Index: include/lld/Core/DefinedAtom.h
===================================================================
--- include/lld/Core/DefinedAtom.h
+++ include/lld/Core/DefinedAtom.h
@@ -229,6 +229,9 @@
   /// not by the OS loader.
   virtual Scope scope() const = 0;
 
+  /// \brief Alter the visibility of this atom.
+  virtual void setScope(Scope);
+
   /// \brief Whether the linker should use direct or indirect access to this
   /// atom.
   virtual Interposable interposable() const = 0;
Index: include/lld/Core/LinkingContext.h
===================================================================
--- include/lld/Core/LinkingContext.h
+++ include/lld/Core/LinkingContext.h
@@ -224,6 +224,21 @@
   }
   InputGraph &getInputGraph() const { return *_inputGraph; }
 
+  /// Notify the LinkingContext when an atom is added to the symbol table.
+  /// This is an opportunity for platform specific work to be done such
+  /// as adjusting the atom's scope.
+  virtual void notifySymbolTableAdd(const Atom *atom) const {
+  }
+
+  /// Notify the LinkingContext when the symbol table found a name collision.
+  /// The useNew parameter specifies which the symbol table plans to keep,
+  /// but that can be changed by the LinkingContext.  This is also an
+  /// opportunity for the platform specific processing.
+  virtual void notifySymbolTableCoalesce(const Atom *existingAtom,
+                                         const Atom *newAtom,
+                                         bool &useNew) const {
+  }
+
   /// This method adds undefined symbols specified by the -u option to the to
   /// the list of undefined symbols known to the linker. This option essentially
   /// forces an undefined symbol to be created. You may also need to call
Index: include/lld/ReaderWriter/MachOLinkingContext.h
===================================================================
--- include/lld/ReaderWriter/MachOLinkingContext.h
+++ include/lld/ReaderWriter/MachOLinkingContext.h
@@ -15,6 +15,7 @@
 #include "lld/ReaderWriter/Writer.h"
 
 #include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MachO.h"
 
@@ -25,7 +26,7 @@
 namespace lld {
 
 namespace mach_o {
-class ArchHandler; 
+class ArchHandler;
 class MachODylibFile;
 }
 
@@ -51,6 +52,12 @@
     iOS_simulator
   };
 
+  enum class ExportMode {
+    globals,    // Default, all global symbols exported.
+    whiteList,  // -exported_symbols_list, only listed symbols exported.
+    blackList   // -unexported_symbols_list, no listed symbol exported.
+  };
+
   /// Initializes the context to sane default values given the specified output
   /// file type, arch, os, and minimum os version.  This should be called before
   /// other setXXX() methods.
@@ -59,6 +66,9 @@
   void addPasses(PassManager &pm) override;
   bool validateImpl(raw_ostream &diagnostics) override;
   bool createImplicitFiles(std::vector<std::unique_ptr<File>> &) const override;
+  void notifySymbolTableAdd(const Atom *atom) const override;
+  void notifySymbolTableCoalesce(const Atom *existingAtom, const Atom *newAtom,
+                                 bool &useNew) const override;
 
   uint32_t getCPUType() const;
   uint32_t getCPUSubType() const;
@@ -79,6 +89,10 @@
   StringRef archName() const { return nameFromArch(_arch); }
   OS os() const { return _os; }
 
+  ExportMode exportMode() { return _exportMode; }
+  void setExportMode(ExportMode mode) { _exportMode = mode; }
+  void addExportedSymbol(StringRef sym);
+
   bool minOS(StringRef mac, StringRef iOS) const;
   void setDoNothing(bool value) { _doNothing = value; }
   bool doNothing() const { return _doNothing; }
@@ -222,6 +236,8 @@
 private:
   Writer &writer() const override;
   mach_o::MachODylibFile* loadIndirectDylib(StringRef path) const;
+  void checkExportWhiteList(const DefinedAtom *atom) const;
+  void checkExportBlackList(const DefinedAtom *atom) const;
 
 
   struct ArchInfo {
@@ -265,6 +281,8 @@
   llvm::StringMap<mach_o::MachODylibFile*> _pathToDylibMap;
   std::set<mach_o::MachODylibFile*> _allDylibs;
   mutable std::vector<std::unique_ptr<class MachOFileNode>> _indirectDylibs;
+  ExportMode _exportMode;
+  llvm::StringSet<> _exportedSymbols;
 };
 
