[docs] Add section 'Half-Precision Floating Point'

Sjoerd Meijer · Sjoerd Meijer · commit 167f2e34d592 · 2017-11-07T10:09:45.000Z
This documents the differences/interactions between _Float16 and __fp16 and is a companion change for the _Float16 type implementation (r312794). Differential Revision: https://reviews.llvm.org/D35295 llvm-svn: 317558
diff --git a/clang/docs/LanguageExtensions.rst b/clang/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
 =========================================================
 
