LIKELY&UNLIKELY 宏提示编译器做分支预测
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LIKELY&UNLIKELY 宏提示编译器做分支预测
folly的Likely.h文件提供了 LIKELY 和 UNLIKELY 宏，提示编译器在分支预测时选择分支。其实现很简单：
#define LIKELY(x) (__builtin_expect((x), 1)) #define UNLIKELY(x) (__builtin_expect((x), 0))
用法如下：
#include <folly/likely.h>
int a = 1;
if (LIKELY(a == 1)) {
a = 2;
}
if (UNLIKELY(a == 3)) {
a = 3;
}
只在gcc下适用。
Q. E. D.
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