 } // end namespace lld
Index: lib/Core/DefinedAtom.cpp
===================================================================
--- lib/Core/DefinedAtom.cpp
+++ lib/Core/DefinedAtom.cpp
@@ -7,10 +7,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/Support/ErrorHandling.h"
 
 #include "lld/Core/DefinedAtom.h"
 
+#include "llvm/Support/ErrorHandling.h"
+
+#include <cassert>
 
 namespace lld {
 
@@ -81,5 +83,10 @@
 }
 
 
+void DefinedAtom::setScope(Scope s) {
+  assert(s == scope() && "setScope() must be overridden to alter scope");
+}
+
+
 } // namespace
 
Index: lib/Core/SymbolTable.cpp
===================================================================
--- lib/Core/SymbolTable.cpp
+++ lib/Core/SymbolTable.cpp
@@ -171,6 +171,7 @@
   const Atom *existing = findByName(name);
   if (existing == nullptr) {
     // Name is not in symbol table yet, add it associate with this atom.
+    _context.notifySymbolTableAdd(&newAtom);
     _nameTable[name] = &newAtom;
     return true;
   }
@@ -299,6 +300,9 @@
     break;
   }
 
+  // Give context a chance to change which is kept.
+  _context.notifySymbolTableCoalesce(existing, &newAtom, useNew);
+
   if (useNew) {
     // Update name table to use new atom.
     _nameTable[name] = &newAtom;
Index: lib/Driver/DarwinLdDriver.cpp
===================================================================
--- lib/Driver/DarwinLdDriver.cpp
+++ lib/Driver/DarwinLdDriver.cpp
@@ -88,6 +88,31 @@
                                           new MachOFileNode(path, forceLoad)));
 }
 
+// Export lists are one symbol per line.  Blank lines are ignored.
+// Trailing comments start with #.
+std::error_code parseExportsList(StringRef exportFilePath,
+                                 MachOLinkingContext &ctx,
+                                 raw_ostream &diagnostics) {
+  // Map in export list file.
+  ErrorOr<std::unique_ptr<MemoryBuffer>> mb =
+                                   MemoryBuffer::getFileOrSTDIN(exportFilePath);
+  if (std::error_code ec = mb.getError())
+    return ec;
+  StringRef buffer = mb->get()->getBuffer();
+  while (!buffer.empty()) {
+    // Split off each line in the file.
+    std::pair<StringRef, StringRef> lineAndRest = buffer.split('\n');
+    StringRef line = lineAndRest.first;
+    // Ignore trailing # comments.
+    std::pair<StringRef, StringRef> symAndComment = line.split('#');
+    StringRef sym = symAndComment.first.trim();
+    if (!sym.empty())
+      ctx.addExportedSymbol(sym);
+    buffer = lineAndRest.second;
+  }
+  return std::error_code();
+}
+
 //
 // There are two variants of the  -filelist option:
 //
@@ -349,7 +374,7 @@
     ctx.appendLLVMOption(llvmArg->getValue());
   }
 
-  // Handle -print_atoms a
+  // Handle -print_atoms
   if (parsedArgs->getLastArg(OPT_print_atoms))
     ctx.setPrintAtoms();
 
