diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -427,16 +427,16 @@
   SmallString<128> DirBuf;
   SmallString<128> FileBuf;
   if (llvm::sys::path::is_absolute(RemappedFile)) {
-    // Strip the common prefix (if it is more than just "/") from current
-    // directory and FileName for a more space-efficient encoding.
+    // Strip the common prefix (if it is more than just "/" or "C:\") from
+    // current directory and FileName for a more space-efficient encoding.
     auto FileIt = llvm::sys::path::begin(RemappedFile);
     auto FileE = llvm::sys::path::end(RemappedFile);
     auto CurDirIt = llvm::sys::path::begin(CurDir);
     auto CurDirE = llvm::sys::path::end(CurDir);
     for (; CurDirIt != CurDirE && *CurDirIt == *FileIt; ++CurDirIt, ++FileIt)
       llvm::sys::path::append(DirBuf, *CurDirIt);
-    if (std::distance(llvm::sys::path::begin(CurDir), CurDirIt) == 1) {
-      // Don't strip the common prefix if it is only the root "/"
+    if (llvm::sys::path::root_path(DirBuf) == DirBuf) {
+      // Don't strip the common prefix if it is only the root ("/" or "C:\")
       // since that would make LLVM diagnostic locations confusing.
       Dir = {};
       File = RemappedFile;
diff --git a/clang/test/CodeGen/debug-prefix-map.c b/clang/test/CodeGen/debug-prefix-map.c
--- a/clang/test/CodeGen/debug-prefix-map.c
+++ b/clang/test/CodeGen/debug-prefix-map.c
@@ -19,23 +19,21 @@
   vprintf("string", argp);
 }
 
-// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|C:\\\\|()}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}<stdin>"
-// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|C:\\\\|()}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}",
-// On POSIX systems "Dir" should actually be empty, but on Windows we
-// can't recognize "/UNLIKELY_PATH" as being an absolute path.
-// CHECK-NO-MAIN-FILE-NAME-SAME:    directory: "{{()|(.*:.*)}}")
-// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|C:\\\\|()}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}Inputs{{/|\\\\}}stdio.h",
-// CHECK-NO-MAIN-FILE-NAME-SAME:    directory: "{{()|(.*:.*)}}")
+// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|C:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}<stdin>"
+// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|C:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}",
+// CHECK-NO-MAIN-FILE-NAME-SAME:    directory: "")
+// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|C:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}Inputs{{/|\\\\}}stdio.h",
+// CHECK-NO-MAIN-FILE-NAME-SAME:    directory: "")
 // CHECK-NO-MAIN-FILE-NAME-NOT: !DIFile(filename:
 
-// CHECK-EVIL: !DIFile(filename: "{{/|C:\\\\|()}}UNLIKELY_PATH=empty{{/|\\\\}}{{.*}}"
-// CHECK-EVIL: !DIFile(filename: "{{/|C:\\\\|()}}UNLIKELY_PATH=empty{{/|\\\\}}{{.*}}Inputs{{/|\\\\}}stdio.h",
-// CHECK-EVIL-SAME:    directory: "{{()|(.*:.*)}}")
+// CHECK-EVIL: !DIFile(filename: "{{/|C:\\\\}}UNLIKELY_PATH=empty{{/|\\\\}}{{.*}}"
+// CHECK-EVIL: !DIFile(filename: "{{/|C:\\\\}}UNLIKELY_PATH=empty{{/|\\\\}}{{.*}}Inputs{{/|\\\\}}stdio.h",
+// CHECK-EVIL-SAME:    directory: "")
 // CHECK-EVIL-NOT: !DIFile(filename:
 
-// CHECK: !DIFile(filename: "{{/|C:\\\\|()}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}"
-// CHECK: !DIFile(filename: "{{/|C:\\\\|()}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}Inputs{{/|\\\\}}stdio.h",
-// CHECK-SAME:    directory: "{{()|(.*:.*)}}")
+// CHECK: !DIFile(filename: "{{/|C:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}"
+// CHECK: !DIFile(filename: "{{/|C:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}Inputs{{/|\\\\}}stdio.h",
+// CHECK-SAME:    directory: ""
 // CHECK-NOT: !DIFile(filename:
 
 // CHECK-COMPILATION-DIR: !DIFile(filename: "{{.*}}", directory: "{{/|C:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty")