rjmccall added inline comments.
================ Comment at: docs/LanguageExtensions.rst:497 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 +code from being ported to architectures other than Arm, though ``_Float16`` is +currently disabled on most platforms until support is added to their published ---------------- This entire documentation section really just needs to be reworked. Here's a starting point: Clang supports two half-precision (16-bit) floating point types: ``__fp16`` and ``_Float16``. These types are supported in all language modes. ``__fp16`` is supported on every target, as it is purely a storage format; see below. ``_Float16`` is currently only supported on the following targets: - 32-bit ARM - 64-bit ARM (AArch64) - SPIR ``_Float16`` will be supported on more targets as they define ABIs for them. ``__fp16`` is a storage and interchange format only. This means that values of ``__fp16`` are immediately promoted to (at least) ``float`` when used in arithmetic operations, so that e.g. the result of adding two ``__fp16`` values has type ``float``. The behavior of ``__fp16`` is specified by the ARM C Language Extensions (`ACLE <http://infocenter.arm.com/help/topic/com.arm.doc.ihi0053d/IHI0053D_acle_2_1.pdf>`_). Clang uses the ``binary16`` format from IEEE 754-2008 for ``__fp16``, not the ARM alternative format. ``_Float16`` is an extended floating-point type. This means that, just like arithmetic on ``float`` or ``double``, arithmetic on ``_Float16`` operands is formally performed in the ``_Float16`` type, so that e.g. the result of adding two ``_Float16`` values has type ``_Float16``. The behavior of ``_Float16`` is specified by ISO/IEC TS 18661-3:2015 ("Floating-point extensions for C"). As with ``__fp16``, Clang uses the ``binary16`` format from IEEE 754-2008 for ``_Float16``. ``_Float16`` arithmetic will be performed using native half-precision support when available on the target (e.g. on ARMv8.2a); otherwise it will be performed at a higher precision (currently always ``float``) and then truncated down to ``_Float16``. Note that C and C++ allow intermediate floating-point operands of an expression to be computed with greater precision than is expressible in their type, so Clang may avoid intermediate truncations in certain cases; this may lead to results that are inconsistent with native arithmetic. It is recommended that portable code use ``_Float16`` instead of ``__fp16``, as it has been defined by the C standards committee and has behavior that is more familiar to most programmers. Because ``__fp16`` operands are always immediately promoted to ``float``, the common real type of ``__fp16`` and ``_Float16`` for the purposes of the usual arithmetic conversions is ``float``. A literal can be given ``_Float16`` type using the suffix ``f16``; for example: ``` 3.14f16 ``` Because default argument promotion only applies to the standard floating-point types, ``_Float16`` values are not promoted to ``double`` when passed as variadic or untyped arguments. As a consequence, some caution must be taken when using certain library facilities with ``_Float16``; for example, there is no ``printf`` format specifier for ``_Float16``, and (unlike ``float``) it will not be implicitly promoted to ``double`` when passed to ``printf``, so the programmer must explicitly cast it to ``double`` before using it with an ``%f`` or similar specifier. ================ Comment at: lib/Basic/Targets/AArch64.cpp:52 HasLegalHalfType = true; + HasFloat16= true; ---------------- spacing ================ Comment at: lib/Basic/Targets/SPIR.h:50 HasLegalHalfType = true; + HasFloat16= true; // Define available target features ---------------- spacing CHANGES SINCE LAST ACTION https://reviews.llvm.org/D57188/new/ https://reviews.llvm.org/D57188 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