@@ -427,6 +452,42 @@
     }
   }
 
+  // Handle -exported_symbols_list <file>
+  for (auto expFile : parsedArgs->filtered(OPT_exported_symbols_list)) {
+    if (ctx.exportMode() == MachOLinkingContext::ExportMode::blackList) {
+      diagnostics << "error: -exported_symbols_list cannot be combined "
+                  << "with -unexported_symbols_list\n";
+       return false;
+    }
+    ctx.setExportMode(MachOLinkingContext::ExportMode::whiteList);
+    if (std::error_code ec = parseExportsList(expFile->getValue(), ctx,
+                                              diagnostics)) {
+      diagnostics << "error: " << ec.message()
+                  << ", processing '-exported_symbols_list "
+                  << expFile->getValue()
+                  << "'\n";
+      return false;
+    }
+  }
+
+  // Handle -unexported_symbols_list <file>
+  for (auto expFile : parsedArgs->filtered(OPT_unexported_symbols_list)) {
+    if (ctx.exportMode() == MachOLinkingContext::ExportMode::whiteList) {
+      diagnostics << "error: -unexported_symbols_list cannot be combined "
+                  << "with -exported_symbols_list\n";
+       return false;
+    }
+    ctx.setExportMode(MachOLinkingContext::ExportMode::blackList);
+    if (std::error_code ec = parseExportsList(expFile->getValue(), ctx,
+                                              diagnostics)) {
+      diagnostics << "error: " << ec.message()
+                  << ", processing '-unexported_symbols_list "
+                  << expFile->getValue()
+                  << "'\n";
+      return false;
+    }
+  }
+
   // Handle input files
   for (auto &arg : *parsedArgs) {
     ErrorOr<StringRef> resolvedPath = StringRef();
@@ -442,7 +503,8 @@
         diagnostics << "Unable to find library -l" << arg->getValue() << "\n";
         return false;
       } else if (ctx.testingFileUsage()) {
-       diagnostics << "Found library " << canonicalizePath(resolvedPath.get()) << '\n';
+       diagnostics << "Found library "
+                   << canonicalizePath(resolvedPath.get()) << '\n';
       }
       addFile(resolvedPath.get(), inputGraph, globalWholeArchive);
       break;
@@ -452,7 +514,8 @@
         diagnostics << "Unable to find -framework " << arg->getValue() << "\n";
         return false;
       } else if (ctx.testingFileUsage()) {
-        diagnostics << "Found framework " << canonicalizePath(resolvedPath.get()) << '\n';
+        diagnostics << "Found framework "
+                    << canonicalizePath(resolvedPath.get()) << '\n';
       }
       addFile(resolvedPath.get(), inputGraph, globalWholeArchive);
       break;
Index: lib/Driver/DarwinLdOptions.td
===================================================================
--- lib/Driver/DarwinLdOptions.td
+++ lib/Driver/DarwinLdOptions.td
@@ -34,6 +34,12 @@
 def mllvm : Separate<["-"], "mllvm">,
      MetaVarName<"<option>">,
      HelpText<"Options to pass to LLVM during LTO">, Group<grp_opts>;
+def exported_symbols_list : Separate<["-"], "exported_symbols_list">,
+     MetaVarName<"<file-path>">,
+     HelpText<"Restricts which symbols will be exported">, Group<grp_opts>;
+def unexported_symbols_list : Separate<["-"], "unexported_symbols_list">,
+     MetaVarName<"<file-path>">,
+     HelpText<"Lists symbols that should not be exported">, Group<grp_opts>;
 
 // main executable options
 def grp_main : OptionGroup<"opts">, HelpText<"MAIN EXECUTABLE OPTIONS">;
Index: lib/ReaderWriter/MachO/Atoms.h
===================================================================
--- lib/ReaderWriter/MachO/Atoms.h
+++ lib/ReaderWriter/MachO/Atoms.h
@@ -38,6 +38,8 @@
 
   Scope scope() const override { return _scope; }
 
+  void setScope(Scope s) override { _scope = s; }
+
   Merge merge() const override { return _merge; }
 
   DeadStripKind deadStrip() const override {
@@ -66,7 +68,7 @@
   const StringRef _name;
   const ArrayRef<uint8_t> _content;
   const ContentType _contentType;
-  const Scope _scope;
+        Scope _scope;
   const Merge _merge;
   const bool _thumb;
 };
Index: lib/ReaderWriter/MachO/MachOLinkingContext.cpp
===================================================================
--- lib/ReaderWriter/MachO/MachOLinkingContext.cpp
+++ lib/ReaderWriter/MachO/MachOLinkingContext.cpp
@@ -572,4 +572,69 @@
   return false;
 }
 
