diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -2191,6 +2191,10 @@
   case SourceLocExpr::File: {
     SmallString<256> Path(PLoc.getFilename());
     Ctx.getLangOpts().remapPathPrefix(Path);
+    if (Ctx.getTargetInfo().getTriple().isOSWindows()) {
+      llvm::sys::path::make_preferred(Path,
+                                      llvm::sys::path::Style::windows_slash);
+    }
     return MakeStringLiteral(Path);
   }
   case SourceLocExpr::Function: {
@@ -2224,6 +2228,10 @@
       if (Name == "_M_file_name") {
         SmallString<256> Path(PLoc.getFilename());
         Ctx.getLangOpts().remapPathPrefix(Path);
+        if (Ctx.getTargetInfo().getTriple().isOSWindows()) {
+          llvm::sys::path::make_preferred(
+              Path, llvm::sys::path::Style::windows_slash);
+        }
         Value.getStructField(F->getFieldIndex()) = MakeStringLiteral(Path);
       } else if (Name == "_M_function_name") {
         // Note: this emits the PrettyFunction name -- different than what
diff --git a/clang/lib/Lex/PPMacroExpansion.cpp b/clang/lib/Lex/PPMacroExpansion.cpp
--- a/clang/lib/Lex/PPMacroExpansion.cpp
+++ b/clang/lib/Lex/PPMacroExpansion.cpp
@@ -1512,6 +1512,10 @@
         FN += PLoc.getFilename();
       }
       getLangOpts().remapPathPrefix(FN);
+      if (getTargetInfo().getTriple().isOSWindows()) {
+        llvm::sys::path::make_preferred(FN,
+                                        llvm::sys::path::Style::windows_slash);
+      }
       Lexer::Stringify(FN);
       OS << '"' << FN << '"';
     }
diff --git a/clang/test/Preprocessor/file_test_windows.c b/clang/test/Preprocessor/file_test_windows.c
--- a/clang/test/Preprocessor/file_test_windows.c
+++ b/clang/test/Preprocessor/file_test_windows.c
@@ -8,19 +8,19 @@
 filename: __FILE__
 #include "Inputs/include-file-test/file_test.h"
 
-// CHECK: filename: "A:\\UNLIKELY_PATH\\empty\\file_test_windows.c"
-// CHECK: filename: "A:\\UNLIKELY_PATH\\empty/Inputs/include-file-test/file_test.h"
-// CHECK: basefile: "A:\\UNLIKELY_PATH\\empty\\file_test_windows.c"
+// CHECK: filename: "A:/UNLIKELY_PATH/empty/file_test_windows.c"
+// CHECK: filename: "A:/UNLIKELY_PATH/empty/Inputs/include-file-test/file_test.h"
+// CHECK: basefile: "A:/UNLIKELY_PATH/empty/file_test_windows.c"
 // CHECK-NOT: filename:
 
-// CHECK-EVIL: filename: "A:\\UNLIKELY_PATH=empty\\file_test_windows.c"
-// CHECK-EVIL: filename: "A:\\UNLIKELY_PATH=empty/Inputs/include-file-test/file_test.h"
-// CHECK-EVIL: basefile: "A:\\UNLIKELY_PATH=empty\\file_test_windows.c"
+// CHECK-EVIL: filename: "A:/UNLIKELY_PATH=empty/file_test_windows.c"
+// CHECK-EVIL: filename: "A:/UNLIKELY_PATH=empty/Inputs/include-file-test/file_test.h"
+// CHECK-EVIL: basefile: "A:/UNLIKELY_PATH=empty/file_test_windows.c"
 // CHECK-EVIL-NOT: filename:
 
-// CHECK-CASE: filename: "A:\\UNLIKELY_PATH_BASE\\file_test_windows.c"
-// CHECK-CASE: filename: "A:\\UNLIKELY_PATH_INC\\include-file-test/file_test.h"
-// CHECK-CASE: basefile: "A:\\UNLIKELY_PATH_BASE\\file_test_windows.c"
+// CHECK-CASE: filename: "A:/UNLIKELY_PATH_BASE/file_test_windows.c"
+// CHECK-CASE: filename: "A:/UNLIKELY_PATH_INC/include-file-test/file_test.h"
+// CHECK-CASE: basefile: "A:/UNLIKELY_PATH_BASE/file_test_windows.c"
 // CHECK-CASE-NOT: filename:
 
 // CHECK-REMOVE: filename: "file_test_windows.c"