Index: llvm/test/tools/llvm-ar/windows-path.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-ar/windows-path.test
@@ -0,0 +1,44 @@
+# Test that windows path seperators are handled correctly
+REQUIRES: system-windows
+
+# Note: many of these tests depend on relative paths, so we have to cd to a
+# test directory first.
+RUN: mkdir -p %t && cd %t
+RUN: rm -rf a b && mkdir a b
+RUN: echo hello-a      > a/foo.txt
+RUN: echo hello-b      > b/foo.txt
+RUN: echo hello-parent > foo.txt
+
+# P is implied when using thin archives.
+# Create an archive.
+RUN: rm -f archive.a
+RUN: llvm-ar rcST archive.a a\foo.txt
+RUN: llvm-ar rcST archive.a foo.txt
+RUN: llvm-ar rcST archive.a b\foo.txt b/foo.txt 
+RUN: llvm-ar dT archive.a foo.txt
+RUN: llvm-ar t archive.a | FileCheck %s --check-prefix=ARCHIVE --implicit-check-not {{.}}
+
+ARCHIVE:      a/foo.txt
+ARCHIVE-NEXT: b/foo.txt
+ARCHIVE-NEXT: b/foo.txt
+
+RUN: llvm-ar t archive.a | FileCheck %s --check-prefix=LIST-BOTH --implicit-check-not {{.}}
+RUN: llvm-ar t archive.a a\foo.txt | FileCheck %s --check-prefix=LIST-A --implicit-check-not {{.}}
+RUN: llvm-ar t archive.a b\foo.txt | FileCheck %s --check-prefix=LIST-B --implicit-check-not {{.}}
+
+LIST-BOTH:       a/foo.txt
+LIST-BOTH-NEXT:  b/foo.txt
+LIST-BOTH-NEXT:  b/foo.txt
+LIST-A:          a/foo.txt
+LIST-B:          b/foo.txt
+
+# Nesting a thin archive with a name conflict.
+RUN: rm -f a\nested.a b\nested.a nested.a
+RUN: llvm-ar rcST a\nested.a a\foo.txt
+RUN: llvm-ar rcST b\nested.a b\foo.txt
+RUN: llvm-ar rcST nested.a a\nested.a foo.txt b\nested.a
+RUN: llvm-ar t nested.a | FileCheck %s --check-prefix=NESTED --implicit-check-not {{.}}
+
+NESTED:      a/foo.txt
+NESTED-NEXT: foo.txt
+NESTED-NEXT: b/foo.txt
Index: llvm/tools/llvm-ar/llvm-ar.cpp
===================================================================
--- llvm/tools/llvm-ar/llvm-ar.cpp
+++ llvm/tools/llvm-ar/llvm-ar.cpp
@@ -474,8 +474,9 @@
   outs() << Name << "\n";
 }
 
-static StringRef normalizePath(StringRef Path) {
-  return CompareFullPath ? Path : sys::path::filename(Path);
+static std::string normalizePath(StringRef Path) {
+  return CompareFullPath ? sys::path::convert_to_slash(Path)
+                         : sys::path::filename(Path);
 }
 
 // Implement the 'x' operation. This function extracts files back to the file
@@ -698,9 +699,8 @@
     return IA_MoveOldMember;
 
   if (Operation == ReplaceOrInsert) {
-    StringRef PosName = normalizePath(RelPos);
     if (!OnlyUpdate) {
-      if (PosName.empty())
+      if (RelPos.empty())
         return IA_AddNewMember;
       return IA_MoveNewMember;
     }
@@ -712,12 +712,12 @@
     auto ModTimeOrErr = Member.getLastModified();
     failIfError(ModTimeOrErr.takeError());
     if (Status.getLastModificationTime() < ModTimeOrErr.get()) {
-      if (PosName.empty())
+      if (RelPos.empty())
         return IA_AddOldMember;
       return IA_MoveOldMember;
     }
 
-    if (PosName.empty())
+    if (RelPos.empty())
       return IA_AddNewMember;
     return IA_MoveNewMember;
   }
@@ -732,8 +732,8 @@
   std::vector<NewArchiveMember> Ret;
   std::vector<NewArchiveMember> Moved;
   int InsertPos = -1;
-  StringRef PosName = normalizePath(RelPos);
   if (OldArchive) {
+    std::string PosName = normalizePath(RelPos);
     Error Err = Error::success();
     StringMap<int> MemberCount;
     for (auto &Child : OldArchive->children(Err)) {
@@ -1009,7 +1009,7 @@
       ArchiveName = Rest;
       break;
     case MRICommand::Delete: {
-      StringRef Name = normalizePath(Rest);
+      std::string Name = normalizePath(Rest);
       llvm::erase_if(NewMembers,
                      [=](NewArchiveMember &M) { return M.MemberName == Name; });
       break;