I was browsing through some codes at the end of the contest and found out that many people were using functions like __gcd(int,int) . What are these functions ? Similar functions include
__builtin_popcount(int)
__builtin_ctz(int)
__builtin_clz(int)
Where can I study about these functions ? Googling it didn't help much.
Those are all GCC specifics. you can read about them here: http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions - but be aware, they're NOT C/C++ Standards.
来源：https://stackoverflow.com/questions/13443463/built-in-functions-in-c