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125 lines
5.0 KiB
125 lines
5.0 KiB
libstdc++: Update from latest fast_float [PR107468]
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The following patch is a cherry-pick from
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https://github.com/fastfloat/fast_float/pull/153
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to restrict fast_float Clinger's fast path to when rounding mode
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is FE_TONEAREST.
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Using std::fegetround showed in benchmarks too slow, so instead
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it uses a check with 2 float additions and comparison to verify
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if rounding is FE_TONEAREST.
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2022-11-20 Jakub Jelinek <jakub@redhat.com>
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PR libstdc++/107468
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* src/c++17/fast_float/fast_float.h (detail::rounds_to_nearest): New
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function, taken from https://github.com/fastfloat/fast_float/pull/153.
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(from_chars_advanced): Only use Clinger's fast path if
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detail::rounds_to_nearest().
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* testsuite/20_util/from_chars/pr107468.cc: New test.
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--- libstdc++-v3/src/c++17/fast_float/fast_float.h.jj 2022-04-28 15:56:18.315632888 +0200
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+++ libstdc++-v3/src/c++17/fast_float/fast_float.h 2022-11-20 18:53:49.570830249 +0100
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@@ -2842,6 +2842,48 @@ from_chars_result parse_infnan(const cha
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return answer;
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}
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+/**
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+ * Returns true if the floating-pointing rounding mode is to 'nearest'.
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+ * It is the default on most system. This function is meant to be inexpensive.
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+ * Credit : @mwalcott3
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+ */
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+fastfloat_really_inline bool rounds_to_nearest() noexcept {
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+ // See
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+ // A fast function to check your floating-point rounding mode
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+ // https://lemire.me/blog/2022/11/16/a-fast-function-to-check-your-floating-point-rounding-mode/
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+ //
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+ // This function is meant to be equivalent to :
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+ // prior: #include <cfenv>
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+ // return fegetround() == FE_TONEAREST;
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+ // However, it is expected to be much faster than the fegetround()
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+ // function call.
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+ //
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+ // The volatile keywoard prevents the compiler from computing the function
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+ // at compile-time.
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+ // There might be other ways to prevent compile-time optimizations (e.g., asm).
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+ // The value does not need to be std::numeric_limits<float>::min(), any small
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+ // value so that 1 + x should round to 1 would do (after accounting for excess
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+ // precision, as in 387 instructions).
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+ static volatile float fmin = std::numeric_limits<float>::min();
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+ float fmini = fmin; // we copy it so that it gets loaded at most once.
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+ //
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+ // Explanation:
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+ // Only when fegetround() == FE_TONEAREST do we have that
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+ // fmin + 1.0f == 1.0f - fmin.
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+ //
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+ // FE_UPWARD:
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+ // fmin + 1.0f > 1
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+ // 1.0f - fmin == 1
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+ //
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+ // FE_DOWNWARD or FE_TOWARDZERO:
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+ // fmin + 1.0f == 1
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+ // 1.0f - fmin < 1
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+ //
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+ // Note: This may fail to be accurate if fast-math has been
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+ // enabled, as rounding conventions may not apply.
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+ return (fmini + 1.0f == 1.0f - fmini);
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+}
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+
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} // namespace detail
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template<typename T>
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@@ -2870,7 +2912,7 @@ from_chars_result from_chars_advanced(co
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answer.ec = std::errc(); // be optimistic
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answer.ptr = pns.lastmatch;
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// Next is Clinger's fast path.
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- 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) {
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+ 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()) {
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value = T(pns.mantissa);
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if (pns.exponent < 0) { value = value / binary_format<T>::exact_power_of_ten(-pns.exponent); }
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else { value = value * binary_format<T>::exact_power_of_ten(pns.exponent); }
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--- libstdc++-v3/testsuite/20_util/from_chars/pr107468.cc.jj
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+++ libstdc++-v3/testsuite/20_util/from_chars/pr107468.cc
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@@ -0,0 +1,42 @@
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+// Copyright (C) 2022 Free Software Foundation, Inc.
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+//
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+// This file is part of the GNU ISO C++ Library. This library is free
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+// software; you can redistribute it and/or modify it under the
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+// terms of the GNU General Public License as published by the
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+// Free Software Foundation; either version 3, or (at your option)
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+// any later version.
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+
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+// This library is distributed in the hope that it will be useful,
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+// but WITHOUT ANY WARRANTY; without even the implied warranty of
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+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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+// GNU General Public License for more details.
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+
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+// You should have received a copy of the GNU General Public License along
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+// with this library; see the file COPYING3. If not see
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+// <http://www.gnu.org/licenses/>.
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+
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+// { dg-do run { target c++17 } }
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+// { dg-add-options ieee }
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+
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+#include <charconv>
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+#include <string>
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+#include <cfenv>
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+#include <testsuite_hooks.h>
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+
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+int
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+main()
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+{
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+ // FP from_char not available otherwise.
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+#if __cpp_lib_to_chars >= 201611L \
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+ && _GLIBCXX_USE_C99_FENV_TR1 \
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+ && defined(FE_DOWNWARD) \
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+ && defined(_GLIBCXX_FLOAT_IS_IEEE_BINARY32)
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+ // PR libstdc++/107468
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+ float f;
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+ char buf[] = "3.355447e+07";
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+ std::fesetround(FE_DOWNWARD);
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+ auto [ptr, ec] = std::from_chars(buf, buf + sizeof(buf) - 1, f, std::chars_format::scientific);
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+ VERIFY( ec == std::errc() && ptr == buf + sizeof(buf) - 1 );
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+ VERIFY( f == 33554472.0f );
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+#endif
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+}
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