Index: lib/Support/Path.cpp
===================================================================
--- lib/Support/Path.cpp
+++ lib/Support/Path.cpp
@@ -90,10 +90,9 @@
     return path.substr(0, end);
   }
 
+  // Returns the first character of the filename in str. For paths ending in
+  // '/', it returns the position of the '/'.
   size_t filename_pos(StringRef str, Style style) {
-    if (str.size() == 2 && is_separator(str[0], style) && str[0] == str[1])
-      return 0;
-
     if (str.size() > 0 && is_separator(str[str.size() - 1], style))
       return str.size() - 1;
 
@@ -110,6 +109,8 @@
     return pos + 1;
   }
 
+  // Returns the position of the root directory in str. If there is no root
+  // directory in str, it returns StringRef::npos.
   size_t root_dir_start(StringRef str, Style style) {
     // case "c:/"
     if (real_style(style) == Style::windows) {
@@ -117,10 +118,6 @@
         return 2;
     }
 
-    // case "//"
-    if (str.size() == 2 && is_separator(str[0], style) && str[0] == str[1])
-      return StringRef::npos;
-
     // case "//net"
     if (str.size() > 3 && is_separator(str[0], style) && str[0] == str[1] &&
         !is_separator(str[2], style)) {
@@ -134,22 +131,29 @@
     return StringRef::npos;
   }
 
+  // Returns the position past the end of the "parent path" of path. The parent
+  // path will not end in '/', unless the parent is the root directory. If the
+  // path has no parent, 0 is returned.
   size_t parent_path_end(StringRef path, Style style) {
     size_t end_pos = filename_pos(path, style);
 
     bool filename_was_sep =
         path.size() > 0 && is_separator(path[end_pos], style);
 
-    // Skip separators except for root dir.
-    size_t root_dir_pos = root_dir_start(path.substr(0, end_pos), style);
-
-    while (end_pos > 0 && (end_pos - 1) != root_dir_pos &&
+    // Skip separators until we reach root dir (or the start of the string).
+    size_t root_dir_pos = root_dir_start(path, style);
+    while (end_pos > 0 &&
+           (root_dir_pos == StringRef::npos || end_pos > root_dir_pos) &&
            is_separator(path[end_pos - 1], style))
       --end_pos;
 
-    if (end_pos == 1 && root_dir_pos == 0 && filename_was_sep)
-      return StringRef::npos;
+    if (end_pos == root_dir_pos && !filename_was_sep) {
+      // We've reached the root dir and the input path was *not* ending in a
+      // sequence of slashes. Include the root dir in the parent path.
+      return root_dir_pos + 1;
+    }
 
+    // Otherwise, just include before the last slash.
     return end_pos;
   }
 } // end unnamed namespace
@@ -281,8 +285,8 @@
       ++Position;
     }
 
-    // Treat trailing '/' as a '.'.
-    if (Position == Path.size()) {
+    // Treat trailing '/' as a '.', unless it is the root dir.
+    if (Position == Path.size() && Component != "/") {
       --Position;
       Component = ".";
       return *this;
@@ -321,23 +325,23 @@
 }
 
 reverse_iterator &reverse_iterator::operator++() {
-  // If we're at the end and the previous char was a '/', return '.' unless
-  // we are the root path.
   size_t root_dir_pos = root_dir_start(Path, S);
-  if (Position == Path.size() && Path.size() > root_dir_pos + 1 &&
-      is_separator(Path[Position - 1], S)) {
-    --Position;
-    Component = ".";
-    return *this;
-  }
 
   // Skip separators unless it's the root directory.
   size_t end_pos = Position;
-
   while (end_pos > 0 && (end_pos - 1) != root_dir_pos &&
          is_separator(Path[end_pos - 1], S))
     --end_pos;
 
+  // Treat trailing '/' as a '.', unless it is the root dir.
+  if (Position == Path.size() && !Path.empty() &&
+      is_separator(Path.back(), S) &&
+      (root_dir_pos == StringRef::npos || end_pos - 1 > root_dir_pos)) {
+    --Position;
+    Component = ".";
+    return *this;
+  }
+
   // Find next separator.
   size_t start_pos = filename_pos(Path.substr(0, end_pos), S);
   Component = Path.slice(start_pos, end_pos);
Index: unittests/Support/Path.cpp
===================================================================
--- unittests/Support/Path.cpp
+++ unittests/Support/Path.cpp
@@ -183,6 +183,15 @@
   EXPECT_EQ("\\", path::filename("c:\\", path::Style::windows));
   EXPECT_EQ("c:", path::parent_path("c:\\", path::Style::windows));
 
+  EXPECT_EQ("/", path::filename("//"));
+  EXPECT_EQ("", path::parent_path("//"));
+
+  EXPECT_EQ("/", path::filename("///"));
+  EXPECT_EQ("", path::parent_path("///"));
+
+  EXPECT_EQ("\\", path::filename("c:\\\\", path::Style::windows));
+  EXPECT_EQ("c:", path::parent_path("c:\\\\", path::Style::windows));
+
   EXPECT_EQ("bar", path::filename("/foo/bar"));
   EXPECT_EQ("/foo", path::parent_path("/foo/bar"));
 
@@ -213,6 +222,8 @@
 TEST(Support, PathIterator) {
   EXPECT_THAT(GetComponents("/foo"), testing::ElementsAre("/", "foo"));
   EXPECT_THAT(GetComponents("/"), testing::ElementsAre("/"));
+  EXPECT_THAT(GetComponents("//"), testing::ElementsAre("/"));
+  EXPECT_THAT(GetComponents("///"), testing::ElementsAre("/"));
   EXPECT_THAT(GetComponents("c/d/e/foo.txt"),
               testing::ElementsAre("c", "d", "e", "foo.txt"));
   EXPECT_THAT(GetComponents(".c/.d/../."),
@@ -233,10 +244,8 @@
   SmallVector<std::pair<StringRef, path::Style>, 4> Paths;
   Paths.emplace_back("/foo/", path::Style::native);
   Paths.emplace_back("/foo//", path::Style::native);
-  Paths.emplace_back("//net//", path::Style::native);
   Paths.emplace_back("//net/foo/", path::Style::native);
   Paths.emplace_back("c:\\foo\\", path::Style::windows);
-  Paths.emplace_back("c:\\\\", path::Style::windows);
 
   for (auto &Path : Paths) {
     SCOPED_TRACE(Path.first);
@@ -248,6 +257,8 @@
   RootPaths.emplace_back("/", path::Style::native);
   RootPaths.emplace_back("//net/", path::Style::native);
   RootPaths.emplace_back("c:\\", path::Style::windows);
+  RootPaths.emplace_back("//net//", path::Style::native);
+  RootPaths.emplace_back("c:\\\\", path::Style::windows);
 
   for (auto &Path : RootPaths) {
     SCOPED_TRACE(Path.first);