likely or unlikely
likely 和 unlikely可以用来优化 指令的预读技术。
likely表示极可能发生， unlikely表示不怎么发生。
(书可以在http://elinux.cn下载)
Branch Annotation
The gcc C compiler has a built-in directive that optimizes conditional branches as either very likely taken or very unlikely taken. The compiler uses the directive to appropriately optimize the branch. The kernel wraps the directive in very easy-to-use macros, likely() and unlikely().
For example, consider an if statement such as the following:
if (foo) {
/* ... */
}
To mark this branch as very unlikely taken (that is, likely not taken):
/* we predict foo is nearly always zero ... */
if (unlikely(foo)) {
/* ... */
}
Conversely, to mark a branch as very likely taken:
/* we predict foo is nearly always nonzero ... */
if (likely(foo)) {
/* ... */
}
You should only use these directives when the branch direction is overwhelmingly a known priori or when you want to optimize a specific case at the cost of the other case. This is an important point: These directives result in a performance boost when the branch is correctly predicted, but a performance loss when the branch is mispredicted. A very common usage for unlikely() and likely() is error conditions. As one might expect, unlikely() finds much more use in the kernel because if statements tend to indicate a special case.
需要添加头文件 <linux/compiler.h>
