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

[Craig topper via cfe-dev]

> On Mar 11, 2021, at 10:43 AM, James Y Knight <jyknight at google.com> wrote:
> 
> 
> 
> On Thu, Mar 11, 2021 at 11:20 AM Sjoerd Meijer via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> In fact there is kind of awkward story behind this, we chose __fp16 rather than _Float16, major reason is GCC supporting, and also we saw ARM/AArch64 deprecated that for a while, but _Float16 problem still there in GCC, so we decide to using __fp16 rather than _Float16.
> I am not really following this.
> And we also considering the compatible issue there, I mean the compatibility of ACLE's __fp16, __fp16 isn't a language extension for GCC, it's a target specific type for ARM/AArch64, so if we don't define that as a formal extension types, then we have broken support/compiler compatibility issue on this due to GCC didn't support __fp16 as language extension.
> And am not entirely sure about this.
> 
> I don't think I have changed my mind on this topic, and I don't know what else/more I can add to what I have already mentioned.
> I won't repeat my arguments, but the bottom line is that we want to deprecate __fp16, while you like to rebrand it to a native half-precision type, and I don't see how this goes together very well. I also remain unconvinced about the arguments against _Float16, which I think is the solution.
> 
> I don't know what your options are now. Maybe someone else can weigh in and shine a light on this, perhaps I am missing something. I don't know if you can go ahead and make these changes if you get an LGTM from someone else, but I hope the community sees the problem of the same type behaving different on different targets.
> 
> I don't know why _Float16 isn't implemented by GCC in C++ mode, but I'd agree that using _Float16 for RISC-V, and adding support for _Float16 to GCC's C++ mode does seem like the better solution here, especially given that such support will also be needed for ARM.
> 

A thread has been started on the gcc mailing list. https://gcc.gnu.org/pipermail/gcc/2021-March/234962.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20210311/761cb6ba/attachment.html>