Index: include/llvm/Support/Path.h
===================================================================
--- include/llvm/Support/Path.h
+++ include/llvm/Support/Path.h
@@ -152,15 +152,21 @@
 /// @code
 ///   /foo, /old, /new => /foo
 ///   /old/foo, /old, /new => /new/foo
+///   /old/foo, /old/, /new => /new/foo
+///   /old/foo, /old/, /new/ => /new/foo
+///   /oldfoo, /old, /new => /oldfoo
 ///   /foo, <empty>, /new => /new/foo
-///   /old/foo, /old, <empty> => /foo
+///   /foo, <empty>, new => new/foo
+///   /old/foo, /old, <empty> => foo
 /// @endcode
 ///
 /// @param Path If \a Path starts with \a OldPrefix modify to instead
 ///        start with \a NewPrefix.
-/// @param OldPrefix The path prefix to strip from \a Path.
+/// @param OldPrefix The path prefix to strip from \a Path. Any trailing
+///        path separator is ignored.
 /// @param NewPrefix The path prefix to replace \a NewPrefix with.
-void replace_path_prefix(SmallVectorImpl<char> &Path,
+/// @result true if \a Path begins with OldPrefix
+bool replace_path_prefix(SmallVectorImpl<char> &Path,
                          const StringRef &OldPrefix, const StringRef &NewPrefix,
                          Style style = Style::native);
 
Index: lib/Support/Path.cpp
===================================================================
--- lib/Support/Path.cpp
+++ lib/Support/Path.cpp
@@ -521,27 +521,44 @@
   path.append(ext.begin(), ext.end());
 }
 
-void replace_path_prefix(SmallVectorImpl<char> &Path,
+bool replace_path_prefix(SmallVectorImpl<char> &Path,
                          const StringRef &OldPrefix, const StringRef &NewPrefix,
                          Style style) {
   if (OldPrefix.empty() && NewPrefix.empty())
-    return;
+    return false;
 
   StringRef OrigPath(Path.begin(), Path.size());
-  if (!OrigPath.startswith(OldPrefix))
-    return;
+  StringRef OldPrefixDir;
 
+  // Ensure OldPrefixDir does not have a trailing separator.
+  if (!OldPrefix.empty() && is_separator(OldPrefix.back()))
+    OldPrefixDir = parent_path(OldPrefix, style);
+  else
+    OldPrefixDir = OldPrefix;
+
+  if (!OrigPath.startswith(OldPrefixDir))
+    return false;
+
+  if (!is_separator(OrigPath[OldPrefixDir.size()], style) &&
+      OrigPath.size() > OldPrefixDir.size())
+    return false;
+
   // If prefixes have the same size we can simply copy the new one over.
-  if (OldPrefix.size() == NewPrefix.size()) {
+  if (OldPrefixDir.size() == NewPrefix.size()) {
     llvm::copy(NewPrefix, Path.begin());
-    return;
+    return true;
   }
 
-  StringRef RelPath = OrigPath.substr(OldPrefix.size());
+  StringRef RelPath = OrigPath.substr(OldPrefixDir.size());
   SmallString<256> NewPath;
   path::append(NewPath, style, NewPrefix);
-  path::append(NewPath, style, RelPath);
+  if (!is_separator(RelPath[0], style))
+    path::append(NewPath, style, RelPath);
+  else
+    path::append(NewPath, style, relative_path(RelPath, style));
   Path.swap(NewPath);
+
+  return true;
 }
 
 void native(const Twine &path, SmallVectorImpl<char> &result, Style style) {
Index: unittests/Support/Path.cpp
===================================================================
--- unittests/Support/Path.cpp
+++ unittests/Support/Path.cpp
@@ -1208,7 +1208,9 @@
 TEST(Support, ReplacePathPrefix) {
   SmallString<64> Path1("/foo");
   SmallString<64> Path2("/old/foo");
+  SmallString<64> Path3("/oldnew/foo");
   SmallString<64> OldPrefix("/old");
+  SmallString<64> OldPrefixSep("/old/");
   SmallString<64> NewPrefix("/new");
   SmallString<64> NewPrefix2("/longernew");
   SmallString<64> EmptyPrefix("");
@@ -1227,7 +1229,25 @@
   EXPECT_EQ(Path, "/new/foo");
   Path = Path2;
   path::replace_path_prefix(Path, OldPrefix, EmptyPrefix);
-  EXPECT_EQ(Path, "/foo");
+  EXPECT_EQ(Path, "foo");
+  Path = Path3;
+  path::replace_path_prefix(Path, OldPrefix, NewPrefix);
+  EXPECT_EQ(Path, "/oldnew/foo");
+  Path = Path3;
+  path::replace_path_prefix(Path, OldPrefixSep, NewPrefix);
+  EXPECT_EQ(Path, "/oldnew/foo");
+  Path = Path1;
+  path::replace_path_prefix(Path, EmptyPrefix, NewPrefix);
+  EXPECT_EQ(Path, "/new/foo");
+  Path = OldPrefix;
+  path::replace_path_prefix(Path, OldPrefix, NewPrefix);
+  EXPECT_EQ(Path, "/new");
+  Path = OldPrefixSep;
+  path::replace_path_prefix(Path, OldPrefix, NewPrefix);
+  EXPECT_EQ(Path, "/new/");
+  Path = OldPrefix;
+  path::replace_path_prefix(Path, OldPrefixSep, NewPrefix);
+  EXPECT_EQ(Path, "/new");
 }
 
 TEST_F(FileSystemTest, OpenFileForRead) {