# __builtin_expect

You can use the `__builtin_expect` built-in function to indicate that an
expression is likely to evaluate to a specified value. The compiler can use this knowledge to direct
optimizations. This built-in function is portable with the GNU C/C++
`__builtin_expect` function.

The prototype of this built-in function is as
follows:

`long __builtin_expect (long exp, long c);`

where `exp`is the integral-type expression to be evaluated and`c`is the expected value of the expression.If `exp` does not actually evaluate at run time to the predicted value
`c`, performance might suffer. Therefore, you must use this built-in function with
caution.