Index: lld/MachO/Driver.h
===================================================================
--- lld/MachO/Driver.h
+++ lld/MachO/Driver.h
@@ -70,7 +70,8 @@
 
 llvm::Optional<InputFile *> loadArchiveMember(MemoryBufferRef, uint32_t modTime,
                                               StringRef archiveName,
-                                              bool objCOnly);
+                                              bool objCOnly,
+                                              uint64_t offsetInArchive);
 
 uint32_t getModTime(llvm::StringRef path);
 
Index: lld/MachO/Driver.cpp
===================================================================
--- lld/MachO/Driver.cpp
+++ lld/MachO/Driver.cpp
@@ -228,6 +228,7 @@
 struct ArchiveMember {
   MemoryBufferRef mbref;
   uint32_t modTime;
+  uint64_t offsetInArchive;
 };
 } // namespace
 
@@ -257,7 +258,7 @@
         CHECK(c.getLastModified(), mb.getBufferIdentifier() +
                                        ": could not get the modification "
                                        "time for a child of the archive"));
-    v.push_back({mbref, modTime});
+    v.push_back({mbref, modTime, c.getChildOffset()});
   }
   if (err)
     fatal(mb.getBufferIdentifier() +
@@ -298,7 +299,8 @@
       if (Optional<MemoryBufferRef> buffer = readFile(path)) {
         for (const ArchiveMember &member : getArchiveMembers(*buffer)) {
           if (Optional<InputFile *> file = loadArchiveMember(
-                  member.mbref, member.modTime, path, /*objCOnly=*/false)) {
+                  member.mbref, member.modTime, path, /*objCOnly=*/false,
+                  member.offsetInArchive)) {
             inputFiles.insert(*file);
             printArchiveMemberLoad(
                 (forceLoadArchive ? "-force_load" : "-all_load"),
@@ -319,7 +321,8 @@
       if (Optional<MemoryBufferRef> buffer = readFile(path)) {
         for (const ArchiveMember &member : getArchiveMembers(*buffer)) {
           if (Optional<InputFile *> file = loadArchiveMember(
-                  member.mbref, member.modTime, path, /*objCOnly=*/true)) {
+                  member.mbref, member.modTime, path, /*objCOnly=*/true,
+                  member.offsetInArchive)) {
             inputFiles.insert(*file);
             printArchiveMemberLoad("-ObjC", inputFiles.back());
           }
@@ -343,7 +346,7 @@
     }
     break;
   case file_magic::bitcode:
-    newFile = make<BitcodeFile>(mbref);
+    newFile = make<BitcodeFile>(mbref, "", 0);
     break;
   case file_magic::macho_executable:
   case file_magic::macho_bundle:
Index: lld/MachO/DriverUtils.cpp
===================================================================
--- lld/MachO/DriverUtils.cpp
+++ lld/MachO/DriverUtils.cpp
@@ -280,7 +280,8 @@
 Optional<InputFile *> macho::loadArchiveMember(MemoryBufferRef mb,
                                                uint32_t modTime,
                                                StringRef archiveName,
-                                               bool objCOnly) {
+                                               bool objCOnly,
+                                               uint64_t offsetInArchive) {
   if (config->zeroModTime)
     modTime = 0;
 
@@ -291,7 +292,7 @@
     return None;
   case file_magic::bitcode:
     if (!objCOnly || check(isBitcodeContainingObjCCategory(mb)))
-      return make<BitcodeFile>(mb);
+      return make<BitcodeFile>(mb, archiveName, offsetInArchive);
     return None;
   default:
     error(archiveName + ": archive member " + mb.getBufferIdentifier() +
Index: lld/MachO/InputFiles.h
===================================================================
--- lld/MachO/InputFiles.h
+++ lld/MachO/InputFiles.h
@@ -196,7 +196,8 @@
 
 class BitcodeFile final : public InputFile {
 public:
-  explicit BitcodeFile(MemoryBufferRef mb);
+  explicit BitcodeFile(MemoryBufferRef mb, StringRef archiveName,
+                       uint64_t offsetInArchive);
   static bool classof(const InputFile *f) { return f->kind() == BitcodeKind; }
 
   std::unique_ptr<llvm::lto::InputFile> obj;
Index: lld/MachO/InputFiles.cpp
===================================================================
--- lld/MachO/InputFiles.cpp
+++ lld/MachO/InputFiles.cpp
@@ -1215,8 +1215,8 @@
   // to it later.
   const object::Archive::Symbol symCopy = sym;
 
-  if (Optional<InputFile *> file =
-          loadArchiveMember(mb, modTime, getName(), /*objCOnly=*/false)) {
+  if (Optional<InputFile *> file = loadArchiveMember(
+          mb, modTime, getName(), /*objCOnly=*/false, c.getChildOffset())) {
     inputFiles.insert(*file);
     // ld64 doesn't demangle sym here even with -demangle.
     // Match that: intentionally don't call toMachOString().
@@ -1255,8 +1255,22 @@
                             /*noDeadStrip=*/false);
 }
 
-BitcodeFile::BitcodeFile(MemoryBufferRef mbref)
-    : InputFile(BitcodeKind, mbref) {
+BitcodeFile::BitcodeFile(MemoryBufferRef mb, StringRef archiveName,
+                         uint64_t offsetInArchive)
+    : InputFile(BitcodeKind, mb) {
+  std::string path = mb.getBufferIdentifier().str();
+  // ThinLTO assumes that all MemoryBufferRefs given to it have a unique
+  // name. If two archives define two members with the same name, this
+  // causes a collision which result in only one of the objects being taken
+  // into consideration at LTO time (which very likely causes undefined
+  // symbols later in the link stage). So we append file offset to make
+  // filename unique.
+  MemoryBufferRef mbref(
+      mb.getBuffer(),
+      saver.save(archiveName.empty() ? path
+                                     : archiveName + sys::path::filename(path) +
+                                           utostr(offsetInArchive)));
+
   obj = check(lto::InputFile::create(mbref));
 
   // Convert LTO Symbols to LLD Symbols in order to perform resolution. The