+
+void MachOLinkingContext::addExportedSymbol(StringRef sym) {
+  // FIXME: Support wildcards.
+  _exportedSymbols.insert(sym);
+}
+
+void MachOLinkingContext::checkExportWhiteList(const DefinedAtom *atom) const {
+  bool currentlyGlobal = (atom->scope() == DefinedAtom::scopeGlobal);
+  bool whiteListed = _exportedSymbols.count(atom->name());
+
+  if (currentlyGlobal != whiteListed) {
+    if (currentlyGlobal) {
+      // Reduce scope of symbols not in export list down to "hidden".
+      const_cast<DefinedAtom *>(atom)->setScope(Atom::scopeLinkageUnit);
+    } else {
+      // Warn, if symbol that is supposed to be exported is hidden.
+      llvm::errs() << "warning: cannot export visibility=hidden symbol "
+                   << atom->name() << "\n";
+    }
+  }
+}
+
+void MachOLinkingContext::checkExportBlackList(const DefinedAtom *atom) const {
+  bool currentlyGlobal = (atom->scope() == DefinedAtom::scopeGlobal);
+  bool blackListed = _exportedSymbols.count(atom->name());
+
+  if (currentlyGlobal == blackListed) {
+    if (currentlyGlobal) {
+      // Reduce scope of symbols in unexport list down to "hidden".
+      const_cast<DefinedAtom *>(atom)->setScope(Atom::scopeLinkageUnit);
+    }
+  }
+}
+
+void MachOLinkingContext::notifySymbolTableAdd(const Atom *atom) const {
+  // Nothing to do if command line did not have -[un]exported_symbols_list.
+  switch (_exportMode) {
+  case ExportMode::globals:
+    // Do nothing by default.
+    break;
+  case ExportMode::whiteList:
+    // If -exported_symbols_list specified, see if scope needs adjusting.
+    if (const DefinedAtom *defAtom = dyn_cast<DefinedAtom>(atom))
+      checkExportWhiteList(defAtom);
+    break;
+  case ExportMode::blackList:
+    // If -unexported_symbols_list specified, see if scope needs adjusting.
+    if (const DefinedAtom *defAtom = dyn_cast<DefinedAtom>(atom))
+      checkExportBlackList(defAtom);
+    break;
+  }
+}
+
+void MachOLinkingContext::notifySymbolTableCoalesce(const Atom *existingAtom,
+                                                    const Atom *newAtom,
+                                                    bool &useNew) const {
+  // Nothing to do newAtom is not going to be used by symbol table.
+  if (!useNew)
+    return;
+
+  // Do same checking done when new atom added to symbol table.
+  notifySymbolTableAdd(newAtom);
+}
+
+
 } // end namespace lld
Index: lib/ReaderWriter/Native/ReaderNative.cpp
===================================================================
--- lib/ReaderWriter/Native/ReaderNative.cpp
+++ lib/ReaderWriter/Native/ReaderNative.cpp
@@ -16,6 +16,7 @@
 #include "lld/ReaderWriter/Reader.h"
 
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -52,9 +53,9 @@
     return _ivarData->contentSize;
   }
 
-  DefinedAtom::Scope scope() const override {
-    return (DefinedAtom::Scope)(attributes().scope);
-  }
+  DefinedAtom::Scope scope() const override;
+
+  void setScope(DefinedAtom::Scope s) override;
 
   DefinedAtom::Interposable interposable() const override {
     return (DefinedAtom::Interposable)(attributes().interposable);
@@ -835,6 +836,11 @@
     uint32_t           elementCount;
   };
 
+  struct Overrides {
+    Atom::Scope             scope;
+  };
+  typedef llvm::DenseMap<const NativeDefinedAtomV1 *, Overrides> AtomAttributeOverrides;
+
   std::unique_ptr<MemoryBuffer>   _buffer;
   const NativeFileHeader*         _header;
   AtomArray<DefinedAtom>          _definedAtoms;
@@ -855,6 +861,7 @@
   uint32_t                        _addendsMaxIndex;
   const uint8_t                  *_contentStart;
   const uint8_t                  *_contentEnd;
+  mutable AtomAttributeOverrides  _atomAttributeOverrides;
 };
 
 inline const lld::File &NativeDefinedAtomV1::file() const {
@@ -870,6 +877,22 @@
   return _file->string(_ivarData->nameOffset);
 }
 
