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

[Patrick Palka <ppalka@redhat.com>]

On Thu, 14 Sep 2023, Jonathan Wakely wrote:

> 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.

Makes sense.

> 
> >  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? :-)

In this specific case it should still be very worthwhile, since the
mangling of _If_is_unsigned_integer<_Tp> is around 200 characters due to
alias template transparency.  IIUC the mangling for the new constraint
__unsigned_integer<_Tp> should be around the same size as written since
we won't normalize concept-ids when mangling them like we do alias
template-ids.

> 
> OK for trunk.

Thanks!

> 
> 
> >
> > 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
> >
> 
>