As a PlayStation3 SPU programmer, I am a big fan of "__builtin_expect()". It is a C/C++ compiler built-in intrinsic that can improve branch prediction. If you are not familiar with the intrinsics, have a look on GCC document here.
According to "RealView" document here, ARM seems to have a way to take advantage of the intrinsic but GCC doesn't seem to generate any special code about it.
I am not sure if there is any hidden magic compiler version or compiler option to utilize __builtin_expect on RaspberryPi but I should conclude that the intrinsic doesn't work on RaspberryPi with GCC.