+inline DefinedAtom::Scope NativeDefinedAtomV1::scope() const {
+  auto pos = _file->_atomAttributeOverrides.find(this);
+  if (pos == _file->_atomAttributeOverrides.end())
+    return (DefinedAtom::Scope)(attributes().scope);
+  else
+    return pos->second.scope;
+}
+
+inline void NativeDefinedAtomV1::setScope(DefinedAtom::Scope s) {
+  // Do nothing is scope is already requested value.
+  if (scope() == s)
+    return;
+  // Changing scope is rare.  Store changes in side table.
+  _file->_atomAttributeOverrides[this].scope = s;
+}
+
 inline const NativeAtomAttributesV1& NativeDefinedAtomV1::attributes() const {
   return _file->attribute(_ivarData->attributesOffset);
 }
Index: lib/ReaderWriter/PECOFF/Atoms.h
===================================================================
--- lib/ReaderWriter/PECOFF/Atoms.h
+++ lib/ReaderWriter/PECOFF/Atoms.h
@@ -161,6 +161,7 @@
   SectionChoice sectionChoice() const override { return sectionCustomRequired; }
   StringRef customSectionName() const override { return _sectionName; }
   Scope scope() const override { return _scope; }
+  void setScope(Scope s) override { _scope = s; }
   ContentType contentType() const override { return _contentType; }
   ContentPermissions permissions() const override { return _permissions; }
   uint64_t ordinal() const override { return _ordinal; }
Index: lib/ReaderWriter/YAML/ReaderWriterYAML.cpp
===================================================================
--- lib/ReaderWriter/YAML/ReaderWriterYAML.cpp
+++ lib/ReaderWriter/YAML/ReaderWriterYAML.cpp
@@ -846,6 +846,7 @@
     StringRef name() const override { return _name; }
     uint64_t size() const override { return _size; }
     Scope scope() const override { return _scope; }
+    void setScope(Scope s) override { _scope = s; }
     Interposable interposable() const override { return _interpose; }
     Merge merge() const override { return _merge; }
     ContentType contentType() const override { return _contentType; }
Index: test/mach-o/Inputs/exported_symbols_list.exp
===================================================================
--- /dev/null
+++ test/mach-o/Inputs/exported_symbols_list.exp
@@ -0,0 +1,6 @@
+#
+# For use with exported_symbols_list.yaml
+#
+_foo
+_b
+
Index: test/mach-o/exported_symbols_list.yaml
===================================================================
--- /dev/null
+++ test/mach-o/exported_symbols_list.yaml
@@ -0,0 +1,57 @@
+# RUN: lld -flavor darwin -arch x86_64 -macosx_version_min 10.8 -dylib \
+# RUN:      %s %p/Inputs/libSystem.yaml -o %t \
+# RUN:      -exported_symbols_list %p/Inputs/exported_symbols_list.exp && \
+# RUN: llvm-nm -m %t | FileCheck %s
+#
+# Test -exported_symbols_list properly changes visibility.
+#
+
+--- !mach-o
+arch:            x86_64
+file-type:       MH_OBJECT
+flags:           [ MH_SUBSECTIONS_VIA_SYMBOLS ]
+sections:
+  - segment:         __TEXT
+    section:         __text
+    type:            S_REGULAR
+    attributes:      [ S_ATTR_PURE_INSTRUCTIONS, S_ATTR_SOME_INSTRUCTIONS ]
+    address:         0x0000000000000000
+    content:         [ 0x55, 0x48, 0x89, 0xE5, 0x5D, 0xC3, 0x55, 0x48, 
+                       0x89, 0xE5, 0x5D, 0xC3 ]
+  - segment:         __DATA
+    section:         __data
+    type:            S_REGULAR
+    attributes:      [  ]
+    alignment:       2
+    address:         0x000000000000000C
+    content:         [ 0x0A, 0x00, 0x00, 0x00, 0x14, 0x00, 0x00, 0x00 ]
+
+global-symbols:
+  - name:            _a
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            2
+    value:           0x000000000000000C
+  - name:            _b
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            2
+    value:           0x0000000000000010
+  - name:            _bar
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            1
+    value:           0x0000000000000006
+  - name:            _foo
+    type:            N_SECT
+    scope:           [ N_EXT ]
+    sect:            1
+    value:           0x0000000000000000
+
+
+...
+
+# CHECK: (__DATA,__data) private external _a
+# CHECK: (__DATA,__data) external _b
+# CHECK: (__TEXT,__text) private external _bar
+# CHECK: (__TEXT,__text) external _foo