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
@@ -424,16 +424,16 @@
   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
@@ -1,10 +1,10 @@
-// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - | FileCheck %s -check-prefix CHECK-NO-MAIN-FILE-NAME
-// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH=empty %s -emit-llvm -o - | FileCheck %s -check-prefix CHECK-EVIL
-// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -main-file-name debug-prefix-map.c | FileCheck %s
-// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -fdebug-compilation-dir %p | FileCheck %s -check-prefix CHECK-COMPILATION-DIR
-// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty %s -emit-llvm -o - -isysroot %p -debugger-tuning=lldb | FileCheck %s -check-prefix CHECK-SYSROOT
-// RUN: %clang -g -fdebug-prefix-map=%p=/UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s
-// RUN: %clang -g -ffile-prefix-map=%p=/UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH%{fs-sep}empty %s -emit-llvm -o - | FileCheck %s -check-prefix CHECK-NO-MAIN-FILE-NAME
+// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH=empty %s -emit-llvm -o - | FileCheck %s -check-prefix CHECK-EVIL
+// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH%{fs-sep}empty %s -emit-llvm -o - -main-file-name debug-prefix-map.c | FileCheck %s
+// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH%{fs-sep}empty %s -emit-llvm -o - -fdebug-compilation-dir %p | FileCheck %s -check-prefix CHECK-COMPILATION-DIR
+// RUN: %clang_cc1 -debug-info-kind=standalone -fdebug-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH%{fs-sep}empty %s -emit-llvm -o - -isysroot %p -debugger-tuning=lldb | FileCheck %s -check-prefix CHECK-SYSROOT
+// RUN: %clang -g -fdebug-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH%{fs-sep}empty -S -c %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang -g -ffile-prefix-map=%p=%{fs-src-root}UNLIKELY_PATH%{fs-sep}empty -S -c %s -emit-llvm -o - | FileCheck %s
 
 #include "Inputs/stdio.h"
 
@@ -19,26 +19,24 @@
   vprintf("string", argp);
 }
 
-// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "/UNLIKELY_PATH/empty{{/|\\\\}}<stdin>"
-// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "/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: "/UNLIKELY_PATH/empty{{/|\\\\}}Inputs/stdio.h",
-// CHECK-NO-MAIN-FILE-NAME-SAME:    directory: "{{()|(.*:.*)}}")
+// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}<stdin>",
+// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}",
+// CHECK-NO-MAIN-FILE-NAME-SAME:    directory: "")
+// CHECK-NO-MAIN-FILE-NAME: !DIFile(filename: "{{/|.:\\\\}}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: "/UNLIKELY_PATH=empty{{/|\\\\}}{{.*}}"
-// CHECK-EVIL: !DIFile(filename: "/UNLIKELY_PATH=empty{{/|\\\\}}{{.*}}Inputs/stdio.h",
-// CHECK-EVIL-SAME:    directory: "{{()|(.*:.*)}}")
+// CHECK-EVIL: !DIFile(filename: "{{/|.:\\\\}}UNLIKELY_PATH=empty{{/|\\\\}}{{.*}}"
+// CHECK-EVIL: !DIFile(filename: "{{/|.:\\\\}}UNLIKELY_PATH=empty{{/|\\\\}}{{.*}}Inputs{{/|\\\\}}stdio.h",
+// CHECK-EVIL-SAME:    directory: "")
 // CHECK-EVIL-NOT: !DIFile(filename:
 
-// CHECK: !DIFile(filename: "/UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}"
-// CHECK: !DIFile(filename: "/UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}Inputs/stdio.h",
-// CHECK-SAME:    directory: "{{()|(.*:.*)}}")
+// CHECK: !DIFile(filename: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}",
+// CHECK: !DIFile(filename: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty{{/|\\\\}}{{.*}}Inputs{{/|\\\\}}stdio.h",
+// CHECK-SAME:    directory: ""
 // CHECK-NOT: !DIFile(filename:
 
-// CHECK-COMPILATION-DIR: !DIFile(filename: "{{.*}}", directory: "/UNLIKELY_PATH/empty")
-// CHECK-COMPILATION-DIR: !DIFile(filename: "{{.*}}Inputs/stdio.h", directory: "/UNLIKELY_PATH/empty")
+// CHECK-COMPILATION-DIR: !DIFile(filename: "{{.*}}", directory: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty")
+// CHECK-COMPILATION-DIR: !DIFile(filename: "{{.*}}Inputs{{/|\\\\}}stdio.h", directory: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty")
 // CHECK-COMPILATION-DIR-NOT: !DIFile(filename:
-// CHECK-SYSROOT: !DICompileUnit({{.*}}sysroot: "/UNLIKELY_PATH/empty"
+// CHECK-SYSROOT: !DICompileUnit({{.*}}sysroot: "{{/|.:\\\\}}UNLIKELY_PATH{{/|\\\\}}empty"