连载：编写高效代码(11) 尽量减少分支 - Linux安全网 - Linux操作系统_Linux 命令_Linux教程

我们在介绍处理器时，已经知道了，现在的处理器都是流水线结构，if和switch等语句会带来跳转，而跳转会打乱流水线的正常执行，影响程序的执行效率。
下面这段代码，把奇数赋一个值，把偶数赋一个值，可以用这种方式实现：
for(i=0; i<100; i++)

{

   if(i%2 == 0)

   {

       a[i] = x;

   }

   e<a href='http://www.linuxso.com/command/ls.html' target='_blank'><u>ls</u></a>e

   {

       a[i] = y;

   }

}
</pre></blockquote>
<p><span style="font-size:18px"> </span></p>
<p><span style="font-size:18px">如果改成如下这种形式就更好了：</span></p>
<textarea readonly name="code"class="cpp" ><span style="font-size:18px;">for(i=0; i<100; i+=2)

{

   a[i] = x;

   a[i+1] = y;

}
</span></pre></blockquote>
<p><br>
<span style="font-size:18px"><strong>  将最可能进入的分支放在 if中，而不是else中</strong></span></p>
<p><span style="font-size:18px">         Intel处理器有分支预测单元，第一次进入一个分支时，由于没有历史信息可供参考，是否跳转取决于Static Pr<a href='http://www.linuxso.com/command/ed.html' target='_blank'><u>ed</u></a>ictor（静态预测器）的预测策略，通常Static Predictor的策略是：向下跳转预测为不跳转，向上跳转预测为跳转。根据这个特性，我们在写if else语句时，也要注意。且看下面这段代码是否合适？</span><span style="font-size:18px"> </span></p>
<textarea readonly name="code"class="cpp" >int a = -5;

int b = 0;

if (a > 0)

{

   b = 1;

}

else

{

   b = 2;

}
</pre></blockquote>
<p><span style="font-size:18px"></span> </p>
<p><span style="font-size:18px">我们来看看汇编语言：</span></p>
<p><span style="font-size:18px"> </span></p>
<textarea readonly name="code"class="cpp" >4:        int a = -5;

00401028   mov         dword p<a href='http://www.linuxso.com/command/tr.html' target='_blank'><u>tr</u></a> [ebp-4],0FFFFFFFBh

5:        int b = 0;

0040102F   mov         dword ptr [ebp-8],0

6:        if (a > 0)

00401036   <a href='http://www.linuxso.com/command.html' target='_blank'><u>cmp</u></a>         dword ptr [ebp-4],0

0040103A   jle         main+35h (00401045)

7:        {

8:            b = 1;

0040103C   mov         dword ptr [ebp-8],1

9:        }

10:       else

00401043   jmp         main+3Ch (0040104c)

11:       {

12:           b = 2;

00401045   mov         dword ptr [ebp-8],2

13:       }
</pre></blockquote><span style="font-size:18px">       读者没必要管这么一大串汇编语言是什么意思，只需要知道jle是个条件跳转指令就可以了，它跳到地址00401045处，是向下跳，根据Static Predictor的策略，它被预测为不跳转，处理器会从0040103C地址处开始取下一条指令。再看看实际的执行情况，a<0，执行else这个分支，于是处理器发现取错了地址，又要从头来过，白白浪费了大量时间。可见，执行概率高的分支，应该放在if分支中。</span>
                
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