Index: docs/UsersManual.rst
===================================================================
--- docs/UsersManual.rst
+++ docs/UsersManual.rst
@@ -1055,6 +1055,18 @@
    the behavior of sanitizers in the ``cfi`` group to allow checking
    of cross-DSO virtual and indirect calls.
 
+.. option:: -ffast-math
+
+   Enable fast-math mode. This defines the ``__FAST_MATH__`` preprocessor
+   macro, and lets the compiler make aggressive, potentially-lossy assumptions
+   about floating-point math.  These include:
+
+   * Floating-point math obeys regular algebraic rules for real numbers (e.g.
+     ``+`` and ``*`` are associative, ``x/y == x * (1/y)``, and
+     ``(a + b) * c == a * c + b * c``),
+   * operands to fp operations are not equal to ``NaN`` and ``Inf``, and
+   * ``+0`` and ``-0`` are interchangeable.
+
 .. option:: -fwhole-program-vtables
 
    Enable whole-program vtable optimizations, such as single-implementation
Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -162,7 +162,7 @@
 COMPATIBLE_LANGOPT(GNUInline         , 1, 0, "GNU inline semantics")
 COMPATIBLE_LANGOPT(NoInlineDefine    , 1, 0, "__NO_INLINE__ predefined macro")
 COMPATIBLE_LANGOPT(Deprecated        , 1, 0, "__DEPRECATED predefined macro")
-COMPATIBLE_LANGOPT(FastMath          , 1, 0, "__FAST_MATH__ predefined macro")
+COMPATIBLE_LANGOPT(FastMath          , 1, 0, "fast fp math optimizations, and __FAST_MATH__ predefined macro")
 COMPATIBLE_LANGOPT(FiniteMathOnly    , 1, 0, "__FINITE_MATH_ONLY__ predefined macro")
 COMPATIBLE_LANGOPT(UnsafeFPMath      , 1, 0, "Unsafe Floating Point Math")
 
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -592,9 +592,7 @@
 def : Flag<["-"], "fno-extended-identifiers">, Group<f_Group>, Flags<[Unsupported]>;
 def fhosted : Flag<["-"], "fhosted">, Group<f_Group>;
 def ffast_math : Flag<["-"], "ffast-math">, Group<f_Group>, Flags<[CC1Option]>,
-  HelpText<"Enable the *frontend*'s 'fast-math' mode. This has no effect on "
-           "optimizations, but provides a preprocessor macro __FAST_MATH__ the "
-           "same as GCC's -ffast-math flag">;
+  HelpText<"Allow aggressive, lossy floating-point optimizations">;
 def fno_fast_math : Flag<["-"], "fno-fast-math">, Group<f_Group>;
 def fmath_errno : Flag<["-"], "fmath-errno">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Require math functions to indicate errors by setting errno">;