likely() and unlikely()
In Linux kernel code, one often find calls to likely() and unlikely(), in conditions, like :
bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx);
if (unlikely(!bvl)) {
mempool_free(bio, bio_pool);
bio = NULL;
goto out;
}In fact, these functions are hints for the compiler that allows it to correctly optimize the branch, by knowing which is the likeliest one. The definitions of these macros, found in include/linux/compiler.h are the following :
#define likely(x) __builtin_expect(!!(x), 1) #define unlikely(x) __builtin_expect(!!(x), 0)
The GCC documentation explains the role of __builtin_except() :