Index: include/lldb/Utility/FileSpec.h
===================================================================
--- include/lldb/Utility/FileSpec.h
+++ include/lldb/Utility/FileSpec.h
@@ -491,7 +491,7 @@
   ///     the static FileSpec::Resolve.
   //------------------------------------------------------------------
   void SetFile(llvm::StringRef path, bool resolve_path,
-               Style style = Style::native);
+               llvm::Optional<Style> style = llvm::None);
 
   void SetFile(llvm::StringRef path, bool resolve_path,
                const llvm::Triple &Triple);
Index: source/Core/Debugger.cpp
===================================================================
--- source/Core/Debugger.cpp
+++ source/Core/Debugger.cpp
@@ -605,8 +605,8 @@
                    const FileSpec &file_spec) {
   Status error;
 
-  static ConstString g_dylibext("dylib");
-  static ConstString g_solibext("so");
+  static ConstString g_dylibext(".dylib");
+  static ConstString g_solibext(".so");
 
   if (!baton)
     return FileSpec::eEnumerateDirectoryResultQuit;
Index: source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp
===================================================================
--- source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp
+++ source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp
@@ -2047,8 +2047,8 @@
   // custom extension and file name makes it highly unlikely that this will
   // collide with anything else.
   ConstString file_extension = m_file.GetFileNameExtension();
-  bool skip_oatdata_oatexec = file_extension == ConstString("oat") ||
-                              file_extension == ConstString("odex");
+  bool skip_oatdata_oatexec = file_extension == ConstString(".oat") ||
+                              file_extension == ConstString(".odex");
 
   ArchSpec arch;
   GetArchitecture(arch);
Index: source/Plugins/Platform/Android/PlatformAndroid.cpp
===================================================================
--- source/Plugins/Platform/Android/PlatformAndroid.cpp
+++ source/Plugins/Platform/Android/PlatformAndroid.cpp
@@ -305,7 +305,7 @@
                                            const FileSpec &dst_file_spec) {
   // For oat file we can try to fetch additional debug info from the device
   ConstString extension = module_sp->GetFileSpec().GetFileNameExtension();
-  if (extension != ConstString("oat") && extension != ConstString("odex"))
+  if (extension != ConstString(".oat") && extension != ConstString(".odex"))
     return Status(
         "Symbol file downloading only supported for oat and odex files");
 
Index: source/Plugins/Platform/MacOSX/PlatformDarwinKernel.cpp
===================================================================
--- source/Plugins/Platform/MacOSX/PlatformDarwinKernel.cpp
+++ source/Plugins/Platform/MacOSX/PlatformDarwinKernel.cpp
@@ -431,8 +431,8 @@
 FileSpec::EnumerateDirectoryResult
 PlatformDarwinKernel::FindKDKandSDKDirectoriesInDirectory(
     void *baton, llvm::sys::fs::file_type ft, const FileSpec &file_spec) {
-  static ConstString g_sdk_suffix = ConstString("sdk");
-  static ConstString g_kdk_suffix = ConstString("kdk");
+  static ConstString g_sdk_suffix = ConstString(".sdk");
+  static ConstString g_kdk_suffix = ConstString(".kdk");
 
   PlatformDarwinKernel *thisp = (PlatformDarwinKernel *)baton;
   if (ft == llvm::sys::fs::file_type::directory_file &&
@@ -492,8 +492,8 @@
 PlatformDarwinKernel::GetKernelsAndKextsInDirectoryHelper(
     void *baton, llvm::sys::fs::file_type ft, const FileSpec &file_spec,
     bool recurse) {
-  static ConstString g_kext_suffix = ConstString("kext");
-  static ConstString g_dsym_suffix = ConstString("dSYM");
+  static ConstString g_kext_suffix = ConstString(".kext");
+  static ConstString g_dsym_suffix = ConstString(".dSYM");
   static ConstString g_bundle_suffix = ConstString("Bundle");
   ConstString file_spec_extension = file_spec.GetFileNameExtension();
 
