[libc++][NFC] Use concepts in <bit>.

[Mordante]

Summary

All supported compilers have concepts support so use that in the C++20
functions in <bit>.

s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/ as drive-by fix.

[jloser]

libcxx/include/__bit/bit_cast.h

27

Nit: can we use west const for consistency in a lot of the other places in libcxx? E.g. s/_FromType const&/const _FromType&.

libcxx/include/bit

94

Since we're concept constraining the public functions like std::rotl and friends, should we just remove these static_asserts in the private functions? I don't see these as ever being called from outside this file from the public function.

[ldionne]

libcxx/include/__bit/bit_cast.h

25–26

[Mordante]

Thanks for the reviews!

libcxx/include/__bit/bit_cast.h

25–26

Fixed, this is clang-format's idea ;-)

27

Nice catch!

libcxx/include/bit

94

I prefer to keep them as-is. These functions are there to allow other parts of the code to use these functions before C++20. When they are unused I think it would make more sense to remove these functions and move the implementation into the public files. We've implemented most of C++17 so there shouldn't be much new pre-C++20 code around.

I'll do that in a separate patch.

libcxx/include/__bit/bit_cast.h

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
-template<class _ToType, class _FromType, class = enable_if_t<
-  sizeof(_ToType) == sizeof(_FromType) &&
+template <class _ToType, class _FromType>
+  requires(sizeof(_ToType) == sizeof(_FromType) &&
            is_trivially_copyable_v<_ToType> &&
-  is_trivially_copyable_v<_FromType>
->>
-_LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI
-constexpr _ToType bit_cast(_FromType const& __from) noexcept {
-    return __builtin_bit_cast(_ToType, __from);
+           is_trivially_copyable_v<_FromType>)
+_LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr _ToType bit_cast(const _FromType& __from) noexcept {
 }
 
 #endif // _LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___BIT_BIT_CAST_H

libcxx/include/bit

 } // namespace std
 
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__bit/bit_cast.h>
 #include <__bit/byteswap.h>
 #include <__bits> // __libcpp_clz
+#include <__concepts/arithmetic.h>
 #include <__config>
 #include <limits>
 #include <type_traits>
 #include <version>
 
 #if defined(__IBMCPP__)
 #include "__support/ibm/support.h"
 #endif
 #if defined(_LIBCPP_COMPILER_MSVC)
 #include <intrin.h>
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 _Tp __rotl(_Tp __t, unsigned int __cnt) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__rotl requires an unsigned integer type");
     const unsigned int __dig = numeric_limits<_Tp>::digits;
     if ((__cnt % __dig) == 0)
         return __t;
     return (__t << (__cnt % __dig)) | (__t >> (__dig - (__cnt % __dig)));
 }
 
 template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 _Tp __rotr(_Tp __t, unsigned int __cnt) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__rotr requires an unsigned integer type");
     const unsigned int __dig = numeric_limits<_Tp>::digits;
     if ((__cnt % __dig) == 0)
         return __t;
     return (__t >> (__cnt % __dig)) | (__t << (__dig - (__cnt % __dig)));
 }
 
 template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 int __countr_zero(_Tp __t) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countr_zero requires an unsigned integer type");
     if (__t == 0)
         return numeric_limits<_Tp>::digits;
 
     if      (sizeof(_Tp) <= sizeof(unsigned int))
         return __libcpp_ctz(static_cast<unsigned int>(__t));
else
             __ret += __ulldigits;
             __t >>= __ulldigits;
         }
         return __ret + __libcpp_ctz(static_cast<unsigned long long>(__t));
     }
 }
 
 template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 int __countl_zero(_Tp __t) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countl_zero requires an unsigned integer type");
     if (__t == 0)
         return numeric_limits<_Tp>::digits;
 
     if      (sizeof(_Tp) <= sizeof(unsigned int))
         return __libcpp_clz(static_cast<unsigned int>(__t))
else
                 break;
             __ret += __iter;
             }
         return __ret + __iter;
     }
 }
 
 template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 int __countl_one(_Tp __t) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countl_one requires an unsigned integer type");
     return __t != numeric_limits<_Tp>::max()
         ? __countl_zero(static_cast<_Tp>(~__t))
         : numeric_limits<_Tp>::digits;
 }
 
 template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 int __countr_one(_Tp __t) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countr_one requires an unsigned integer type");
     return __t != numeric_limits<_Tp>::max()
         ? __countr_zero(static_cast<_Tp>(~__t))
         : numeric_limits<_Tp>::digits;
 }
 
