On Thu, Dec 01, 2016 at 11:09:07AM +0000, James Greenhalgh wrote:
>
> On Wed, Nov 30, 2016 at 05:58:13PM +0000, Joseph Myers wrote:
> > On Wed, 30 Nov 2016, James Greenhalgh wrote:
> >
> > > +@code{_Float16} type defined by ISO/IEC TS18661:3-2005
> >
> > Add a space after "TS", and it's -3:2015 not :3-2005.
>
> You would think that after 2 months of having the specification sitting
> on my desk I'd have got that right... Fixed in this revision.
*ping*
Thanks,
James
>
> > I think the -mfp16-format documentation in invoke.texi should also be
> > updated to reflect that it affects availability of _Float16.
>
> I'm working on something larger for -mfp16-format, I'll update invoke.texi
> at that point (or otherwise before GCC 7 releases).
>
> Thanks,
> James
>
> ---
>
> 2016-12-01 James Greenhalgh <[email protected]>
>
> * doc/extend.texi (Half-Precision): Update to document current
> compiler behaviour.
>
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 7d3d17a..23d03bd 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -1012,11 +1012,12 @@ that handle conversions if/when long double is
> changed to be IEEE
> @cindex half-precision floating point
> @cindex @code{__fp16} data type
>
> -On ARM targets, GCC supports half-precision (16-bit) floating point via
> -the @code{__fp16} type. You must enable this type explicitly
> -with the @option{-mfp16-format} command-line option in order to use it.
> +On ARM and AArch64 targets, GCC supports half-precision (16-bit) floating
> +point via the @code{__fp16} type defined in the ARM C Language Extensions.
> +On ARM systems, you must enable this type explicitly with the
> +@option{-mfp16-format} command-line option in order to use it.
>
> -ARM supports two incompatible representations for half-precision
> +ARM targets support two incompatible representations for half-precision
> floating-point values. You must choose one of the representations and
> use it consistently in your program.
>
> @@ -1031,22 +1032,20 @@ format, but does not support infinities or NaNs.
> Instead, the range
> of exponents is extended, so that this format can represent normalized
> values in the range of @math{2^{-14}} to 131008.
>
> -The @code{__fp16} type is a storage format only. For purposes
> -of arithmetic and other operations, @code{__fp16} values in C or C++
> -expressions are automatically promoted to @code{float}. In addition,
> -you cannot declare a function with a return value or parameters
> -of type @code{__fp16}.
> +The GCC port for AArch64 only supports the IEEE 754-2008 format, and does
> +not require use of the @option{-mfp16-format} command-line option.
>
> -Note that conversions from @code{double} to @code{__fp16}
> -involve an intermediate conversion to @code{float}. Because
> -of rounding, this can sometimes produce a different result than a
> -direct conversion.
> +The @code{__fp16} type may only be used as an argument to intrinsics defined
> +in @code{<arm_fp16.h>}, or as a storage format. For purposes of
> +arithmetic and other operations, @code{__fp16} values in C or C++
> +expressions are automatically promoted to @code{float}.
>
> -ARM provides hardware support for conversions between
> +The ARM target provides hardware support for conversions between
> @code{__fp16} and @code{float} values
> -as an extension to VFP and NEON (Advanced SIMD). GCC generates
> -code using these hardware instructions if you compile with
> -options to select an FPU that provides them;
> +as an extension to VFP and NEON (Advanced SIMD), and from ARMv8 provides
> +hardware support for conversions between @code{__fp16} and @code{double}
> +values. GCC generates code using these hardware instructions if you
> +compile with options to select an FPU that provides them;
> for example, @option{-mfpu=neon-fp16 -mfloat-abi=softfp},
> in addition to the @option{-mfp16-format} option to select
> a half-precision format.
> @@ -1054,8 +1053,12 @@ a half-precision format.
> Language-level support for the @code{__fp16} data type is
> independent of whether GCC generates code using hardware floating-point
> instructions. In cases where hardware support is not specified, GCC
> -implements conversions between @code{__fp16} and @code{float} values
> -as library calls.
> +implements conversions between @code{__fp16} and other types as library
> +calls.
> +
> +It is recommended that code which is intended to be portable use the
> +@code{_Float16} type defined by ISO/IEC TS 18661-3:2015
> +(@xref{Floating Types}).
>
> @node Decimal Float
> @section Decimal Floating Types
