From 3ffead7761acaf55a28654a8e97cee6f5c53d82d Mon Sep 17 00:00:00 2001 From: James Greenhalgh Date: Tue, 13 Dec 2016 10:43:02 +0000 Subject: [PATCH] [Patch Doc] Update documentation for __fp16 type gcc/ * doc/extend.texi (Half-Precision): Update to document current compiler behaviour. From-SVN: r243602 --- gcc/ChangeLog | 5 +++++ gcc/doc/extend.texi | 40 +++++++++++++++++++++------------------- 2 files changed, 26 insertions(+), 19 deletions(-) diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 16c1ecad8eb..5fd41549487 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,5 +1,10 @@ 2016-12-13 James Greenhalgh + * doc/extend.texi (Half-Precision): Update to document current + compiler behaviour. + +2016-12-13 James Greenhalgh + * doc/extend.texi (Floating Types): Document availability of _Float16 on ARM/AArch64. diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index d82444978fa..5f7f676ccaa 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{}, 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 +1053,11 @@ 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 portable code use the @code{_Float16} type defined +by ISO/IEC TS 18661-3:2015 (@xref{Floating Types}). @node Decimal Float @section Decimal Floating Types -- 2.11.4.GIT