I'm not an expert at all, but from what I've read about the C standard and how compilers interpret it, it seems dangerous to implement things like this (constant time integer multiplication) as C macros.

It should probably be done in (inline) assembly, to avoid the compiler optimizing away the extra "pointless" bits for instance. Just because a favorite compiler doesn't do it today, doesn't mean it's safe.

I'm sure the actual authors of code like this have analyzed the resulting code and know more than I do about the safety of the operations involved, but I didn't see that covered in the article.