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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.mene...@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
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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