Index: lib/Core/Reproduce.cpp
===================================================================
--- lib/Core/Reproduce.cpp
+++ lib/Core/Reproduce.cpp
@@ -52,9 +52,11 @@
 
 // Converts path to use unix path separators so the cpio can be extracted on
 // both unix and windows.
+#ifdef LLVM_ON_WIN32
 static void convertToUnixPathSeparator(SmallString<128> &Path) {
   std::replace(Path.begin(), Path.end(), '\\', '/');
 }
+#endif
 
 void CpioFile::append(StringRef Path, StringRef Data) {
   if (!Seen.insert(Path).second)
@@ -65,7 +67,10 @@
   // (i.e. in that case we are creating baz.cpio.)
   SmallString<128> Fullpath;
   path::append(Fullpath, Basename, Path);
+
+#ifdef LLVM_ON_WIN32
   convertToUnixPathSeparator(Fullpath);
+#endif
 
   writeMember(*OS, Fullpath, Data);
 
@@ -98,7 +103,9 @@
     Res = Root.substr(2);
 
   path::append(Res, path::relative_path(Abs));
+#ifdef LLVM_ON_WIN32
   convertToUnixPathSeparator(Res);
+#endif
 
   return Res.str();
 }
Index: test/ELF/reproduce-backslash.s
===================================================================
--- /dev/null
+++ test/ELF/reproduce-backslash.s
@@ -0,0 +1,13 @@
+# REQUIRES: x86, cpio, shell
+
+# Test that we don't erroneously replace \ with / on UNIX, as it's
+# legal for a filename to contain backslashes.
+# RUN: rm -rf %t.dir
+# RUN: mkdir -p %t.dir/build
+# RUN: llvm-mc %s -o %t.dir/build/foo\\.o -filetype=obj -triple=x86_64-pc-linux
+# RUN: cd %t.dir
+# RUN: ld.lld build/foo\\.o --reproduce repro
+# RUN: cpio -t < repro.cpio | FileCheck %s
+
+# CHECK: repro/response.txt
+# CHECK: repro/{{.*}}/build/foo\.o