Index: llvm/lib/Support/Path.cpp
===================================================================
--- llvm/lib/Support/Path.cpp
+++ llvm/lib/Support/Path.cpp
@@ -885,13 +885,13 @@
   }
 
   if (rootName && !rootDirectory) {
-    StringRef pRootName      = path::root_name(p);
+    StringRef bRootName      = path::root_name(current_dir);
     StringRef bRootDirectory = path::root_directory(current_dir);
     StringRef bRelativePath  = path::relative_path(current_dir);
     StringRef pRelativePath  = path::relative_path(p);
 
     SmallString<128> res;
-    path::append(res, pRootName, bRootDirectory, bRelativePath, pRelativePath);
+    path::append(res, bRootName, bRootDirectory, bRelativePath, pRelativePath);
     path.swap(res);
     return;
   }
Index: llvm/unittests/Support/Path.cpp
===================================================================
--- llvm/unittests/Support/Path.cpp
+++ llvm/unittests/Support/Path.cpp
@@ -185,10 +185,19 @@
     path::native(*i, temp_store);
   }
 
-  SmallString<32> Relative("foo.cpp");
-  sys::fs::make_absolute("/root", Relative);
-  Relative[5] = '/'; // Fix up windows paths.
-  ASSERT_EQ("/root/foo.cpp", Relative);
+  {
+    SmallString<32> Relative("foo.cpp");
+    sys::fs::make_absolute("/root", Relative);
+    Relative[5] = '/'; // Fix up windows paths.
+    ASSERT_EQ("/root/foo.cpp", Relative);
+  }
+
+  {
+    SmallString<32> Relative("foo.cpp");
+    sys::fs::make_absolute("//root", Relative);
+    Relative[6] = '/'; // Fix up windows paths.
+    ASSERT_EQ("//root/foo.cpp", Relative);
+  }
 }
 
 TEST(Support, FilenameParent) {