Update -ffast-math documentation to match reality.

[jlebar]

Update per Richard's review.

[rsmith]

docs/UsersManual.rst

1067

fp -> floating-point

include/clang/Basic/LangOptions.def

165

fp -> FP

[jlebar]

Thank you for the review!

docs/UsersManual.rst

.. option:: -fsanitize-undefined-trap-on-error
    Deprecated alias for ``-fsanitize-trap=undefined``.
 
 .. option:: -fsanitize-cfi-cross-dso
 
    Enable cross-DSO control flow integrity checks. This flag modifies
    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
    devirtualization and virtual constant propagation, for classes with
    :doc:`hidden LTO visibility <LTOVisibility>`. Requires ``-flto``.
 
 .. option:: -fno-assume-sane-operator-new
 

include/clang/Basic/LangOptions.def

 VALUE_LANGOPT(MaxTypeAlign  , 32, 0,
               "default maximum alignment for types")
 VALUE_LANGOPT(AlignDouble            , 1, 0, "Controls if doubles should be aligned to 8 bytes (x86 only)")
 COMPATIBLE_VALUE_LANGOPT(PICLevel    , 2, 0, "__PIC__ level")
 COMPATIBLE_VALUE_LANGOPT(PIELevel    , 2, 0, "__PIE__ level")
 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")
 
 BENIGN_LANGOPT(ObjCGCBitmapPrint , 1, 0, "printing of GC's bitmap layout for __weak/__strong ivars")
 
 BENIGN_LANGOPT(AccessControl     , 1, 1, "C++ access control")
 LANGOPT(CharIsSigned      , 1, 1, "signed char")
 LANGOPT(ShortWChar        , 1, 0, "unsigned short wchar_t")

include/clang/Driver/Options.td

 def : Flag<["-"], "fno-expensive-optimizations">, Group<clang_ignored_gcc_optimization_f_Group>;
 def fextdirs_EQ : Joined<["-"], "fextdirs=">, Group<f_Group>;
 def : Flag<["-"], "fdefer-pop">, Group<clang_ignored_gcc_optimization_f_Group>;
 def : Flag<["-"], "fno-defer-pop">, Group<clang_ignored_gcc_optimization_f_Group>;
 def : Flag<["-"], "fextended-identifiers">, Group<clang_ignored_f_Group>;
 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">;
 def fno_math_errno : Flag<["-"], "fno-math-errno">, Group<f_Group>;
 def fbracket_depth_EQ : Joined<["-"], "fbracket-depth=">, Group<f_Group>;
 def fsignaling_math : Flag<["-"], "fsignaling-math">, Group<f_Group>;
 def fno_signaling_math : Flag<["-"], "fno-signaling-math">, Group<f_Group>;
 def fjump_tables : Flag<["-"], "fjump-tables">, Group<f_Group>;