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diff --git a/docs/LanguageExtensions.rst b/docs/LanguageExtensions.rst index 652145f66f..9583a51a3b 100644 --- a/docs/LanguageExtensions.rst +++ b/docs/LanguageExtensions.rst @@ -436,6 +436,49 @@ const_cast no no no no See also :ref:`langext-__builtin_shufflevector`, :ref:`langext-__builtin_convertvector`. +Half-Precision Floating Point +============================= + +Clang supports two half-precision (16-bit) floating point types: ``__fp16`` and +``_Float16``. ``__fp16`` is defined in the ARM C Language Extensions (`ACLE +<http://infocenter.arm.com/help/topic/com.arm.doc.ihi0053d/IHI0053D_acle_2_1.pdf>`_) +and ``_Float16`` in ISO/IEC TS 18661-3:2015. + +``__fp16`` is a storage and interchange format only. This means that values of +``__fp16`` promote to (at least) float when used in arithmetic operations. +There are two ``__fp16`` formats. Clang supports the IEEE 754-2008 format and +not the ARM alternative format. + +ISO/IEC TS 18661-3:2015 defines C support for additional floating point types. +``_FloatN`` is defined as a binary floating type, where the N suffix denotes +the number of bits and is 16, 32, 64, or greater and equal to 128 and a +multiple of 32. Clang supports ``_Float16``. The difference from ``__fp16`` is +that arithmetic on ``_Float16`` is performed in half-precision, thus it is not +a storage-only format. ``_Float16`` is available as a source language type in +both C and C++ mode. + +It is recommended that portable code use the ``_Float16`` type because +``__fp16`` is an ARM C-Language Extension (ACLE), whereas ``_Float16`` is +defined by the C standards committee, so using ``_Float16`` will not prevent +code from being ported to architectures other than Arm. Also, ``_Float16`` +arithmetic and operations will directly map on half-precision instructions when +they are available (e.g. Armv8.2-A), avoiding conversions to/from +single-precision, and thus will result in more performant code. If +half-precision instructions are unavailable, values will be promoted to +single-precision, similar to the semantics of ``__fp16`` except that the +results will be stored in single-precision. + +In an arithmetic operation where one operand is of ``__fp16`` type and the +other is of ``_Float16`` type, the ``_Float16`` type is first converted to +``__fp16`` type and then the operation is completed as if both operands were of +``__fp16`` type. + +To define a ``_Float16`` literal, suffix ``f16`` can be appended to the compile-time +constant declaration. There is no default argument promotion for ``_Float16``; this +applies to the standard floating types only. As a consequence, for example, an +explicit cast is required for printing a ``_Float16`` value (there is no string +format specifier for ``_Float16``). + Messages on ``deprecated`` and ``unavailable`` Attributes ========================================================= |