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
@@ -760,11 +760,15 @@
     }
   }
 
+  SmallString<256> buffer = root;
+  // "root" could be "/", which may need to be translated into "\".
+  make_preferred(buffer, style);
+  needs_change |= root != buffer;
+
   // Avoid rewriting the path unless we have to.
   if (!needs_change)
     return false;
 
-  SmallString<256> buffer = root;
   if (!components.empty()) {
     buffer += components[0];
     for (StringRef C : makeArrayRef(components).drop_front()) {
diff --git a/llvm/unittests/Support/Path.cpp b/llvm/unittests/Support/Path.cpp
--- a/llvm/unittests/Support/Path.cpp
+++ b/llvm/unittests/Support/Path.cpp
@@ -1544,16 +1544,14 @@
   EXPECT_EQ("C:\\a\\c", remove_dots("C:\\foo\\bar//..\\..\\a\\c", true,
                                     path::Style::windows));
 
-  // FIXME: These leading forward slashes are emergent behavior. VFS depends on
-  // this behavior now.
-  EXPECT_EQ("C:/bar",
+  EXPECT_EQ("C:\\bar",
             remove_dots("C:/foo/../bar", true, path::Style::windows));
-  EXPECT_EQ("C:/foo\\bar",
+  EXPECT_EQ("C:\\foo\\bar",
             remove_dots("C:/foo/bar", true, path::Style::windows));
-  EXPECT_EQ("C:/foo\\bar",
+  EXPECT_EQ("C:\\foo\\bar",
             remove_dots("C:/foo\\bar", true, path::Style::windows));
-  EXPECT_EQ("/", remove_dots("/", true, path::Style::windows));
-  EXPECT_EQ("C:/", remove_dots("C:/", true, path::Style::windows));
+  EXPECT_EQ("\\", remove_dots("/", true, path::Style::windows));
+  EXPECT_EQ("C:\\", remove_dots("C:/", true, path::Style::windows));
 
   // Some clients of remove_dots expect it to remove trailing slashes. Again,
   // this is emergent behavior that VFS relies on, and not inherently part of
@@ -1564,7 +1562,7 @@
             remove_dots("/foo/bar/", true, path::Style::posix));
 
   // A double separator is rewritten.
-  EXPECT_EQ("C:/foo\\bar", remove_dots("C:/foo//bar", true, path::Style::windows));
+  EXPECT_EQ("C:\\foo\\bar", remove_dots("C:/foo//bar", true, path::Style::windows));
 
   SmallString<64> Path1(".\\.\\c");
   EXPECT_TRUE(path::remove_dots(Path1, true, path::Style::windows));