Re: [PATCH] libstdc++: _Bfloat16 for <compare>

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 2 Nov 2022 at 09:39, Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> Jon pointed out that we have TODO: _Bfloat16 in <compare>.
> Right now _S_fp_fmt() returns _Binary16 for _Float16, __fp16 as well
> as __bf16 and it actually works because we don't have a special handling
> of _Binary16.  So, either we could just document that, but I'm a little bit
> afraid if HPPA or MIPS don't start supporting _Float16 and/or __bf16.
> If they do, we have the
> #if defined __hppa__ || (defined __mips__ && !defined __mips_nan2008)
>                   // IEEE 754-1985 allowed the meaning of the quiet/signaling
>                   // bit to be reversed. Flip that to give desired ordering.
>                   if (__builtin_isnan(__x) && __builtin_isnan(__y))
>                     {
>                       using _Int = decltype(__ix);
>
>                       constexpr int __nantype = __fmt == _Binary32  ?  22
>                                               : __fmt == _Binary64  ?  51
>                                               : __fmt == _Binary128 ? 111
>                                               : -1;
>                       constexpr _Int __bit = _Int(1) << __nantype;
>                       __ix ^= __bit;
>                       __iy ^= __bit;
>                     }
> #endif
> code, the only one where we actually care whether something is
> _Binary{32,64,128} (elsewhere we just care about the x86 and m68k 80bits
> or double double or just floating point type's sizeof) and we'd need
> to handle there _Binary16 and/or _Bfloat16.
>
> So this patch uses different enum for it even when it isn't needed right
> now, after all _Binary16 isn't needed either and we could just use
> _Binary32...
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK, thanks for taking care of it.


>
> 2022-11-02  Jakub Jelinek  <jakub@redhat.com>
>
>         * libsupc++/compare (_Strong_order::_Fp_fmt): Add _Bfloat16.
>         (_Strong_order::_Bfloat16): New static data member.
>         (_Strong_order::_S_fp_fmt): Return _Bfloat16 for std::bfloat16_t.
>
> --- libstdc++-v3/libsupc++/compare.jj   2022-05-09 09:09:21.196461093 +0200
> +++ libstdc++-v3/libsupc++/compare      2022-11-01 22:13:16.771219615 +0100
> @@ -672,7 +672,7 @@ namespace std _GLIBCXX_VISIBILITY(defaul
>         _X86_80bit,  // x86 80-bit extended precision
>         _M68k_80bit, // m68k 80-bit extended precision
>         _Dbldbl, // IBM 128-bit double-double
> -       // TODO: _Bfloat16,
> +       _Bfloat16,   // std::bfloat16_t
>        };
>
>  #ifndef __cpp_using_enum
> @@ -684,6 +684,7 @@ namespace std _GLIBCXX_VISIBILITY(defaul
>        static constexpr _Fp_fmt _X86_80bit = _Fp_fmt::_X86_80bit;
>        static constexpr _Fp_fmt _M68k_80bit = _Fp_fmt::_M68k_80bit;
>        static constexpr _Fp_fmt _Dbldbl = _Fp_fmt::_Dbldbl;
> +      static constexpr _Fp_fmt _Bfloat16 = _Fp_fmt::_Bfloat16;
>  #endif
>
>        // Identify the format used by a floating-point type.
> @@ -714,6 +715,10 @@ namespace std _GLIBCXX_VISIBILITY(defaul
>           if constexpr (__is_same(_Tp, __float80))
>             return _X86_80bit;
>  #endif
> +#ifdef __STDCPP_BFLOAT16_T__
> +         if constexpr (__is_same(_Tp, decltype(0.0bf16)))
> +           return _Bfloat16;
> +#endif
>
>           constexpr int __width = sizeof(_Tp) * __CHAR_BIT__;
>
>
>         Jakub
>