[PATCH] D98012: [RFC][doc] Document that RISC-V's __fp16 has different behavior

[Kito Cheng via Phabricator via cfe-commits]

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RISC-V has a draft extension for half-precision proposed last year[1],
so we think adding a new type for that would be great to make this
easier to use that extension.

We found there is `_Float16` and `__fp16` types are supported on GCC and
Clang, but `_Float16` has C++ supporting issue on GCC site, so we think
support both type is a reasonable choice to us.

However we would like have slight different behavior for `__fp16` other
than ACLE: The evaluation format of `__fp16` set same as `_Float16`,
which means no promotion are performed if there is no hardware half-precision
supported.

The only concern to us is it's defined differnt with clang's document,
so we would put this RFC patch here, to make sure it's OK for make
`__fp16` has differnt behavior between differnt targets.

This patch contain document change only, implementation would be in
another patches.

[1] https://github.com/riscv/riscv-isa-manual/pull/496
[2] https://github.com/riscv/riscv-elf-psabi-doc/pull/172

Co-authored-by: Evandro Menezes <evandro.menezes at sifive.com>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D98012

Files:
  clang/docs/LanguageExtensions.rst


Index: clang/docs/LanguageExtensions.rst
===================================================================
--- clang/docs/LanguageExtensions.rst
+++ clang/docs/LanguageExtensions.rst
@@ -526,6 +526,7 @@
 * 64-bit ARM (AArch64)
 * AMDGPU
 * SPIR
+* RISC-V
 
 ``_Float16`` will be supported on more targets as they define ABIs for it.
 
@@ -535,13 +536,18 @@
 
 The ``__bf16`` type is only available when supported in hardware.
 
-``__fp16`` is a storage and interchange format only.  This means that values of
+``__fp16`` is a storage and interchange format only on most targets.  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.
 
+On RISC-V, the evaluation format of ``__fp16`` will depend on the presence of
+the ``Zfh``  (half-precision) extension. ``__fp16`` will perform arithmetic
+without promotion when the Zfh extension is enabled, and promoted to float
+otherwise.
+
 ``_Float16`` is an interchange 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


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