[gcc r13-3606] libstdc++: _Bfloat16 for <compare>

[gcc r13-3606] libstdc++: _Bfloat16 for <compare>

[Jakub Jelinek]

https://gcc.gnu.org/g:0c86a667486362e8bc6a92e5d36acaac4242c59f

commit r13-3606-g0c86a667486362e8bc6a92e5d36acaac4242c59f
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Nov 2 13:35:53 2022 +0100

    libstdc++: _Bfloat16 for <compare>
    
    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...
    
    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.

Diff:
---
 libstdc++-v3/libsupc++/compare | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
index 066867e9ce1..e25f4ef52cb 100644
--- a/libstdc++-v3/libsupc++/compare
+++ b/libstdc++-v3/libsupc++/compare
@@ -672,7 +672,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 	_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(default)
       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(default)
 	  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__;