3 files changed, 25 insertions, 7 deletions

diff --git a/lld/lib/ReaderWriter/Native/NativeFileFormat.h b/lld/lib/ReaderWriter/Native/NativeFileFormat.h
index 5a715e54e37..6686eb340cd 100644
--- a/lld/lib/ReaderWriter/Native/NativeFileFormat.h
+++ b/lld/lib/ReaderWriter/Native/NativeFileFormat.h
@@ -164,10 +164,10 @@ struct NativeAtomAttributesV1 {
 struct NativeUndefinedAtomIvarsV1 {
   uint32_t  nameOffset;
   uint32_t  flags;
+  uint32_t  fallbackNameOffset;
 };
 
 
-
 //
 // The NCS_SharedLibraryAtomsV1 chunk contains an array of these structs
 //
diff --git a/lld/lib/ReaderWriter/Native/ReaderNative.cpp b/lld/lib/ReaderWriter/Native/ReaderNative.cpp
index 31024a06fab..0554dcecd1f 100644
--- a/lld/lib/ReaderWriter/Native/ReaderNative.cpp
+++ b/lld/lib/ReaderWriter/Native/ReaderNative.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "lld/ReaderWriter/Reader.h"
+#include "lld/ReaderWriter/Simple.h"
 
 #include "lld/Core/Atom.h"
 #include "lld/Core/Error.h"
@@ -134,10 +135,12 @@ public:
     return (CanBeNull)(_ivarData->flags & 0x3);
   }
 
+  virtual const UndefinedAtom *fallback() const;
 
 private:
   const File                        *_file;
   const NativeUndefinedAtomIvarsV1  *_ivarData;
+  mutable std::unique_ptr<const SimpleUndefinedAtom> _fallback;
 };
 
 
@@ -860,8 +863,14 @@ inline StringRef NativeUndefinedAtomV1::name() const {
   return _file->string(_ivarData->nameOffset);
 }
 
-
-
+inline const UndefinedAtom *NativeUndefinedAtomV1::fallback() const {
+  if (!_ivarData->fallbackNameOffset)
+    return nullptr;
+  if (!_fallback)
+    _fallback.reset(new SimpleUndefinedAtom(
+        *_file, _file->string(_ivarData->fallbackNameOffset)));
+  return _fallback.get();
+}
 
 inline const lld::File& NativeSharedLibraryAtomV1::file() const {
   return *_file;
diff --git a/lld/lib/ReaderWriter/YAML/ReaderWriterYAML.cpp b/lld/lib/ReaderWriter/YAML/ReaderWriterYAML.cpp
index 404bad73d99..5109097bd2f 100644
--- a/lld/lib/ReaderWriter/YAML/ReaderWriterYAML.cpp
+++ b/lld/lib/ReaderWriter/YAML/ReaderWriterYAML.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "lld/ReaderWriter/Reader.h"
+#include "lld/ReaderWriter/Simple.h"
 #include "lld/ReaderWriter/Writer.h"
 
 #include "lld/Core/ArchiveLibraryFile.h"
@@ -957,10 +958,13 @@ template <> struct MappingTraits<const lld::UndefinedAtom *> {
   class NormalizedAtom : public lld::UndefinedAtom {
   public:
     NormalizedAtom(IO &io)
-        : _file(fileFromContext(io)), _name(), _canBeNull(canBeNullNever) {}
+        : _file(fileFromContext(io)), _name(), _canBeNull(canBeNullNever),
+          _fallback(nullptr) {}
+
     NormalizedAtom(IO &io, const lld::UndefinedAtom *atom)
         : _file(fileFromContext(io)), _name(atom->name()),
-          _canBeNull(atom->canBeNull()) {}
+          _canBeNull(atom->canBeNull()), _fallback(atom->fallback()) {}
+
     const lld::UndefinedAtom *denormalize(IO &io) {
       ContextInfo *info = reinterpret_cast<ContextInfo *>(io.getContext());
       assert(info != nullptr);
@@ -972,10 +976,11 @@ template <> struct MappingTraits<const lld::UndefinedAtom *> {
 
       DEBUG_WITH_TYPE("WriterYAML",
                       llvm::dbgs() << "created UndefinedAtom named: '" << _name
-                                   << "' (" << (void *)_name.data() << ", "
-                                   << _name.size() << ")\n");
+                      << "' (" << (void *)_name.data() << ", "
+                      << _name.size() << ")\n");
       return this;
     }
+
     // Extract current File object from YAML I/O parsing context
     const lld::File &fileFromContext(IO &io) {
       ContextInfo *info = reinterpret_cast<ContextInfo *>(io.getContext());
@@ -987,10 +992,12 @@ template <> struct MappingTraits<const lld::UndefinedAtom *> {
     virtual const lld::File &file() const { return _file; }
     virtual StringRef name() const { return _name; }
     virtual CanBeNull canBeNull() const { return _canBeNull; }
+    virtual const UndefinedAtom *fallback() const { return _fallback; }
 
     const lld::File &_file;
     StringRef _name;
     CanBeNull _canBeNull;
+    const UndefinedAtom *_fallback;
   };
 
   static void mapping(IO &io, const lld::UndefinedAtom *&atom) {
@@ -1000,6 +1007,8 @@ template <> struct MappingTraits<const lld::UndefinedAtom *> {
     io.mapRequired("name", keys->_name);
     io.mapOptional("can-be-null", keys->_canBeNull,
                    lld::UndefinedAtom::canBeNullNever);
+    io.mapOptional("fallback", keys->_fallback,
+                   (const lld::UndefinedAtom *)nullptr);
   }
 };