How do the likely/unlikely macros in the Linux kernel work and what is their benefit?

Question

I\'ve been digging through some parts of the Linux kernel, and found calls like this:

if (unlikely(fd < 0))
{
    /* Do something */
}

o

Answer 1

In many linux release, you can find complier.h in /usr/linux/ , you can include it for use simply. And another opinion, unlikely() is more useful rather than likely(), because

if ( likely( ... ) ) {
     doSomething();
}

it can be optimized as well in many compiler.

And by the way, if you want to observe the detail behavior of the code, you can do simply as follow:

gcc -c test.c objdump -d test.o > obj.s

Then, open obj.s, you can find the answer.