Revert "[libc++] Replace __libcpp_popcount by __builtin_popcountg (llvm#133937)"

This reverts commit 703cfe7.

Author: AnthonyLatsis

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
 
@@ -24,10 +27,50 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned __x) _NOEXCEPT {
+  return __builtin_popcount(__x);
+}
+
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned long __x) _NOEXCEPT {
+  return __builtin_popcountl(__x);
+}
+
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned long long __x) _NOEXCEPT {
+  return __builtin_popcountll(__x);
+}
+
+template <class _Tp>
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __popcount_impl(_Tp __t) _NOEXCEPT {
+  if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned int)) {
+    return std::__libcpp_popcount(static_cast<unsigned int>(__t));
+  } else if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned long)) {
+    return std::__libcpp_popcount(static_cast<unsigned long>(__t));
+  } else if _LIBCPP_CONSTEXPR (sizeof(_Tp) <= sizeof(unsigned long long)) {
+    return std::__libcpp_popcount(static_cast<unsigned long long>(__t));
+  } else {
+#if _LIBCPP_STD_VER == 11
+    return __t != 0 ? std::__libcpp_popcount(static_cast<unsigned long long>(__t)) +
+                          std::__popcount_impl<_Tp>(__t >> numeric_limits<unsigned long long>::digits)
+                    : 0;
+#else
+    int __ret = 0;
+    while (__t != 0) {
+      __ret += std::__libcpp_popcount(static_cast<unsigned long long>(__t));
+      __t >>= std::numeric_limits<unsigned long long>::digits;
+    }
+    return __ret;
+#endif
+  }
+}
+
 template <class _Tp>
 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __popcount(_Tp __t) _NOEXCEPT {
   static_assert(is_unsigned<_Tp>::value, "__popcount only works with unsigned types");
+#if __has_builtin(__builtin_popcountg) // TODO (LLVM 21): This can be dropped once we only support Clang >= 19.
   return __builtin_popcountg(__t);
+#else
+  return std::__popcount_impl(__t);
+#endif
 }
 
 #if _LIBCPP_STD_VER >= 20

libcxx/include/__stop_token/atomic_unique_lock.h

@@ -28,7 +28,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // and LockedBit is the value of State when the lock bit is set, e.g  1 << 2
 template <class _State, _State _LockedBit>
 class _LIBCPP_AVAILABILITY_SYNC __atomic_unique_lock {
-  static_assert(std::__popcount(static_cast<unsigned long long>(_LockedBit)) == 1,
+  static_assert(std::__libcpp_popcount(static_cast<unsigned long long>(_LockedBit)) == 1,
                 "LockedBit must be an integer where only one bit is set");
 
   std::atomic<_State>& __state_;