviews.llvm.org/D70598

2 files changed, 66 insertions, 0 deletions

diff --git a/llvm/lib/ObjectYAML/ELFEmitter.cpp b/llvm/lib/ObjectYAML/ELFEmitter.cpp
index 1a6f0a8337a..e8b54a7e602 100644
--- a/llvm/lib/ObjectYAML/ELFEmitter.cpp
+++ b/llvm/lib/ObjectYAML/ELFEmitter.cpp
@@ -203,6 +203,9 @@ template <class ELFT> class ELFState {
   void writeSectionContent(Elf_Shdr &SHeader,
                            const ELFYAML::LinkerOptionsSection &Section,
                            ContiguousBlobAccumulator &CBA);
+  void writeSectionContent(Elf_Shdr &SHeader,
+                           const ELFYAML::DependentLibrariesSection &Section,
+                           ContiguousBlobAccumulator &CBA);
 
   void writeFill(ELFYAML::Fill &Fill, ContiguousBlobAccumulator &CBA);
 
@@ -475,6 +478,8 @@ void ELFState<ELFT>::initSectionHeaders(std::vector<Elf_Shdr> &SHeaders,
       writeSectionContent(SHeader, *S, CBA);
     } else if (auto S = dyn_cast<ELFYAML::GnuHashSection>(Sec)) {
       writeSectionContent(SHeader, *S, CBA);
+    } else if (auto S = dyn_cast<ELFYAML::DependentLibrariesSection>(Sec)) {
+      writeSectionContent(SHeader, *S, CBA);
     } else {
       llvm_unreachable("Unknown section type");
     }
@@ -922,6 +927,28 @@ void ELFState<ELFT>::writeSectionContent(
 }
 
 template <class ELFT>
+void ELFState<ELFT>::writeSectionContent(
+    Elf_Shdr &SHeader, const ELFYAML::DependentLibrariesSection &Section,
+    ContiguousBlobAccumulator &CBA) {
+  raw_ostream &OS =
+      CBA.getOSAndAlignedOffset(SHeader.sh_offset, SHeader.sh_addralign);
+
+  if (Section.Content) {
+    SHeader.sh_size = writeContent(OS, Section.Content, None);
+    return;
+  }
+
+  if (!Section.Libs)
+    return;
+
+  for (StringRef Lib : *Section.Libs) {
+    OS.write(Lib.data(), Lib.size());
+    OS.write('\0');
+    SHeader.sh_size += Lib.size() + 1;
+  }
+}
+
+template <class ELFT>
 void ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
                                          const ELFYAML::HashSection &Section,
                                          ContiguousBlobAccumulator &CBA) {
diff --git a/llvm/lib/ObjectYAML/ELFYAML.cpp b/llvm/lib/ObjectYAML/ELFYAML.cpp
index bc546b19ff3..a5e5894af04 100644
--- a/llvm/lib/ObjectYAML/ELFYAML.cpp
+++ b/llvm/lib/ObjectYAML/ELFYAML.cpp
@@ -863,6 +863,24 @@ template <> struct SequenceElementTraits<StOtherPiece> {
   static const bool flow = true;
 };
 
+template <> struct ScalarTraits<ELFYAML::YAMLFlowString> {
+  static void output(const ELFYAML::YAMLFlowString &Val, void *,
+                     raw_ostream &Out) {
+    Out << Val;
+  }
+  static StringRef input(StringRef Scalar, void *,
+                         ELFYAML::YAMLFlowString &Val) {
+    Val = Scalar;
+    return {};
+  }
+  static QuotingType mustQuote(StringRef S) {
+    return ScalarTraits<StringRef>::mustQuote(S);
+  }
+};
+template <> struct SequenceElementTraits<ELFYAML::YAMLFlowString> {
+  static const bool flow = true;
+};
+
 namespace {
 
 struct NormalizedOther {
@@ -1106,6 +1124,13 @@ static void sectionMapping(IO &IO, ELFYAML::LinkerOptionsSection &Section) {
   IO.mapOptional("Content", Section.Content);
 }
 
+static void sectionMapping(IO &IO,
+                           ELFYAML::DependentLibrariesSection &Section) {
+  commonSectionMapping(IO, Section);
+  IO.mapOptional("Libraries", Section.Libs);
+  IO.mapOptional("Content", Section.Content);
+}
+
 void MappingTraits<ELFYAML::SectionOrType>::mapping(
     IO &IO, ELFYAML::SectionOrType &sectionOrType) {
   IO.mapRequired("SectionOrType", sectionOrType.sectionNameOrType);
@@ -1228,6 +1253,12 @@ void MappingTraits<std::unique_ptr<ELFYAML::Chunk>>::mapping(
       Section.reset(new ELFYAML::LinkerOptionsSection());
     sectionMapping(IO, *cast<ELFYAML::LinkerOptionsSection>(Section.get()));
     break;
+  case ELF::SHT_LLVM_DEPENDENT_LIBRARIES:
+    if (!IO.outputting())
+      Section.reset(new ELFYAML::DependentLibrariesSection());
+    sectionMapping(IO,
+                   *cast<ELFYAML::DependentLibrariesSection>(Section.get()));
+    break;
   default:
     if (!IO.outputting()) {
       StringRef Name;
@@ -1372,6 +1403,14 @@ StringRef MappingTraits<std::unique_ptr<ELFYAML::Chunk>>::validate(
     return {};
   }
 
+  if (const auto *Sec = dyn_cast<ELFYAML::DependentLibrariesSection>(C.get())) {
+    if (Sec->Libs && Sec->Content)
+      return "SHT_LLVM_DEPENDENT_LIBRARIES: \"Libraries\" and \"Content\" "
+             "can't "
+             "be used together";
+    return {};
+  }
+
   if (const auto *F = dyn_cast<ELFYAML::Fill>(C.get())) {
     if (!F->Pattern)
       return {};