libstdc++: Update from latest fast_float [PR107468]

The following patch is a cherry-pick from

https://github.com/fastfloat/fast_float/pull/153

to restrict fast_float Clinger's fast path to when rounding mode

is FE_TONEAREST.

Using std::fegetround showed in benchmarks too slow, so instead

it uses a check with 2 float additions and comparison to verify

if rounding is FE_TONEAREST.

2022-11-20  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/107468

	* src/c++17/fast_float/fast_float.h (detail::rounds_to_nearest): New

	function, taken from https://github.com/fastfloat/fast_float/pull/153.

	(from_chars_advanced): Only use Clinger's fast path if

	detail::rounds_to_nearest().

	* testsuite/20_util/from_chars/pr107468.cc: New test.

--- libstdc++-v3/src/c++17/fast_float/fast_float.h.jj	2022-04-28 15:56:18.315632888 +0200

+++ libstdc++-v3/src/c++17/fast_float/fast_float.h	2022-11-20 18:53:49.570830249 +0100

@@ -2842,6 +2842,48 @@ from_chars_result parse_infnan(const cha

   return answer;

 }

+/**

+ * Returns true if the floating-pointing rounding mode is to 'nearest'.

+ * It is the default on most system. This function is meant to be inexpensive.

+ * Credit : @mwalcott3

+ */

+fastfloat_really_inline bool rounds_to_nearest() noexcept {

+  // See

+  // A fast function to check your floating-point rounding mode

+  // https://lemire.me/blog/2022/11/16/a-fast-function-to-check-your-floating-point-rounding-mode/

+  //

+  // This function is meant to be equivalent to :

+  // prior: #include <cfenv>

+  //  return fegetround() == FE_TONEAREST;

+  // However, it is expected to be much faster than the fegetround()

+  // function call.

+  //

+  // The volatile keywoard prevents the compiler from computing the function

+  // at compile-time.

+  // There might be other ways to prevent compile-time optimizations (e.g., asm).

+  // The value does not need to be std::numeric_limits<float>::min(), any small

+  // value so that 1 + x should round to 1 would do (after accounting for excess

+  // precision, as in 387 instructions).

+  static volatile float fmin = std::numeric_limits<float>::min();

+  float fmini = fmin; // we copy it so that it gets loaded at most once.

+  //

+  // Explanation:

+  // Only when fegetround() == FE_TONEAREST do we have that

+  // fmin + 1.0f == 1.0f - fmin.

+  //

+  // FE_UPWARD:

+  //  fmin + 1.0f > 1

+  //  1.0f - fmin == 1

+  //

+  // FE_DOWNWARD or  FE_TOWARDZERO:

+  //  fmin + 1.0f == 1

+  //  1.0f - fmin < 1

+  //

+  // Note: This may fail to be accurate if fast-math has been

+  // enabled, as rounding conventions may not apply.

+  return (fmini + 1.0f == 1.0f - fmini);

+}

+

 } // namespace detail

 template<typename T>

@@ -2870,7 +2912,7 @@ from_chars_result from_chars_advanced(co

   answer.ec = std::errc(); // be optimistic

   answer.ptr = pns.lastmatch;

   // Next is Clinger's fast path.

-  if (binary_format<T>::min_exponent_fast_path() <= pns.exponent && pns.exponent <= binary_format<T>::max_exponent_fast_path() && pns.mantissa <=binary_format<T>::max_mantissa_fast_path() && !pns.too_many_digits) {

+  if (binary_format<T>::min_exponent_fast_path() <= pns.exponent && pns.exponent <= binary_format<T>::max_exponent_fast_path() && pns.mantissa <=binary_format<T>::max_mantissa_fast_path() && !pns.too_many_digits && detail::rounds_to_nearest()) {

     value = T(pns.mantissa);

     if (pns.exponent < 0) { value = value / binary_format<T>::exact_power_of_ten(-pns.exponent); }

     else { value = value * binary_format<T>::exact_power_of_ten(pns.exponent); }

--- libstdc++-v3/testsuite/20_util/from_chars/pr107468.cc.jj

+++ libstdc++-v3/testsuite/20_util/from_chars/pr107468.cc

@@ -0,0 +1,42 @@

+// Copyright (C) 2022 Free Software Foundation, Inc.

+//

+// This file is part of the GNU ISO C++ Library.  This library is free

+// software; you can redistribute it and/or modify it under the

+// terms of the GNU General Public License as published by the

+// Free Software Foundation; either version 3, or (at your option)

+// any later version.

+

+// This library is distributed in the hope that it will be useful,

+// but WITHOUT ANY WARRANTY; without even the implied warranty of

+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

+// GNU General Public License for more details.

+

+// You should have received a copy of the GNU General Public License along

+// with this library; see the file COPYING3.  If not see

+// <http://www.gnu.org/licenses/>.

+

+// { dg-do run { target c++17 } }

+// { dg-add-options ieee }

+

+#include <charconv>

+#include <string>

+#include <cfenv>

+#include <testsuite_hooks.h>

+

+int

+main()

+{

+  // FP from_char not available otherwise.

+#if __cpp_lib_to_chars >= 201611L \

+    && _GLIBCXX_USE_C99_FENV_TR1 \

+    && defined(FE_DOWNWARD) \

+    && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)

+  // PR libstdc++/107468

+  float f;

+  char buf[] = "3.355447e+07";

+  std::fesetround(FE_DOWNWARD);

+  auto [ptr, ec] = std::from_chars(buf, buf + sizeof(buf) - 1, f, std::chars_format::scientific);

+  VERIFY( ec == std::errc() && ptr == buf + sizeof(buf) - 1 );

+  VERIFY( f == 33554472.0f );

+#endif

+}