[PATCH] D35295: [docs] Add section 'Half-Precision Floating Point'

[Sjoerd Meijer via Phabricator via cfe-commits]

SjoerdMeijer created this revision.

This documents the differences and interactions between _Float16 and __fp16.


https://reviews.llvm.org/D35295

Files:
  docs/LanguageExtensions.rst


Index: docs/LanguageExtensions.rst
===================================================================
--- docs/LanguageExtensions.rst
+++ docs/LanguageExtensions.rst
@@ -436,6 +436,37 @@
 
 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 artimethic 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 egual to 128 and a
+multiple of 32. Clang supports ``_Float16``.  The difference with ``__fp16`` is
+that arithmetic is performed in half-precision, thus it is not a storage-only
+format. It is recommended that portable code use the ``_Float16`` type.
+
+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
 =========================================================
 


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