Presumably it is defined to __builtin_expect(x, 1)[0], a hint to gcc that one branch (in this case the first) of the if is more likely.

[0] http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

macro for gcc __builtin_expect? http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html