[libcxx] Use generic builtins for popcount, clz and ctz (#86563)

Fixes #86556

Use `__builtin_popcountg` instead of `__buildin_popcount{l|ll}`
Use `__builtin_clzg instead` of `__buildin_clz{l|ll}`
Use `__builtin_ctzg instead` of `__builtin_ctz{l|ll}`

The generic variant of the builtins can be used to simplify some logic
with >= Clang 19 or >= GCC 14, where these generic variants are
available. As for backwards compatibility reasons, we can't completely
remove the old logic. Therefore, I left ToDo comments to address this,
as soon as support for pre Clang 19 as well as pre GCC 14 is dropped.

---------

Co-authored-by: Nick Desaulniers <nickdesaulniers@users.noreply.github.com>

libcxx/include/__bit/countl.h

@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// TODO: __builtin_clzg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can
+//  refactor this code to exclusively use __builtin_clzg.
+
 #ifndef _LIBCPP___BIT_COUNTL_H
 #define _LIBCPP___BIT_COUNTL_H
 
@@ -38,6 +41,9 @@ _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_cl
 
 #ifndef _LIBCPP_HAS_NO_INT128
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(__uint128_t __x) _NOEXCEPT {
+#  if __has_builtin(__builtin_clzg)
+  return __builtin_clzg(__x);
+#  else
   // The function is written in this form due to C++ constexpr limitations.
   // The algorithm:
   // - Test whether any bit in the high 64-bits is set
@@ -49,12 +55,16 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(__uint128_t __x)
   //     zeros in the high 64-bits.
   return ((__x >> 64) == 0) ? (64 + __builtin_clzll(static_cast<unsigned long long>(__x)))
                             : __builtin_clzll(static_cast<unsigned long long>(__x >> 64));
+#  endif
 }
 #endif // _LIBCPP_HAS_NO_INT128
 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countl_zero(_Tp __t) _NOEXCEPT {
   static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countl_zero requires an unsigned integer type");
+#if __has_builtin(__builtin_clzg)
+  return __builtin_clzg(__t, numeric_limits<_Tp>::digits);
+#else  // __has_builtin(__builtin_clzg)
   if (__t == 0)
     return numeric_limits<_Tp>::digits;
 
@@ -79,6 +89,7 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countl_zero(_Tp __t) _
     }
     return __ret + __iter;
   }
+#endif // __has_builtin(__builtin_clzg)
 }
 
 #if _LIBCPP_STD_VER >= 20

libcxx/include/__bit/countr.h

@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// TODO: __builtin_ctzg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can
+//  refactor this code to exclusively use __builtin_ctzg.
+
 #ifndef _LIBCPP___BIT_COUNTR_H
 #define _LIBCPP___BIT_COUNTR_H
 
@@ -37,9 +40,11 @@ _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_ct
 
 template <class _Tp>
 _LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countr_zero(_Tp __t) _NOEXCEPT {
+#if __has_builtin(__builtin_ctzg)
+  return __builtin_ctzg(__t, numeric_limits<_Tp>::digits);
+#else  // __has_builtin(__builtin_ctzg)
   if (__t == 0)
     return numeric_limits<_Tp>::digits;
-
   if (sizeof(_Tp) <= sizeof(unsigned int))
     return std::__libcpp_ctz(static_cast<unsigned int>(__t));
   else if (sizeof(_Tp) <= sizeof(unsigned long))
@@ -55,6 +60,7 @@ _LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __coun
     }
     return __ret + std::__libcpp_ctz(static_cast<unsigned long long>(__t));
   }
+#endif // __has_builtin(__builtin_ctzg)
 }
 
 #if _LIBCPP_STD_VER >= 20

libcxx/include/__bit/popcount.h

@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// TODO: __builtin_popcountg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can
+//  refactor this code to exclusively use __builtin_popcountg.
+
 #ifndef _LIBCPP___BIT_POPCOUNT_H
 #define _LIBCPP___BIT_POPCOUNT_H
 
@@ -39,6 +42,9 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned lo
 
 template <__libcpp_unsigned_integer _Tp>
 _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr int popcount(_Tp __t) noexcept {
+#  if __has_builtin(__builtin_popcountg)
+  return __builtin_popcountg(__t);
+#  else  // __has_builtin(__builtin_popcountg)
   if (sizeof(_Tp) <= sizeof(unsigned int))
     return std::__libcpp_popcount(static_cast<unsigned int>(__t));
   else if (sizeof(_Tp) <= sizeof(unsigned long))
@@ -53,6 +59,7 @@ _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr int popcount(_Tp __t) noex
     }
     return __ret;
   }
+#  endif // __has_builtin(__builtin_popcountg)
 }
 
 #endif // _LIBCPP_STD_VER >= 20