Don't do this. libc string functions are usually done with hand tweaked assembly or as a builtin by the compiler. They will be faster than this.

This sort of practice is very outdated. The last time it made sense for performance reasons was probably the early 90s or earlier.

Additionally, strlen is not one of the C string functions you want to replace due to defects, the same way you would want to do with crusty old strcpy. If you're working with C strings there is nothing wrong with strlen. (Just don't call it redundantly in a loop body ...)