[cfe-dev] [RFC] Having different behavior for __fp16 on RISC-V

[Kito Cheng via cfe-dev]

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 issues on GCC site, so we think
support both types 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 contains document change only, implementation would be in
other patches.

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


Corresponding phabricator entry for this RFC:

https://reviews.llvm.org/D98012
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