uclinux-2008R1-RC8(bf561)到VDSP5的移植(28)：likely

在include/linux/compiler.h中定义了两个宏：

/*
* Generic compiler-dependent macros required for kernel
* build go below this comment. Actual compiler/compiler version
* specific implementations come from the above header files
*/

#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)
__builtin_expect(exp, val)是GCC的一个内建函数，而VDSP无此函数，它用于为编译器提供分支预测信息，其返回值是整数表达式exp的值。这个内建函数的语义是 exp 的预期值是 val，编译器可以根据这个信息适当地重排语句块的顺序，使程序在预期的情况下有更高的执行效率。
在VDSP中，提供了两个相似的函数：
You can use the expected_true and expected_false built-in functions to control the compiler’s behavior for specific cases. By using these functions, you can tell the compiler which way a condition is most likely to evaluate. This influences the default flow of execution.
For example,
if (buffer_valid(data_buffer))
if (send_msg(data_buffer))
system_failure();
shows two nested conditional statements. If it was known that, for this example, buffer_valid() would usually return true, but that send_msg() would rarely do so, the code could be written as
if (expected_true(buffer_valid(data_buffer)))
if (expected_false(send_msg(data_buffer)))
system_failure();
因此，我们将这两个宏定义改为：

/*
* Generic compiler-dependent macros required for kernel
* build go below this comment. Actual compiler/compiler version
* specific implementations come from the above header files
*/

#define likely(x) expected_true(!!(x))
#define unlikely(x) expected_false(!!(x))