Index: source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp
===================================================================
--- source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp
+++ source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp
@@ -2590,9 +2590,9 @@
       // strip .py or .pyc extension
       ConstString extension = target_file.GetFileNameExtension();
       if (extension) {
-        if (::strcmp(extension.GetCString(), "py") == 0)
+        if (::strcmp(extension.GetCString(), ".py") == 0)
           basename.resize(basename.length() - 3);
-        else if (::strcmp(extension.GetCString(), "pyc") == 0)
+        else if (::strcmp(extension.GetCString(), ".pyc") == 0)
           basename.resize(basename.length() - 4);
       }
     } else {
Index: source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
===================================================================
--- source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
+++ source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
@@ -1653,7 +1653,7 @@
             // (corresponding to .dwo) so we simply skip it.
             if (m_obj_file->GetFileSpec()
                         .GetFileNameExtension()
-                        .GetStringRef() == "dwo" &&
+                        .GetStringRef() == ".dwo" &&
                 llvm::StringRef(m_obj_file->GetFileSpec().GetPath())
                     .endswith(dwo_module_spec.GetFileSpec().GetPath())) {
               continue;
Index: source/Utility/FileSpec.cpp
===================================================================
--- source/Utility/FileSpec.cpp
+++ source/Utility/FileSpec.cpp
@@ -15,9 +15,9 @@
 #include "llvm/ADT/SmallString.h" // for SmallString
 #include "llvm/ADT/SmallVector.h" // for SmallVectorTemplat...
 #include "llvm/ADT/StringRef.h"
-#include "llvm/ADT/Triple.h"         // for Triple
-#include "llvm/ADT/Twine.h"          // for Twine
-#include "llvm/Support/ErrorOr.h"    // for ErrorOr
+#include "llvm/ADT/Triple.h"      // for Triple
+#include "llvm/ADT/Twine.h"       // for Twine
+#include "llvm/Support/ErrorOr.h" // for ErrorOr
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/raw_ostream.h" // for raw_ostream, fs
@@ -50,7 +50,7 @@
 }
 
 const char *GetPathSeparators(FileSpec::Style style) {
-  return PathStyleIsPosix(style) ? "/" : "\\/";
+  return llvm::sys::path::get_separator(style).data();
 }
 
 char GetPreferredPathSeparator(FileSpec::Style style) {
@@ -67,30 +67,6 @@
 
   std::replace(path.begin(), path.end(), '/', '\\');
 }
-  
-bool PathIsRelative(llvm::StringRef path, FileSpec::Style style) {
-  
-  if (path.empty())
-    return false;
-
-  if (PathStyleIsPosix(style)) {
-    // If the path doesn't start with '/' or '~', return true
-    switch (path[0]) {
-      case '/':
-      case '~':
-        return false;
-      default:
-        return true;
-    }
-  } else {
-    if (path.size() >= 2 && path[1] == ':')
-      return false;
-    if (path[0] == '/')
-      return false;
-    return true;
-  }
-  return false;
-}
 
 } // end anonymous namespace
 
@@ -239,7 +215,7 @@
   return false;
 }
 
-  
+
 }
 //------------------------------------------------------------------
 // Assignment operator.
@@ -259,11 +235,15 @@
 // up into a directory and filename and stored as uniqued string values for
 // quick comparison and efficient memory usage.
 //------------------------------------------------------------------
-void FileSpec::SetFile(llvm::StringRef pathname, bool resolve, Style style) {
+void FileSpec::SetFile(llvm::StringRef pathname, bool resolve,
+                       llvm::Optional<Style> style) {
   m_filename.Clear();
   m_directory.Clear();
   m_is_resolved = false;
-  m_style = (style == Style::native) ? GetNativeStyle() : style;
+
+  // Only update style if explicitly requested.
+  if (style)
+    m_style = (*style == Style::native) ? GetNativeStyle() : *style;
 
   if (pathname.empty())
     return;
@@ -286,15 +266,19 @@
   if (resolved.empty()) {
     // If we have no path after normalization set the path to the current
     // directory. This matches what python does and also a few other path
-    // utilities. 
+    // utilities.
     m_filename.SetString(".");
     return;
   }
 
-  m_filename.SetString(llvm::sys::path::filename(resolved, m_style));
-  llvm::StringRef dir = llvm::sys::path::parent_path(resolved, m_style);
-  if (!dir.empty())
-    m_directory.SetString(dir);
+  // Split path into filename and directory. We rely on the underlying char
+  // pointer to be nullptr when the components are empty.
+  llvm::StringRef filename = llvm::sys::path::filename(resolved, m_style);
+  if(!filename.empty())
+    m_filename.SetString(filename);
+  llvm::StringRef directory = llvm::sys::path::parent_path(resolved, m_style);
+  if(!directory.empty())
+    m_directory.SetString(directory);
 }
 
 void FileSpec::SetFile(llvm::StringRef path, bool resolve,
@@ -441,16 +425,15 @@
 }
 
 bool FileSpec::Equal(const FileSpec &a, const FileSpec &b, bool full) {
-
   // case sensitivity of equality test
   const bool case_sensitive = a.IsCaseSensitive() || b.IsCaseSensitive();
-  
+
   const bool filenames_equal = ConstString::Equals(a.m_filename,
                                                    b.m_filename,
                                                    case_sensitive);
 
   if (!filenames_equal)
-      return false;
+    return false;
 
   if (!full && (a.GetDirectory().IsEmpty() || b.GetDirectory().IsEmpty()))
     return filenames_equal;
@@ -606,25 +589,15 @@
 }
 
 ConstString FileSpec::GetFileNameExtension() const {
-  if (m_filename) {
-    const char *filename = m_filename.GetCString();
-    const char *dot_pos = strrchr(filename, '.');
-    if (dot_pos && dot_pos[1] != '\0')
-      return ConstString(dot_pos + 1);
-  }
-  return ConstString();
+  llvm::SmallString<64> current_path;
+  GetPath(current_path, false);
+  return ConstString(llvm::sys::path::extension(current_path, m_style));
 }
 
