[Bug libstdc++/118855] Simplify when __builtin_*g builtins are available

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118855

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'll test this properly tomorrow, but it passes the 26_numerics/bit/* tests:

--- a/libstdc++-v3/include/std/bit
+++ b/libstdc++-v3/include/std/bit
@@ -205,6 +205,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       using __gnu_cxx::__int_traits;
       constexpr auto _Nd = __int_traits<_Tp>::__digits;

+#if __has_builtin(__builtin_clzg)
+      return __builtin_clzg(__x, _Nd);
+#else
       if (__x == 0)
         return _Nd;

@@ -242,6 +245,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          unsigned long long __low = __x & __max_ull;
          return (_Nd - _Nd_ull) + __builtin_clzll(__low);
        }
+#endif
     }

   template<typename _Tp>
@@ -258,6 +262,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       using __gnu_cxx::__int_traits;
       constexpr auto _Nd = __int_traits<_Tp>::__digits;

+#if __has_builtin(__builtin_ctzg)
+      return __builtin_ctzg(__x, _Nd);
+#else
       if (__x == 0)
         return _Nd;

@@ -283,6 +290,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          unsigned long long __high = __x >> _Nd_ull;
          return __builtin_ctzll(__high) + _Nd_ull;
        }
+#endif
     }

   template<typename _Tp>
@@ -296,6 +304,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr int
     __popcount(_Tp __x) noexcept
     {
+#if __has_builtin(__builtin_popcountg)
+      return __builtin_popcountg(__x);
+#else
       using __gnu_cxx::__int_traits;
       constexpr auto _Nd = __int_traits<_Tp>::__digits;

@@ -319,6 +330,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          unsigned long long __high = __x >> _Nd_ull;
          return __builtin_popcountll(__low) + __builtin_popcountll(__high);
        }
+#endif
     }

   template<typename _Tp>