diff --git a/llvm/include/llvm/Support/Path.h b/llvm/include/llvm/Support/Path.h
--- a/llvm/include/llvm/Support/Path.h
+++ b/llvm/include/llvm/Support/Path.h
@@ -25,7 +25,7 @@
 namespace sys {
 namespace path {
 
-enum class Style { windows, posix, native };
+enum class Style { windows, windows_forward, posix, native };
 
 /// @name Lexical Component Iterator
 /// @{
@@ -241,6 +241,13 @@
 /// @param path A path that is transformed to native format.
 void native(SmallVectorImpl<char> &path, Style style = Style::native);
 
+/// Convert path to the native form in place. On Windows, where both forward and
+/// backwards slashes are path separators, convert to the preferred form.
+/// On other platforms, this does nothing.
+///
+/// @param path A path that is transformed to preferred format.
+void make_preferred(SmallVectorImpl<char> &path, Style style = Style::native);
+
 /// Replaces backslashes with slashes if Windows.
 ///
 /// @param path processed path
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
@@ -37,15 +37,23 @@
   using llvm::sys::path::Style;
 
   inline Style real_style(Style style) {
+    if (style == Style::native) {
 #ifdef _WIN32
-    return (style == Style::posix) ? Style::posix : Style::windows;
+      return Style::windows;
 #else
-    return (style == Style::windows) ? Style::windows : Style::posix;
+      return Style::posix;
 #endif
+    }
+    return style;
+  }
+
+  bool is_style_windows(Style style) {
+    style = real_style(style);
+    return style == Style::windows || style == Style::windows_forward;
   }
 
   inline const char *separators(Style style) {
-    if (real_style(style) == Style::windows)
+    if (is_style_windows(style))
       return "\\/";
     return "/";
   }
@@ -66,7 +74,7 @@
     if (path.empty())
       return path;
 
-    if (real_style(style) == Style::windows) {
+    if (is_style_windows(style)) {
       // C:
       if (path.size() >= 2 &&
           std::isalpha(static_cast<unsigned char>(path[0])) && path[1] == ':')
@@ -98,7 +106,7 @@
 
     size_t pos = str.find_last_of(separators(style), str.size() - 1);
 
-    if (real_style(style) == Style::windows) {
+    if (is_style_windows(style)) {
       if (pos == StringRef::npos)
         pos = str.find_last_of(':', str.size() - 2);
     }
@@ -113,7 +121,7 @@
   // directory in str, it returns StringRef::npos.
   size_t root_dir_start(StringRef str, Style style) {
     // case "c:/"
-    if (real_style(style) == Style::windows) {
+    if (is_style_windows(style)) {
       if (str.size() > 2 && str[1] == ':' && is_separator(str[2], style))
         return 2;
     }
@@ -259,7 +267,7 @@
     // Root dir.
     if (was_net ||
         // c:/
-        (real_style(S) == Style::windows && Component.endswith(":"))) {
+        (is_style_windows(S) && Component.endswith(":"))) {
       Component = Path.substr(Position, 1);
       return *this;
     }
@@ -348,7 +356,7 @@
   if (b != e) {
     bool has_net =
         b->size() > 2 && is_separator((*b)[0], style) && (*b)[1] == (*b)[0];
-    bool has_drive = (real_style(style) == Style::windows) && b->endswith(":");
+    bool has_drive = is_style_windows(style) && b->endswith(":");
 
     if (has_net || has_drive) {
       if ((++pos != e) && is_separator((*pos)[0], style)) {
@@ -373,7 +381,7 @@
   if (b != e) {
     bool has_net =
         b->size() > 2 && is_separator((*b)[0], style) && (*b)[1] == (*b)[0];
-    bool has_drive = (real_style(style) == Style::windows) && b->endswith(":");
+    bool has_drive = is_style_windows(style) && b->endswith(":");
 
     if (has_net || has_drive) {
       // just {C:,//net}, return the first component.
@@ -390,7 +398,7 @@
   if (b != e) {
     bool has_net =
         b->size() > 2 && is_separator((*b)[0], style) && (*b)[1] == (*b)[0];
-    bool has_drive = (real_style(style) == Style::windows) && b->endswith(":");
+    bool has_drive = is_style_windows(style) && b->endswith(":");
 
     if ((has_net || has_drive) &&
         // {C:,//net}, skip to the next component.
@@ -497,7 +505,7 @@
 static bool starts_with(StringRef Path, StringRef Prefix,
                         Style style = Style::native) {
   // Windows prefix matching : case and separator insensitive
-  if (real_style(style) == Style::windows) {
+  if (is_style_windows(style)) {
     if (Path.size() < Prefix.size())
       return false;
     for (size_t I = 0, E = Prefix.size(); I != E; ++I) {
@@ -548,8 +556,10 @@
 void native(SmallVectorImpl<char> &Path, Style style) {
   if (Path.empty())
     return;
-  if (real_style(style) == Style::windows) {
-    std::replace(Path.begin(), Path.end(), '/', '\\');
+  if (is_style_windows(style)) {
+    std::replace_if(Path.begin(), Path.end(),
+                    std::bind(is_separator, std::placeholders::_1, style),
+                    preferred_separator(style));
     if (Path[0] == '~' && (Path.size() == 1 || is_separator(Path[1], style))) {
       SmallString<128> PathHome;
       home_directory(PathHome);
@@ -561,8 +571,14 @@
   }
 }
 
