[PATCH] D57188: Disable _Float16 for non ARM/SPIR Targets

[John McCall via Phabricator via cfe-commits]

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