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.
> 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
