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
@@ -1895,9 +1895,9 @@
   LangOpts.remapPathPrefix(Path);
   if (LangOpts.UseTargetPathSeparator) {
     if (TI.getTriple().isOSWindows())
-      llvm::sys::path::make_preferred(
-          Path, llvm::sys::path::Style::windows_backslash);
+      llvm::sys::path::remove_dots(Path, false,
+                                   llvm::sys::path::Style::windows_backslash);
     else
-      llvm::sys::path::make_preferred(Path, llvm::sys::path::Style::posix);
+      llvm::sys::path::remove_dots(Path, false, llvm::sys::path::Style::posix);
   }
 }
diff --git a/llvm/lib/Support/Path.cpp b/llvm/lib/Support/Path.cpp
--- a/llvm/lib/Support/Path.cpp
+++ b/llvm/lib/Support/Path.cpp
@@ -765,6 +765,9 @@
     return false;
 
   SmallString<256> buffer = root;
+  // "root" could be "/", which may need to be translated into "\".
+  make_preferred(buffer, style);
+
   if (!components.empty()) {
     buffer += components[0];
     for (StringRef C : makeArrayRef(components).drop_front()) {