__builtin_expect from GCC can be used by programmer to show which variants are expected to be very often and which are rare. But __builtin_expect have only "true" and "false" (0% or 100% probability)
For some big projects it is vary hard to get profile feedback (-fprofile-arcs), and sometimes programmer does know, what probability of branch he have in some point of program.
It is possible to give a hint to compiler that a branch have probability >0% and <100% ?
True and false really mean that "the first variant is more likely" and "the second variant is more likely". There's no practical need for any values other than these. The compiler won't be able to use that information.