Re: [committed] libstdc++: Define C++26 saturation arithmetic functions (P0543R3)

["Daniel Krügler" <daniel.kruegler@gmail.com>]

Am Fr., 17. Nov. 2023 um 16:32 Uhr schrieb Jonathan Wakely <jwakely@redhat.com>:
>
> Tested x86_64-linux. Pushed to trunk.
>
> GCC generates better code for add_sat<unsigned> if we use:
>
> unsigned z = x + y;
> z |= -(z < x);
> return z;
>
> If the compiler can't be improved we should consider using that instead
> of __builtin_add_overflow.
>
>
> -- >8 --
>
>
> This was approved for C++26 last week at the WG21 meeting in Kona.
>
> libstdc++-v3/ChangeLog:
>
>         * include/Makefile.am: Add new header.
>         * include/Makefile.in: Regenerate.
>         * include/bits/version.def (saturation_arithmetic): Define.
>         * include/bits/version.h: Regenerate.
>         * include/std/numeric: Include new header.
>         * include/bits/sat_arith.h: New file.
>         * testsuite/26_numerics/saturation/add.cc: New test.
>         * testsuite/26_numerics/saturation/cast.cc: New test.
>         * testsuite/26_numerics/saturation/div.cc: New test.
>         * testsuite/26_numerics/saturation/mul.cc: New test.
>         * testsuite/26_numerics/saturation/sub.cc: New test.
>         * testsuite/26_numerics/saturation/version.cc: New test.
> ---
>  libstdc++-v3/include/Makefile.am              |   1 +
>  libstdc++-v3/include/Makefile.in              |   1 +
>  libstdc++-v3/include/bits/sat_arith.h         | 148 ++++++++++++++++++
>  libstdc++-v3/include/bits/version.def         |   8 +
>  libstdc++-v3/include/bits/version.h           |  11 ++
>  libstdc++-v3/include/std/numeric              |   5 +
>  .../testsuite/26_numerics/saturation/add.cc   |  73 +++++++++
>  .../testsuite/26_numerics/saturation/cast.cc  |  24 +++
>  .../testsuite/26_numerics/saturation/div.cc   |  45 ++++++
>  .../testsuite/26_numerics/saturation/mul.cc   |  34 ++++
>  .../testsuite/26_numerics/saturation/sub.cc   |  86 ++++++++++
>  .../26_numerics/saturation/version.cc         |  19 +++
>  12 files changed, 455 insertions(+)
>  create mode 100644 libstdc++-v3/include/bits/sat_arith.h
>  create mode 100644 libstdc++-v3/testsuite/26_numerics/saturation/add.cc
>  create mode 100644 libstdc++-v3/testsuite/26_numerics/saturation/cast.cc
>  create mode 100644 libstdc++-v3/testsuite/26_numerics/saturation/div.cc
>  create mode 100644 libstdc++-v3/testsuite/26_numerics/saturation/mul.cc
>  create mode 100644 libstdc++-v3/testsuite/26_numerics/saturation/sub.cc
>  create mode 100644 libstdc++-v3/testsuite/26_numerics/saturation/version.cc
>
> diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
> index dab9f720cbb..17d9d9cec31 100644
> --- a/libstdc++-v3/include/Makefile.am
> +++ b/libstdc++-v3/include/Makefile.am
> @@ -142,6 +142,7 @@ bits_freestanding = \
>         ${bits_srcdir}/ranges_uninitialized.h \
>         ${bits_srcdir}/ranges_util.h \
>         ${bits_srcdir}/refwrap.h \
> +       ${bits_srcdir}/sat_arith.h \
>         ${bits_srcdir}/stl_algo.h \
>         ${bits_srcdir}/stl_algobase.h \
>         ${bits_srcdir}/stl_construct.h \
> diff --git a/libstdc++-v3/include/Makefile.in b/libstdc++-v3/include/Makefile.in
> index 4f7ab2dfbab..f038af709cc 100644
> --- a/libstdc++-v3/include/Makefile.in
> +++ b/libstdc++-v3/include/Makefile.in
> @@ -497,6 +497,7 @@ bits_freestanding = \
>         ${bits_srcdir}/ranges_uninitialized.h \
>         ${bits_srcdir}/ranges_util.h \
>         ${bits_srcdir}/refwrap.h \
> +       ${bits_srcdir}/sat_arith.h \
>         ${bits_srcdir}/stl_algo.h \
>         ${bits_srcdir}/stl_algobase.h \
>         ${bits_srcdir}/stl_construct.h \
> diff --git a/libstdc++-v3/include/bits/sat_arith.h b/libstdc++-v3/include/bits/sat_arith.h
> new file mode 100644
> index 00000000000..71793467984
> --- /dev/null
> +++ b/libstdc++-v3/include/bits/sat_arith.h
> @@ -0,0 +1,148 @@
> +// Saturation arithmetic -*- C++ -*-
> +
> +// Copyright The GNU Toolchain Authors.
> +//
> +// 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.
> +
> +// Under Section 7 of GPL version 3, you are granted additional
> +// permissions described in the GCC Runtime Library Exception, version
> +// 3.1, as published by the Free Software Foundation.
> +
> +// You should have received a copy of the GNU General Public License and
> +// a copy of the GCC Runtime Library Exception along with this program;
> +// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
> +// <http://www.gnu.org/licenses/>.
> +
> +/** @file include/bits/sat_arith.h
> + *  This is an internal header file, included by other library headers.
> + *  Do not attempt to use it directly. @headername{numeric}
> + */
> +
> +#ifndef _GLIBCXX_SAT_ARITH_H
> +#define _GLIBCXX_SAT_ARITH_H 1
> +
> +#pragma GCC system_header
> +
> +#include <bits/version.h>
> +
> +#ifdef __glibcxx_saturation_arithmetic // C++ >= 26
> +
> +#include <concepts>
> +#include <ext/numeric_traits.h>
> +
> +namespace std _GLIBCXX_VISIBILITY(default)
> +{
> +_GLIBCXX_BEGIN_NAMESPACE_VERSION
> +
> +  /// Add two integers, with saturation in case of overflow.
> +  template<typename _Tp> requires __is_standard_integer<_Tp>::value
> +    constexpr _Tp
> +    add_sat(_Tp __x, _Tp __y) noexcept
> +    {
> +      _Tp __z;
> +      if (!__builtin_add_overflow(__x, __y, &__z))
> +       return __z;
> +      if constexpr (is_unsigned_v<_Tp>)
> +       return __gnu_cxx::__int_traits<_Tp>::__max;
> +      else if (__x < 0)
> +       return __gnu_cxx::__int_traits<_Tp>::__min;

My apologies, but why does the sign of x decide the direction of the
result, shouldn't that be the sign of the returned value of z?

- Daniel