On Thu, 14 Sep 2023, Jonathan Wakely wrote:
> On Thu, 14 Sept 2023 at 15:13, Patrick Palka via Libstdc++
> <libstd...@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
> >
>
>