 ConstString FileSpec::GetFileNameStrippingExtension() const {
-  const char *filename = m_filename.GetCString();
-  if (filename == NULL)
-    return ConstString();
-
-  const char *dot_pos = strrchr(filename, '.');
-  if (dot_pos == NULL)
-    return m_filename;
-
-  return ConstString(filename, dot_pos - filename);
+  llvm::SmallString<64> current_path;
+  GetPath(current_path, false);
+  return ConstString(llvm::sys::path::stem(current_path, m_style));
 }
 
 //------------------------------------------------------------------
@@ -760,7 +733,7 @@
   std::string result =
       join_path_components(m_style, {component, m_directory.GetStringRef(),
                                      m_filename.GetStringRef()});
-  SetFile(result, resolve, m_style);
+  SetFile(result, resolve);
 }
 
 void FileSpec::PrependPathComponent(const FileSpec &new_path) {
@@ -778,7 +751,7 @@
       join_path_components(m_style, {m_directory.GetStringRef(),
                                      m_filename.GetStringRef(), component});
 
-  SetFile(result, false, m_style);
+  SetFile(result, false);
 }
 
 void FileSpec::AppendPathComponent(const FileSpec &new_path) {
@@ -789,8 +762,7 @@
   llvm::SmallString<64> current_path;
   GetPath(current_path, false);
   if (llvm::sys::path::has_parent_path(current_path, m_style)) {
-    SetFile(llvm::sys::path::parent_path(current_path, m_style), false,
-            m_style);
+    SetFile(llvm::sys::path::parent_path(current_path, m_style), false);
     return true;
   }
   return false;
@@ -810,7 +782,7 @@
     return false;
 
   static RegularExpression g_source_file_regex(llvm::StringRef(
-      "^([cC]|[mM]|[mM][mM]|[cC][pP][pP]|[cC]\\+\\+|[cC][xX][xX]|[cC][cC]|["
+      "^.([cC]|[mM]|[mM][mM]|[cC][pP][pP]|[cC]\\+\\+|[cC][xX][xX]|[cC][cC]|["
       "cC][pP]|[sS]|[aA][sS][mM]|[fF]|[fF]77|[fF]90|[fF]95|[fF]03|[fF][oO]["
       "rR]|[fF][tT][nN]|[fF][pP][pP]|[aA][dD][aA]|[aA][dD][bB]|[aA][dD][sS])"
       "$"));
@@ -818,13 +790,23 @@
 }
 
 bool FileSpec::IsRelative() const {
-  if (m_directory)
-    return PathIsRelative(m_directory.GetStringRef(), m_style);
-  else
-    return PathIsRelative(m_filename.GetStringRef(), m_style);
+  return !IsAbsolute();
 }
 
-bool FileSpec::IsAbsolute() const { return !FileSpec::IsRelative(); }
+bool FileSpec::IsAbsolute() const {
+  llvm::SmallString<64> current_path;
+  GetPath(current_path, false);
+
+  // Early return if the path is empty.
+  if (current_path.empty())
+    return false;
+
+  // We consider paths starting with ~ to be absolute.
+  if (current_path[0] == '~')
+    return true;
+
+  return llvm::sys::path::is_absolute(current_path, m_style);
+}
 
 void llvm::format_provider<FileSpec>::format(const FileSpec &F,
                                              raw_ostream &Stream,
Index: unittests/Utility/FileSpecTest.cpp
===================================================================
--- unittests/Utility/FileSpecTest.cpp
+++ unittests/Utility/FileSpecTest.cpp
@@ -30,6 +30,16 @@
   EXPECT_EQ(nullptr, fs_posix_root.GetDirectory().GetCString());
   EXPECT_STREQ("/", fs_posix_root.GetFilename().GetCString());
 
+  FileSpec fs_net_drive("//net", false, FileSpec::Style::posix);
+  EXPECT_STREQ("//net", fs_net_drive.GetCString());
+  EXPECT_EQ(nullptr, fs_net_drive.GetDirectory().GetCString());
+  EXPECT_STREQ("//net", fs_net_drive.GetFilename().GetCString());
+
+  FileSpec fs_net_root("//net/", false, FileSpec::Style::posix);
+  EXPECT_STREQ("//net/", fs_net_root.GetCString());
+  EXPECT_STREQ("//net", fs_net_root.GetDirectory().GetCString());
+  EXPECT_STREQ("/", fs_net_root.GetFilename().GetCString());
+
   FileSpec fs_windows_drive("F:", false, FileSpec::Style::windows);
   EXPECT_STREQ("F:", fs_windows_drive.GetCString());
   EXPECT_EQ(nullptr, fs_windows_drive.GetDirectory().GetCString());
@@ -281,7 +291,6 @@
     "/a/b",
     "/a/b/",
     "//",
-    "//a",
     "//a/",
     "//a/b",
     "//a/b/",