[gcc r14-2661] libstdc++: Do not define inaccurate from_chars for _Float128 [PR110077]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:0867d30a68de68f4c809757348447bef94ef1491

commit r14-2661-g0867d30a68de68f4c809757348447bef94ef1491
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jul 19 21:15:17 2023 +0100

    libstdc++: Do not define inaccurate from_chars for _Float128 [PR110077]
    
    I think r14-1431-g7037e7b6e4ac41 was wrong to try to define the
    _Float128 overload unconditionally. Not all targets need it, so defining
    the lossy fallback using long double is not useful (and caused an ABI
    change on Solaris x86).
    
    Making the definition depend on USE_STRTOF128_FOR_FROM_CHARS again
    partially reverts the change for PR 109921, however that should still be
    fixed because the changes to make USE_STRTOF128_FOR_FROM_CHARS depend on
    USE_STRTOD_FOR_FROM_CHARS are not reverted.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/110077
            * src/c++17/floating_from_chars.cc (from_chars): Only define
            _Float128 overload when using __strfromf128.

Diff:
---
 libstdc++-v3/src/c++17/floating_from_chars.cc | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc b/libstdc++-v3/src/c++17/floating_from_chars.cc
index 3152d64c67c..e421c5adc3e 100644
--- a/libstdc++-v3/src/c++17/floating_from_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
@@ -1325,24 +1325,14 @@ _ZSt10from_charsPKcS0_RDF128_St12chars_format(const char* first,
 					      __ieee128& value,
 					      chars_format fmt) noexcept
 __attribute__((alias ("_ZSt10from_charsPKcS0_Ru9__ieee128St12chars_format")));
-#elif __FLT128_MANT_DIG__ == 113 && __LDBL_MANT_DIG__ != 113
+#elif defined(USE_STRTOF128_FOR_FROM_CHARS)
 // Overload for _Float128 is not defined inline in <charconv>, define it here.
 from_chars_result
 from_chars(const char* first, const char* last, _Float128& value,
 	   chars_format fmt) noexcept
 {
-#ifdef USE_STRTOF128_FOR_FROM_CHARS
   // fast_float doesn't support IEEE binary128 format, but we can use strtold.
   return from_chars_strtod(first, last, value, fmt);
-#else
-  // Read a long double. This might give an incorrect result (e.g. values
-  // out of range of long double give an error, even if they fit in _Float128).
-  long double ldbl_val;
-  auto res = std::from_chars(first, last, ldbl_val, fmt);
-  if (res.ec == errc{})
-    value = ldbl_val;
-  return res;
-#endif
 }
 #endif