亚洲免费在线-亚洲免费在线播放-亚洲免费在线观看-亚洲免费在线观看视频-亚洲免费在线看-亚洲免费在线视频

likely()與unlikely()

系統 2058 0

he 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.

???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? from Linux Kernel Development Second Edition

?

?

---------------------------------------------------------------------------------------------

?

在linux 中判斷語句經常會看到likely和unlikely,例如:

      
        if(likely(
        
          value)){
        
      
    
      }
    
      
        else{
      
    
      }
    
      ?
    

簡單從表面上看if(likely(value)) == if(value),if(unlikely(value)) == if(value)。

也就是likely和unlikely是一樣的,但是實際上執行是不同的,加likely的意思是value的值為真的可能性更大一些,那么執行if的機會大,而unlikely表示value的值為假的可能性大一些,執行else機會大一些。 加上這種修飾,編譯成二進制代碼時likely使得if后面的執行語句緊跟著前面的程序,unlikely使得else后面的語句緊跟著前面的程序,這樣就會被cache預讀取,增加程序的執行速度,likely和unlikely的實現在include/linux/compiler.h中:

            9 
      
        #if __GNUC__ == 2 && __GNUC_MINOR__ < 96 
      
    
           10 
      
        #define __builtin_expect(x, expected_value) (x) 
      
    
           11 
      
        #endif 
      
    
           12 
    
           13 
      
        #define likely(x)   __builtin_expect((x),1) 
      
    
           14 
      
        #define unlikely(x) __builtin_expect((x),0)
      
    

__builtin_expect是gcc的一個預處理命令,其解釋如下:

long __builtin_expect (long exp, long c)
You may use __builtin_expect to provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this
(‘-fprofile-arcs’), as programmers are notoriously bad at predicting how their programs actually perform. However, there are applications in which this data is hard to collect.
The return value is the value of exp, which should be an integral expression. The value of c must be a compile-time constant. The semantics of the built-in are that it is expected that exp == c. For example:
if (__builtin_expect (x, 0))
foo ();
would indicate that we do not expect to call foo, since we expect x to be zero. Since you are limited to integral expressions for exp, you should use constructions such as
if (__builtin_expect (ptr != NULL, 1))
error ();
when testing pointer or floating-point values.

轉自:http://www.cnblogs.com/yangzd/archive/2010/09/27/1837202.html

likely()與unlikely()


更多文章、技術交流、商務合作、聯系博主

微信掃碼或搜索:z360901061

微信掃一掃加我為好友

QQ號聯系: 360901061

您的支持是博主寫作最大的動力,如果您喜歡我的文章,感覺我的文章對您有幫助,請用微信掃描下面二維碼支持博主2元、5元、10元、20元等您想捐的金額吧,狠狠點擊下面給點支持吧,站長非常感激您!手機微信長按不能支付解決辦法:請將微信支付二維碼保存到相冊,切換到微信,然后點擊微信右上角掃一掃功能,選擇支付二維碼完成支付。

【本文對您有幫助就好】

您的支持是博主寫作最大的動力,如果您喜歡我的文章,感覺我的文章對您有幫助,請用微信掃描上面二維碼支持博主2元、5元、10元、自定義金額等您想捐的金額吧,站長會非常 感謝您的哦!!!

發表我的評論
最新評論 總共0條評論
主站蜘蛛池模板: 日韩一区二区在线视频 | 久久久久久久网站 | 久久综合久久美利坚合众国 | 精品一区中文字幕 | 免费在线观看福利 | 精品四虎免费观看国产高清午夜 | 国产伦精品一区二区三区免费观看 | 高清视频 一区二区三区四区 | 日b黄色| 伊人久久综合谁合综合久久 | 香蕉eeww99国产在线观看 | 婷婷在线视频国产综合 | 亚洲在线播放 | 亚洲一区二区三区四区五区 | 国产视频一区二区三区四区 | 中文字幕在线一区二区在线 | 离线枕边人国语在线影视 | 亚洲精品久久婷婷爱久久婷婷 | 精品成人 | 精品国产_亚洲人成在线高清 | 曰本人一级毛片免费完整视频 | 亚洲国产成人久久笫一页 | 免费观看h片 | 日本成人tv | 国产乱码精品一区二区三区卡 | 国产精品夜色7777青苹果 | 4虎影院午夜在线观看 | 奇米网7777 | 亚洲综合色丁香麻豆 | 欧美一级日本一级韩国一级 | 波多野结衣3女同在线观看 波多野结衣av1区2区3区 | 天天狠狠色综合图片区 | 日产国产欧美视频一区精品 | 欧美一级毛片免费看视频 | 人人爽天天碰天天躁夜夜躁 | 一级在线免费视频 | 夜夜爽天天操 | 羞羞网站在线免费观看 | 红色一级毛片 | 精品视频中文字幕 | 国产亚洲欧美一区 |