Index: ELF/InputFiles.h
===================================================================
--- ELF/InputFiles.h
+++ ELF/InputFiles.h
@@ -66,6 +66,12 @@
   // string for creating error messages.
   StringRef ArchiveName;
 
+  // If this file is in an archive, the member contains the offset of
+  // the file in the archive. Otherwise, it's just zero. We store this
+  // field so that we can pass it to lib/LTO in order to disambiguate
+  // between objects.
+  uint64_t OffsetInArchive;
+
   // If this is an architecture-specific file, the following members
   // have ELF type (i.e. ELF{32,64}{LE,BE}) and target machine type.
   ELFKind EKind = ELFNoneKind;
@@ -239,10 +245,11 @@
   static bool classof(const InputFile *F) { return F->kind() == ArchiveKind; }
   template <class ELFT> void parse();
 
-  // Returns a memory buffer for a given symbol. An empty memory buffer
+  // Returns a memory buffer for a given symbol and the offset in the archive
+  // for the member. An empty memory buffer and an offset of zero
   // is returned if we have already returned the same memory buffer.
   // (So that we don't instantiate same members more than once.)
-  MemoryBufferRef getMember(const Archive::Symbol *Sym);
+  std::pair<MemoryBufferRef, uint64_t> getMember(const Archive::Symbol *Sym);
 
 private:
   std::unique_ptr<Archive> File;
@@ -324,7 +331,8 @@
   std::vector<uint8_t> ELFData;
 };
 
-InputFile *createObjectFile(MemoryBufferRef MB, StringRef ArchiveName = "");
+InputFile *createObjectFile(MemoryBufferRef MB, StringRef ArchiveName = "",
+                            uint64_t OffsetInArchive = 0);
 InputFile *createSharedFile(MemoryBufferRef MB);
 
 } // namespace elf
Index: ELF/InputFiles.cpp
===================================================================
--- ELF/InputFiles.cpp
+++ ELF/InputFiles.cpp
@@ -471,13 +471,14 @@
 }
 
 // Returns a buffer pointing to a member file containing a given symbol.
-MemoryBufferRef ArchiveFile::getMember(const Archive::Symbol *Sym) {
+std::pair<MemoryBufferRef, uint64_t>
+ArchiveFile::getMember(const Archive::Symbol *Sym) {
   Archive::Child C =
       check(Sym->getMember(),
             "could not get the member for symbol " + Sym->getName());
 
   if (!Seen.insert(C.getChildOffset()).second)
-    return MemoryBufferRef();
+    return {MemoryBufferRef(), 0};
 
   MemoryBufferRef Ret =
       check(C.getMemoryBufferRef(),
@@ -487,8 +488,7 @@
   if (C.getParent()->isThin() && Driver->Cpio)
     Driver->Cpio->append(relativeToRoot(check(C.getFullName())),
                          Ret.getBuffer());
-
-  return Ret;
+  return {Ret, C.getChildOffset()};
 }
 
 template <class ELFT>
@@ -713,7 +713,18 @@
 
 template <class ELFT>
 void BitcodeFile::parse(DenseSet<StringRef> &ComdatGroups) {
-  Obj = check(lto::InputFile::create(MB));
+
+  // Here we pass a new MemoryBufferRef which is identified by ArchiveName
+  // (the fully resolved path of the archive) + member name + offset of the
+  // member in the archive.
+  // ThinLTO uses the MemoryBufferRef identifier to access its internal
+  // data structures and 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).
+  Obj = check(lto::InputFile::create(MemoryBufferRef(
+      MB.getBuffer(), Saver.save(ArchiveName + MB.getBufferIdentifier() +
+                                 utostr(OffsetInArchive)))));
   DenseSet<const Comdat *> KeptComdats;
   for (const auto &P : Obj->getComdatSymbolTable()) {
     StringRef N = Saver.save(P.first());
@@ -803,10 +814,12 @@
   return identify_magic(MB.getBuffer()) == file_magic::bitcode;
 }
 
-InputFile *elf::createObjectFile(MemoryBufferRef MB, StringRef ArchiveName) {
+InputFile *elf::createObjectFile(MemoryBufferRef MB, StringRef ArchiveName,
+                                 uint64_t OffsetInArchive) {
   InputFile *F =
       isBitcode(MB) ? new BitcodeFile(MB) : createELFFile<ObjectFile>(MB);
   F->ArchiveName = ArchiveName;
+  F->OffsetInArchive = OffsetInArchive;
   return F;
 }
 
Index: ELF/SymbolTable.cpp
===================================================================
--- ELF/SymbolTable.cpp
+++ ELF/SymbolTable.cpp
@@ -508,9 +508,9 @@
     replaceBody<LazyArchive>(S, *F, Sym, S->body()->Type);
     return;
   }
-  MemoryBufferRef MBRef = F->getMember(&Sym);
-  if (!MBRef.getBuffer().empty())
-    addFile(createObjectFile(MBRef, F->getName()));
+  std::pair<MemoryBufferRef, uint64_t> MBInfo = F->getMember(&Sym);
+  if (!MBInfo.first.getBuffer().empty())
+    addFile(createObjectFile(MBInfo.first, F->getName(), MBInfo.second));
 }
 
 template <class ELFT>
Index: ELF/Symbols.cpp
===================================================================
--- ELF/Symbols.cpp
+++ ELF/Symbols.cpp
@@ -244,13 +244,13 @@
 }
 
 InputFile *LazyArchive::fetch() {
-  MemoryBufferRef MBRef = file()->getMember(&Sym);
+  std::pair<MemoryBufferRef, uint64_t> MBInfo = file()->getMember(&Sym);
 
   // getMember returns an empty buffer if the member was already
   // read from the library.
-  if (MBRef.getBuffer().empty())
+  if (MBInfo.first.getBuffer().empty())
     return nullptr;
-  return createObjectFile(MBRef, file()->getName());
+  return createObjectFile(MBInfo.first, file()->getName(), MBInfo.second);
 }
 
 InputFile *LazyObject::fetch() {
Index: test/ELF/lto/thin-archivecollision.ll
===================================================================
--- test/ELF/lto/thin-archivecollision.ll
+++ test/ELF/lto/thin-archivecollision.ll
@@ -7,7 +7,9 @@
 ; RUN: ld.lld %t.o %t1.a %t2.a -o %t
 ; RUN: llvm-nm %t | FileCheck %s
 
-; CHECK: patatino
+; CHECK: T _start
+; CHECK: T blah
+; CHECK: T foo
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-scei-ps4"