Index: llvm/trunk/lib/Object/ArchiveWriter.cpp
===================================================================
--- llvm/trunk/lib/Object/ArchiveWriter.cpp
+++ llvm/trunk/lib/Object/ArchiveWriter.cpp
@@ -205,6 +205,12 @@
   for (auto ToE = sys::path::end(To); ToI != ToE; ++ToI)
     sys::path::append(Relative, *ToI);
 
+#ifdef LLVM_ON_WIN32
+  // Replace backslashes with slashes so that the path is portable between *nix
+  // and Windows.
+  std::replace(Relative.begin(), Relative.end(), '\\', '/');
+#endif
+
   return Relative.str();
 }
 
Index: llvm/trunk/test/Object/archive-thin-paths.test
===================================================================
--- llvm/trunk/test/Object/archive-thin-paths.test
+++ llvm/trunk/test/Object/archive-thin-paths.test
@@ -0,0 +1,9 @@
+REQUIRES: system-windows
+
+RUN: cd %T
+RUN: mkdir -p archive-thin-paths.dir
+RUN: echo foo > archive-thin-paths.dir/foo.o
+
+RUN: rm -f archive-thin-paths.a
+RUN: llvm-ar rcsT archive-thin-paths.a archive-thin-paths.dir\foo.o
+RUN: grep archive-thin-paths.dir/foo.o archive-thin-paths.a