Index: ELF/InputFiles.cpp
===================================================================
--- ELF/InputFiles.cpp
+++ ELF/InputFiles.cpp
@@ -713,7 +713,16 @@
 
 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 +
+  // member name + size of the archive. ThinLTO internally uses the memory buffer
+  // ref identifier to access its internal data structures and if two archives
+  // define two members with the same name, this causes a collision which results
+  // in only one of the objects taken in consideration at LTO time (which very likely
+  // causes undefined symbols later in the link).
+  Obj = check(lto::InputFile::create(MemoryBufferRef(MB.getBuffer(), Saver.save(ArchiveName +
+                                             MB.getBufferIdentifier() +
+                                             utostr(MB.getBuffer().size())))));
   DenseSet<const Comdat *> KeptComdats;
   for (const auto &P : Obj->getComdatSymbolTable()) {
     StringRef N = Saver.save(P.first());
Index: test/ELF/lto/Inputs/thin1.ll
===================================================================
--- /dev/null
+++ test/ELF/lto/Inputs/thin1.ll
@@ -0,0 +1,12 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-scei-ps4"
+
+define i32 @foo(i32 %goo) {
+entry:
+  %goo.addr = alloca i32, align 4
+  store i32 %goo, i32* %goo.addr, align 4
+  %0 = load i32, i32* %goo.addr, align 4
+  %1 = load i32, i32* %goo.addr, align 4
+  %mul = mul nsw i32 %0, %1
+  ret i32 %mul
+}
Index: test/ELF/lto/Inputs/thin2.ll
===================================================================
--- /dev/null
+++ test/ELF/lto/Inputs/thin2.ll
@@ -0,0 +1,11 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-scei-ps4"
+
+define i32 @blah(i32 %meh) #0 {
+entry:
+  %meh.addr = alloca i32, align 4
+  store i32 %meh, i32* %meh.addr, align 4
+  %0 = load i32, i32* %meh.addr, align 4
+  %sub = sub nsw i32 %0, 48
+  ret i32 %sub
+}
Index: test/ELF/lto/thin-archivecollision.ll
===================================================================
--- /dev/null
+++ test/ELF/lto/thin-archivecollision.ll
@@ -0,0 +1,25 @@
+; RUN: opt -module-summary %s -o %t.o
+; RUN: opt -module-summary %p/Inputs/thin1.ll -o %t.coll.o
+; RUN: llvm-ar rcs %t1.a %t.coll.o
+; RUN: opt -module-summary %p/Inputs/thin2.ll -o %t.coll.o
+; RUN: llvm-ar rcsc %t2.a %t.coll.o
+
+; RUN: ld.lld %t.o %t1.a %t2.a -o %t
+; RUN: llvm-nm %t | FileCheck %s
+
+; 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"
+
+define i32 @_start() {
+entry:
+  %call = call i32 @foo(i32 23)
+  %call1 = call i32 @blah(i32 37)
+  ret i32 0
+}
+
+declare i32 @foo(i32) #1
+declare i32 @blah(i32) #1