How to force Visual Studio compiler to generate inline instructions for intrinsic functions?

I could not reproduce your observation that intrinsic functions were not used by VS2022.

Minimal test code using VS2022 17.14.16 (Release Build)-

#include <iostream>
#include <memory.h>

int main()
{
    char buffer[20]{};
    memset(buffer, 'A', _countof(buffer) - 1);
    std::cout << "buffer is " << buffer << std::endl;
}

Release Build Optimizations -

Disassembly showing intrinsic used for memset function-

Force function call by inserting #pragma function directive -

#include <iostream>
#include <memory.h>
#pragma function(memset)

int main()
{
    char buffer[20]{};
    memset(buffer, 'A', _countof(buffer) - 1);
    std::cout << "buffer is " << buffer << std::endl;
}

Disassembly showing function call of memset -

Even with optimization turned off and only /Oi enabled the generated code did not include a function call -

In some cases, like memset, you can use the _stosb intrinsic function for x64 platform (defined in <intrin.h>), but the result will probably be less efficient in case of small arrays.

Based on documented options, it does not seem possible to ensure that the functions under discussion are always inlined for any data.

Try making the buffer larger. A buffer of 20 means that there would be very little difference between using the intrinsic and using the SIMD instructions. But a large buffer means that copying 8 or 16 bytes at a time increases the performance.