C语言 告诉 gcc 专门展开一个循环

Tell gcc to specifically unroll a loop

提问by Nils

How can I tell GCC to unroll a particular loop? I have used the CUDA SDK where loops can be unrolled manually using #pragma unroll. Is there a similar feature for gcc? I googled a bit but could not find anything.

如何告诉 GCC 展开特定循环？我使用了 CUDA SDK，其中可以使用#pragma unroll. gcc 有类似的功能吗？我用谷歌搜索了一下，但找不到任何东西。

回答by Philip Conrad

GCC gives you a few different ways of handling this:

GCC 为您提供了几种不同的处理方式：

• Use #pragma directives, like #pragma GCC optimize ("string"...), as seen in the GCC docs. Note that the pragma makes the optimizations globalfor the remaining functions. If you used #pragma push_optionsand pop_optionsmacros cleverly, you could probably define this around just one function like so:

  #pragma GCC push_options
  #pragma GCC optimize ("unroll-loops")
  
  //add 5 to each element of the int array.
  void add5(int a[20]) {
      int i = 19;
      for(; i > 0; i--) {
          a[i] += 5;
      }
  }
  
  #pragma GCC pop_options
  

• Annotate individual functions with GCC's attribute syntax: check the GCC function attributedocs for a more detailed dissertation on the subject. An example:

  //add 5 to each element of the int array.
  __attribute__((optimize("unroll-loops")))
  void add5(int a[20]) {
      int i = 19;
      for(; i > 0; i--) {
          a[i] += 5;
      }
  }
  

• 使用#pragma 指令，如GCC 文档中#pragma GCC optimize ("string"...)所见。请注意，编译指示使其余函数的优化全局化。如果您巧妙地使用和宏，您可能可以仅围绕一个函数定义它，如下所示：#pragma push_optionspop_options

  #pragma GCC push_options
  #pragma GCC optimize ("unroll-loops")
  
  //add 5 to each element of the int array.
  void add5(int a[20]) {
      int i = 19;
      for(; i > 0; i--) {
          a[i] += 5;
      }
  }
  
  #pragma GCC pop_options
  

• 使用GCC 的属性语法注释单个函数：查看GCC 函数属性文档以获得关于该主题的更详细的论文。一个例子：

  //add 5 to each element of the int array.
  __attribute__((optimize("unroll-loops")))
  void add5(int a[20]) {
      int i = 19;
      for(; i > 0; i--) {
          a[i] += 5;
      }
  }
  

Note:I'm not sure how good GCC is at unrolling reverse-iterated loops (I did it to get Markdown to play nice with my code). The examples should compile fine, though.

注意：我不确定 GCC 在展开反向迭代循环方面有多好（我这样做是为了让 Markdown 能够很好地处理我的代码）。不过，这些示例应该可以很好地编译。

回答by Frederik Deweerdt

GCC 8 has gained a new pragma that allows you to control how loop unrolling is done:

GCC 8 获得了一个新的编译指示，允许您控制循环展开的方式：

#pragma GCC unroll n

#pragma GCC unroll n

Quoting from the manual:

引用手册：

You can use this pragma to control how many times a loop should be unrolled. It must be placed immediately before a for, while or do loop or a #pragma GCC ivdep, and applies only to the loop that follows. n is an integer constant expression specifying the unrolling factor. The values of 0 and 1 block any unrolling of the loop.

您可以使用此编译指示来控制应该展开循环的次数。它必须紧接在 for、while 或 do 循环或 #pragma GCC ivdep 之前，并且仅适用于随后的循环。n 是指定展开因子的整数常量表达式。0 和 1 的值会阻止循环的任何展开。

回答by Jerry Coffin

-funroll-loopsmight be helpful (though it turns on loop-unrolling globally, not per-loop). I'm not sure whether there's a #pragmato do the same...

-funroll-loops可能会有所帮助（尽管它会全局打开循环展开，而不是每个循环）。我不确定是否有#pragma同样的事情...