Re: [PATCH 2/6] [i386] Enable _Float16 type for TARGET_SSE2 and above.

[Hongtao Liu <crazylht@gmail.com>]

On Mon, Sep 6, 2021 at 8:13 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Mon, Sep 06, 2021 at 10:05:00AM +0800, Hongtao Liu wrote:
>
> > @@ -1076,9 +1076,11 @@ 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, and also on x86 systems with @code{target("sse2")}
> > +for both C and C++.
> > +On ARM systems when the IEEE format for 16-bit floating-point types is
> > +selected with @option{-mfp16-format=ieee}.
>
> This isn't a sentence.  I think it should be:
>
> The @code{_Float16} type is supported on AArch64 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.
>
> >  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 +1110,10 @@ 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 @code{target("sse2")} and above,  GCC supports
> > +half-precision (16-bit) floating point via the @code{_Float16} type.
> > +For C++, x86 provides a builtin type named @code{_Float16} which contains
> > +same data format as C.
>
> Again, I'd write with SSE2 enabled, there are many ways to enable SSE2,
> -msse2, -mavx, -mavx512f, ... on the command line, or various target
> attributes.

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
+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.
+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
>


-- 
BR,
Hongtao