https://zolk3ri.name/cgit/cpudetect/about/ (direct link to the header file: https://zolk3ri.name/cgit/cpudetect/tree/cpudetect.h)

If you are unsure about something, or it misses something you'd like added, feel free to e-mail the author. I am in touch with him at times and he seems friendly.

I remember he wanted to implement more than just detecting CPU instructions, but he did not want to make it bloated with checking for cache sizes and whatnot (can't remember the details).