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
@@ -451,10 +451,48 @@
 
 /// Is path absolute?
 ///
+/// According to cppreference.com, C++17 states: "An absolute path is a path
+/// that unambiguously identifies the location of a file without reference to
+/// an additional starting location."
+///
+/// In other words, the rules are:
+/// 1) POSIX style paths with nonempty root directory are absolute.
+/// 2) Windows style paths with nonempty root name and root directory are
+///    absolute.
+/// 3) No other paths are absolute.
+///
+/// \see has_root_name
+/// \see has_root_directory
+///
 /// @param path Input path.
 /// @result True if the path is absolute, false if it is not.
 bool is_absolute(const Twine &path, Style style = Style::native);
 
+/// Is path absolute using GNU rules?
+///
+/// GNU rules are:
+/// 1) Paths starting with a path separator are absolute.
+/// 2) Windows style paths are also absolute if they start with a character
+///    followed by ':'.
+/// 3) No other paths are absolute.
+///
+/// On Windows style the path "C:\Users\Default" has "C:" as root name and "\"
+/// as root directory.
+///
+/// Hence "C:" on Windows is absolute under GNU rules and not absolute under
+/// C++17 because it has no root directory. Likewise "/" and "\" on Windows are
+/// absolute under GNU and are not absolute under C++17 due to empty root name.
+///
+/// \see has_root_name
+/// \see has_root_directory
+///
+/// @param path Input path.
+/// @param style The style of \p path (e.g. Windows or POSIX). "native" style
+/// means to derive the style from the host.
+/// @result True if the path is absolute following GNU rules, false if it is
+/// not.
+bool is_absolute_gnu(const Twine &path, Style style = Style::native);
+
 /// Is path relative?
 ///
 /// @param path Input 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
@@ -683,6 +683,24 @@
   return rootDir && rootName;
 }
 
+bool is_absolute_gnu(const Twine &path, Style style) {
+  SmallString<128> path_storage;
+  StringRef p = path.toStringRef(path_storage);
+
+  // Handle '/' which is absolute for both Windows and POSIX systems.
+  // Handle '\\' on Windows.
+  if (!p.empty() && is_separator(p.front(), style))
+    return true;
+
+  if (real_style(style) == Style::windows) {
+    // Handle drive letter pattern (a character followed by ':') on Windows.
+    if (p.size() >= 2 && (p[0] && p[1] == ':'))
+      return true;
+  }
+
+  return false;
+}
+
 bool is_relative(const Twine &path, Style style) {
   return !is_absolute(path, style);
 }
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
@@ -85,6 +85,23 @@
 #endif
 }
 
+TEST(is_absolute_gnu, Works) {
+  // Test tuple <Path, ExpectedPosixValue, ExpectedWindowsValue>.
+  const std::tuple<StringRef, bool, bool> Paths[] = {
+      {"", false, false},  {"/", true, true},      {"/foo", true, true},
+      {"\\", false, true}, {"\\foo", false, true}, {"foo", false, false},
+      {"c", false, false}, {"c:", false, true},    {"c:\\", false, true},
+      {"!:", false, true}, {"xx:", false, false},  {"c:abc\\", false, true},
+      {":", false, false}};
+
+  for (const auto &Path : Paths) {
+    EXPECT_EQ(path::is_absolute_gnu(std::get<0>(Path), path::Style::posix),
+              std::get<1>(Path));
+    EXPECT_EQ(path::is_absolute_gnu(std::get<0>(Path), path::Style::windows),
+              std::get<2>(Path));
+  }
+}
+
 TEST(Support, Path) {
   SmallVector<StringRef, 40> paths;
   paths.push_back("");
@@ -171,6 +188,7 @@
     (void)path::has_extension(*i);
     (void)path::extension(*i);
     (void)path::is_absolute(*i);
+    (void)path::is_absolute_gnu(*i);
     (void)path::is_relative(*i);
 
     SmallString<128> temp_store;