+void make_preferred(SmallVectorImpl<char> &path, Style style) {
+  if (!is_style_windows(style))
+    return;
+  native(path, style);
+}
+
 std::string convert_to_slash(StringRef path, Style style) {
-  if (real_style(style) != Style::windows)
+  if (!is_style_windows(style))
     return std::string(path);
 
   std::string s = path.str();
@@ -597,7 +613,7 @@
 bool is_separator(char value, Style style) {
   if (value == '/')
     return true;
-  if (real_style(style) == Style::windows)
+  if (is_style_windows(style))
     return value == '\\';
   return false;
 }
@@ -670,7 +686,7 @@
 
   bool rootDir = has_root_directory(p, style);
   bool rootName =
-      (real_style(style) != Style::windows) || has_root_name(p, style);
+      !is_style_windows(style) || has_root_name(p, style);
 
   return rootDir && rootName;
 }
@@ -684,7 +700,7 @@
   if (!p.empty() && is_separator(p.front(), style))
     return true;
 
-  if (real_style(style) == Style::windows) {
+  if (is_style_windows(style)) {
     // Handle drive letter pattern (a character followed by ':') on Windows.
     if (p.size() >= 2 && (p[0] && p[1] == ':'))
       return true;
@@ -904,7 +920,7 @@
   bool rootName = path::has_root_name(p);
 
   // Already absolute.
-  if ((rootName || real_style(Style::native) != Style::windows) &&
+  if ((rootName || !is_style_windows(Style::native)) &&
       rootDirectory)
     return;
 
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
@@ -76,6 +76,7 @@
   EXPECT_FALSE(path::is_separator(' '));
 
   EXPECT_TRUE(path::is_separator('\\', path::Style::windows));
+  EXPECT_TRUE(path::is_separator('\\', path::Style::windows_forward));
   EXPECT_FALSE(path::is_separator('\\', path::Style::posix));
 
 #ifdef _WIN32
@@ -363,6 +364,8 @@
               testing::ElementsAre("/", ".c", ".d", "..", "."));
   EXPECT_THAT(GetComponents("c:\\c\\e\\foo.txt", path::Style::windows),
               testing::ElementsAre("c:", "\\", "c", "e", "foo.txt"));
+  EXPECT_THAT(GetComponents("c:\\c\\e\\foo.txt", path::Style::windows_forward),
+              testing::ElementsAre("c:", "\\", "c", "e", "foo.txt"));
   EXPECT_THAT(GetComponents("//net/"), testing::ElementsAre("//net", "/"));
   EXPECT_THAT(GetComponents("//net/c/foo.txt"),
               testing::ElementsAre("//net", "/", "c", "foo.txt"));
@@ -1405,10 +1408,25 @@
   for (auto &T : Tests) {
     SmallString<64> Win(std::get<0>(T));
     SmallString<64> Posix(Win);
+    SmallString<64> WinForward(Win);
     path::native(Win, path::Style::windows);
     path::native(Posix, path::Style::posix);
+    path::native(WinForward, path::Style::windows_forward);
     EXPECT_EQ(std::get<1>(T), Win);
     EXPECT_EQ(std::get<2>(T), Posix);
+    EXPECT_EQ(std::get<2>(T), WinForward);
+  }
+
+  for (auto &T : Tests) {
+    SmallString<64> Win(std::get<0>(T));
+    SmallString<64> Posix(Win);
+    SmallString<64> WinForward(Win);
+    path::make_preferred(Win, path::Style::windows);
+    path::make_preferred(Posix, path::Style::posix);
+    path::make_preferred(WinForward, path::Style::windows_forward);
+    EXPECT_EQ(std::get<1>(T), Win);
+    EXPECT_EQ(std::get<0>(T), Posix); // Posix remains unchanged here
+    EXPECT_EQ(std::get<2>(T), WinForward);
   }
 
 #if defined(_WIN32)
@@ -1417,10 +1435,15 @@
 
   const char *Path7a = "~/aaa";
   SmallString<64> Path7(Path7a);
-  path::native(Path7);
+  path::native(Path7, path::Style::windows);
   EXPECT_TRUE(Path7.endswith("\\aaa"));
   EXPECT_TRUE(Path7.startswith(PathHome));
   EXPECT_EQ(Path7.size(), PathHome.size() + strlen(Path7a + 1));
+  Path7 = Path7a;
+  path::native(Path7, path::Style::windows_forward);
+  EXPECT_TRUE(Path7.endswith("/aaa"));
+  EXPECT_TRUE(Path7.startswith(PathHome));
+  EXPECT_EQ(Path7.size(), PathHome.size() + strlen(Path7a + 1));
 
   const char *Path8a = "~";
   SmallString<64> Path8(Path8a);
@@ -1434,7 +1457,7 @@
 
   const char *Path10a = "aaa/~/b";
   SmallString<64> Path10(Path10a);
-  path::native(Path10);
+  path::native(Path10, path::Style::windows);
   EXPECT_EQ(Path10, "aaa\\~\\b");
 #endif
 }