 template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 int __popcount(_Tp __t) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__popcount requires an unsigned integer type");
     if      (sizeof(_Tp) <= sizeof(unsigned int))
         return __libcpp_popcount(static_cast<unsigned int>(__t));
     else if (sizeof(_Tp) <= sizeof(unsigned long))
         return __libcpp_popcount(static_cast<unsigned long>(__t));
     else if (sizeof(_Tp) <= sizeof(unsigned long long))
         return __libcpp_popcount(static_cast<unsigned long long>(__t));
     else
     {
         int __ret = 0;
         while (__t != 0)
         {
             __ret += __libcpp_popcount(static_cast<unsigned long long>(__t));
             __t >>= numeric_limits<unsigned long long>::digits;
         }
         return __ret;
     }
 }
 
 // integral log base 2
 template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 unsigned __bit_log2(_Tp __t) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__bit_log2 requires an unsigned integer type");
     return numeric_limits<_Tp>::digits - 1 - __countl_zero(__t);
 }
 
 template <class _Tp>
-_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
 bool __has_single_bit(_Tp __t) _NOEXCEPT
 {
     static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__has_single_bit requires an unsigned integer type");
     return __t != 0 && (((__t & (__t - 1)) == 0));
 }
 
 #if _LIBCPP_STD_VER > 17
 
-template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, _Tp>
-rotl(_Tp __t, unsigned int __cnt) noexcept
-{
-    return __rotl(__t, __cnt);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp rotl(_Tp __t, unsigned int __cnt) noexcept {
 }
 
-template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, _Tp>
-rotr(_Tp __t, unsigned int __cnt) noexcept
-{
-    return __rotr(__t, __cnt);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp rotr(_Tp __t, unsigned int __cnt) noexcept {
 }
 
-template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, int>
-countl_zero(_Tp __t) noexcept
-{
-    return __countl_zero(__t);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr int countl_zero(_Tp __t) noexcept {
 }
 
-template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, int>
-countl_one(_Tp __t) noexcept
-{
-    return __countl_one(__t);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr int countl_one(_Tp __t) noexcept {
 }
 
-template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, int>
-countr_zero(_Tp __t) noexcept
-{
-    return __countr_zero(__t);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr int countr_zero(_Tp __t) noexcept {
 }
 
-template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, int>
-countr_one(_Tp __t) noexcept
-{
-    return __countr_one(__t);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr int countr_one(_Tp __t) noexcept {
 }
 
-template<class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, int>
-popcount(_Tp __t) noexcept
-{
-    return __popcount(__t);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr int popcount(_Tp __t) noexcept {
 }
 
-template <class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, bool>
-has_single_bit(_Tp __t) noexcept
-{
-    return __has_single_bit(__t);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr bool has_single_bit(_Tp __t) noexcept {
 }
 
-template <class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, _Tp>
-bit_floor(_Tp __t) noexcept
-{
-    return __t == 0 ? 0 : _Tp{1} << __bit_log2(__t);
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp bit_floor(_Tp __t) noexcept {
 }
 
-template <class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, _Tp>
-bit_ceil(_Tp __t) noexcept
-{
-    if (__t < 2) return 1;
-    const unsigned __n = numeric_limits<_Tp>::digits - countl_zero((_Tp)(__t - 1u));
-    _LIBCPP_ASSERT(__n != numeric_limits<_Tp>::digits, "Bad input to bit_ceil");
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp bit_ceil(_Tp __t) noexcept {
+  if (__t < 2)
+    return 1;
 
-    if constexpr (sizeof(_Tp) >= sizeof(unsigned))
-        return _Tp{1} << __n;
-    else
-    {
-        const unsigned __extra = numeric_limits<unsigned>::digits  - numeric_limits<_Tp>::digits;
-        const unsigned __retVal = 1u << (__n + __extra);
-        return (_Tp) (__retVal >> __extra);
-    }
+  else {
+    const unsigned __extra = numeric_limits<unsigned>::digits - numeric_limits<_Tp>::digits;
+    return (_Tp)(__retVal >> __extra);
 }
 
-template <class _Tp>
-_LIBCPP_INLINE_VISIBILITY constexpr
-enable_if_t<__libcpp_is_unsigned_integer<_Tp>::value, int>
-bit_width(_Tp __t) noexcept
-{
-    return __t == 0 ? 0 : __bit_log2(__t) + 1;
+template <__libcpp_unsigned_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr int bit_width(_Tp __t) noexcept {
 }
 
 enum class endian
 {
     little = 0xDEAD,
     big    = 0xFACE,
 #if defined(_LIBCPP_LITTLE_ENDIAN)
     native = little