Re: [PATCH] libstdc++: Use C++20 constraints in <bit>

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 14 Sept 2023 at 15:13, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> -- >8 --
>
> By now it's probably safe to assume supported compilers have full
> concepts support in C++20 mode.

Clang 14.0.0 is good enough for these uses. It doesn't handle our
<ranges> properly, or conditionally trivial special member functions,
but that's not a problem here.

>  And using a requires-clase instead
> enable_if_t inside the return type greatly reduces the sizes of the
> corresponding symbol names.

Will that change when we start to mangle constraints? :-)

OK for trunk.


>
> libstdc++-v3/ChangeLog:
>
>         * include/std/bit: Include <concepts>.
>         (byteswap): Use a requires-clause instead of enable_if_t
>         inside the return type.
>         (_If_is_unsigned_integer): Replace with ...
>         (__unsigned_integer): ... this.
>         (rotl): Use a requires-clause instead of enable_if_t
>         inside the return type.
>         (countl_zero): Likewise.
>         (countl_one): Likewise.
>         (countr_zero): Likewise.
>         (countr_one): Likewise.
>         (popcount): Likewise.
>         (has_single_bit): Likewise.
>         (bit_ceil): Likewise.
>         (bit_floor): Likewise.
>         (bit_width): Likewise.
> ---
>  libstdc++-v3/include/std/bit | 54 ++++++++++++++++++------------------
>  1 file changed, 27 insertions(+), 27 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/bit b/libstdc++-v3/include/std/bit
> index 987b6cdbb35..dce61b440c5 100644
> --- a/libstdc++-v3/include/std/bit
> +++ b/libstdc++-v3/include/std/bit
> @@ -33,6 +33,7 @@
>
>  #if __cplusplus >= 201402L
>
> +#include <concepts> // for std::integral
>  #include <type_traits>
>
>  #if _GLIBCXX_HOSTED || __has_include(<ext/numeric_traits.h>)
> @@ -103,9 +104,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     * @return         An object of the same type, with the bytes reversed.
>     * @since C++23
>     */
> -  template<typename _Tp>
> +  template<integral _Tp>
>      [[nodiscard]]
> -    constexpr enable_if_t<is_integral<_Tp>::value, _Tp>
> +    constexpr _Tp
>      byteswap(_Tp __value) noexcept
>      {
>        if constexpr (sizeof(_Tp) == 1)
> @@ -378,54 +379,53 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #ifdef __cpp_lib_bitops // C++ >= 20
>
>    /// @cond undocumented
> -  template<typename _Tp, typename _Up = _Tp>
> -    using _If_is_unsigned_integer
> -      = enable_if_t<__is_unsigned_integer<_Tp>::value, _Up>;
> +  template<typename _Tp>
> +    concept __unsigned_integer = __is_unsigned_integer<_Tp>::value;
>    /// @endcond
>
>    // [bit.rot], rotating
>
>    /// Rotate `x` to the left by `s` bits.
> -  template<typename _Tp>
> -    [[nodiscard]] constexpr _If_is_unsigned_integer<_Tp>
> +  template<__unsigned_integer _Tp>
> +    [[nodiscard]] constexpr _Tp
>      rotl(_Tp __x, int __s) noexcept
>      { return std::__rotl(__x, __s); }
>
>    /// Rotate `x` to the right by `s` bits.
> -  template<typename _Tp>
> -    [[nodiscard]] constexpr _If_is_unsigned_integer<_Tp>
> +  template<__unsigned_integer _Tp>
> +    [[nodiscard]] constexpr _Tp
>      rotr(_Tp __x, int __s) noexcept
>      { return std::__rotr(__x, __s); }
>
>    // [bit.count], counting
>
>    /// The number of contiguous zero bits, starting from the highest bit.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp, int>
> +  template<__unsigned_integer _Tp>
> +    constexpr int
>      countl_zero(_Tp __x) noexcept
>      { return std::__countl_zero(__x); }
>
>    /// The number of contiguous one bits, starting from the highest bit.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp, int>
> +  template<__unsigned_integer _Tp>
> +    constexpr int
>      countl_one(_Tp __x) noexcept
>      { return std::__countl_one(__x); }
>
>    /// The number of contiguous zero bits, starting from the lowest bit.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp, int>
> +  template<__unsigned_integer _Tp>
> +    constexpr int
>      countr_zero(_Tp __x) noexcept
>      { return std::__countr_zero(__x); }
>
>    /// The number of contiguous one bits, starting from the lowest bit.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp, int>
> +  template<__unsigned_integer _Tp>
> +    constexpr int
>      countr_one(_Tp __x) noexcept
>      { return std::__countr_one(__x); }
>
>    /// The number of bits set in `x`.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp, int>
> +  template<__unsigned_integer _Tp>
> +    constexpr int
>      popcount(_Tp __x) noexcept
>      { return std::__popcount(__x); }
>  #endif // __cpp_lib_bitops
> @@ -434,28 +434,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    // [bit.pow.two], integral powers of 2
>
>    /// True if `x` is a power of two, false otherwise.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp, bool>
> +  template<__unsigned_integer _Tp>
> +    constexpr bool
>      has_single_bit(_Tp __x) noexcept
>      { return std::__has_single_bit(__x); }
>
>    /// The smallest power-of-two not less than `x`.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp>
> +  template<__unsigned_integer _Tp>
> +    constexpr _Tp
>      bit_ceil(_Tp __x) noexcept
>      { return std::__bit_ceil(__x); }
>
>    /// The largest power-of-two not greater than `x`.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp>
> +  template<__unsigned_integer _Tp>
> +    constexpr _Tp
>      bit_floor(_Tp __x) noexcept
>      { return std::__bit_floor(__x); }
>
>    // _GLIBCXX_RESOLVE_LIB_DEFECTS
>    // 3656. Inconsistent bit operations returning a count
>    /// The smallest integer greater than the base-2 logarithm of `x`.
> -  template<typename _Tp>
> -    constexpr _If_is_unsigned_integer<_Tp, int>
> +  template<__unsigned_integer _Tp>
> +    constexpr int
>      bit_width(_Tp __x) noexcept
>      { return std::__bit_width(__x); }
>  #endif // defined (__cpp_lib_int_pow2)
> --
> 2.42.0.158.g94e83dcf5b
>