On Tue, Sep 07, 2021 at 09:52:57AM +0800, Hongtao Liu wrote:
> Adjust the wording for x86 _Float16 type.
>
> gcc/ChangeLog:
>
> * doc/extend.texi: (@node Floating Types): Adjust the wording.
> (@node Half-Precision): Ditto.
>
> 1 file changed, 15 insertions(+), 13 deletions(-)
> gcc/doc/extend.texi | 28 +++++++++++++++-------------
>
> modified gcc/doc/extend.texi
> @@ -1076,9 +1076,10 @@ systems where @code{__float128} is supported.
> The @code{_Float32}
> type is supported on all systems supporting IEEE binary32; the
> @code{_Float64} and @code{_Float32x} types are supported on all systems
> supporting IEEE binary64. The @code{_Float16} type is supported on AArch64
> -systems by default, and on ARM systems when the IEEE format for 16-bit
> -floating-point types is selected with @option{-mfp16-format=ieee}.
> -GCC does not currently support @code{_Float128x} on any systems.
> +systems by default when the IEEE format for 16-bit floating-point types is
The AArch64 case now has the ARM case restriction and ARM is lost. It
should be
+systems by default, on ARM systems when the IEEE format for 16-bit
+floating-point-types is
> +selected with @option{-mfp16-format=ieee} and, for both C and C++, on x86
> +systems with SSE2 enabled. GCC does not currently support
> +@code{_Float128x} on any systems.
>
> On the i386, x86_64, IA-64, and HP-UX targets, you can declare complex
> types using the corresponding internal complex type, @code{XCmode} for
> @@ -1108,6 +1109,12 @@ 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.
> +On x86 targets with SSE2 enabled, GCC supports half-precision (16-bit)
> +floating point via the @code{_Float16} type, there are many ways to enable
> +SSE2, @option{-msse2, -mavx, -mavx512f, ...} on the command line, or various
> +target attributes.
The ", there are many ways ... attributes" was just meant as explanation for
the "with SSE2 enabled" wording, not something that should be literally in
the documentation. It is documented elsewhere...
> +For C++, x86 provides a builtin type named @code{_Float16} which contains
> +same data format as C.
>
> ARM targets support two incompatible representations for half-precision
> floating-point values. You must choose one of the representations and
> @@ -1151,16 +1158,11 @@ calls.
> It is recommended that portable code use the @code{_Float16} type defined
> by ISO/IEC TS 18661-3:2015. @xref{Floating Types}.
>
> -On x86 targets with @code{target("sse2")} and above, GCC supports
> half-precision
> -(16-bit) floating point via the @code{_Float16} type which is defined by
> -18661-3:2015. For C++, x86 provide a builtin type named @code{_Float16}
> -which contains same data format as C.
> -
> -Without @option{-mavx512fp16}, @code{_Float16} type is storage only, all
> -operations will be emulated by software emulation and the @code{float}
> -instructions. The default behavior for @code{FLT_EVAL_METHOD} is to keep
> -the intermediate result of the operation as 32-bit precision. This may lead
> -to inconsistent behavior between software emulation and AVX512-FP16
> +On x86 targets, without @option{-mavx512fp16}, @code{_Float16} type is
> +storage only, all operations will be emulated by software emulation and the
> +@code{float} instructions. The default behavior for @code{FLT_EVAL_METHOD} is
> +to keep the intermediate result of the operation as 32-bit precision. This
> may
> +lead to inconsistent behavior between software emulation and AVX512-FP16
> instructions.
>
> @node Decimal Float
Jakub
