RtlNumberOfSetBitsUlongPtr - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H
#if (PHNT_VERSION >= PHNT_VISTA)

NTSYSAPI
ULONG
NTAPI
RtlNumberOfSetBitsUlongPtr(
    _In_ ULONG_PTR Target
    );

#endif
#endif

View code on GitHub

This function is documented in Windows Driver Kit.