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diff --git a/include/lld/Core/UndefinedAtom.h b/include/lld/Core/UndefinedAtom.h
new file mode 100644
index 000000000000..7a835a4ebaa8
--- /dev/null
+++ b/include/lld/Core/UndefinedAtom.h
@@ -0,0 +1,74 @@
+//===- Core/UndefinedAtom.h - An Undefined Atom ---------------------------===//
+//
+//                             The LLVM Linker
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_CORE_UNDEFINED_ATOM_H
+#define LLD_CORE_UNDEFINED_ATOM_H
+
+#include "lld/Core/Atom.h"
+
+namespace lld {
+
+/// An UndefinedAtom has no content.
+/// It exists as a placeholder for a future atom.
+class UndefinedAtom : public Atom {
+public:
+  /// Whether this undefined symbol needs to be resolved,
+  /// or whether it can just evaluate to nullptr.
+  /// This concept is often called "weak", but that term
+  /// is overloaded to mean other things too.
+  enum CanBeNull {
+    /// Normal symbols must be resolved at build time
+    canBeNullNever,
+
+    /// This symbol can be missing at runtime and will evalute to nullptr.
+    /// That is, the static linker still must find a definition (usually
+    /// is some shared library), but at runtime, the dynamic loader
+    /// will allow the symbol to be missing and resolved to nullptr.
+    ///
+    /// On Darwin this is generated using a function prototype with
+    /// __attribute__((weak_import)).
+    /// On linux this is generated using a function prototype with
+    ///  __attribute__((weak)).
+    /// On Windows this feature is not supported.
+    canBeNullAtRuntime,
+
+    /// This symbol can be missing at build time.
+    /// That is, the static linker will not error if a definition for
+    /// this symbol is not found at build time. Instead, the linker
+    /// will build an executable that lets the dynamic loader find the
+    /// symbol at runtime.
+    /// This feature is not supported on Darwin nor Windows.
+    /// On linux this is generated using a function prototype with
+    ///  __attribute__((weak)).
+    canBeNullAtBuildtime
+  };
+
+  virtual CanBeNull canBeNull() const = 0;
+
+  static bool classof(const Atom *a) {
+    return a->definition() == definitionUndefined;
+  }
+
+  static bool classof(const UndefinedAtom *) { return true; }
+
+  /// Returns an undefined atom if this undefined symbol has a synonym.  This is
+  /// mainly used in COFF. In COFF, an unresolved external symbol can have up to
+  /// one optional name (sym2) in addition to its regular name (sym1). If a
+  /// definition of sym1 exists, sym1 is resolved normally. Otherwise, all
+  /// references to sym1 refer to sym2 instead. In that case sym2 must be
+  /// resolved, or link will fail.
+  virtual const UndefinedAtom *fallback() const { return nullptr; }
+
+protected:
+  UndefinedAtom() : Atom(definitionUndefined) {}
+};
+
+} // namespace lld
+
+#endif // LLD_CORE_UNDEFINED_ATOM_H