On Fri, 12 Mar 2021 at 09:10, Kito Cheng via Gcc <gcc@gcc.gnu.org> wrote: > > Hi: > > It's Kito, a RISC-V folks, recently RISC-V has an ISA extension draft > for IEEE half-precision operations [4], including arithmetic, > conversion, comparison...a full set of support for IEEE > half-precision. > > So we are seeking the data type for IEEE half-precision, and then we > have few candidates for that: > 1. _Float16 > 2. __fp16 (Same as ACLE) > 3. Other type names maybe __float16_t > > _Float16 was the best candidate since it kind of standard types from > ISO/IEC TS 18661-3, and it already supported by both clang and gcc, > however the issue is GCC don't support that on C++ mode and seems like > [1-2] it not intend to support that due to the decimal floating point > issue,
I am not aware of any motivation in the C++ committee for using class types for new floating-point types. I don't think the C++ decimal floating-point TR is considered a success. > C++ also has another proposal for extended floating-point types > (https://wg21.link/p1467), which included half-precision types, so in > my understanding, _Float16 won't be a portable typen between C/C++. Why not just make _Float16 available in C++ as a GCC extension? P1467 specifically says: "The C++ implementation could use _Float16, _Float32, etc. as the names of the extended floating-point types behind the std:: type aliases, allowing the use of the C names in both languages." If/when P1467 (or something like it) gets into the C++ standard we can add std::float16 to libstdc++ as a typedef for _Float16, but even before then we can start to add support to std::to_chars, std::is_floating_point, std::complex, std::abs etc. > And the second candidate __fp16, which is the typename come from ACLE, > it has no C/C++ portable issue due to its target specific type, the This isn't a standard name, so to use it would mean relying on GCC-specific extensions, so I don't see why it would be better than relying on "_Float16" as a GCC extension for C++ (and standard for C). Similarly for __float16_t.
