告诉GCC专门展开循环 [英] Tell gcc to specifically unroll a loop

问题描述

我怎么能告诉GCC解开一个特定的循环？
我已经使用CUDA SDK，其中循环可以使用手动解开的#pragma解开。是否有GCC类似的功能？我GOOGLE了一下，但没有找到任何东西。

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.

推荐答案

GCC给你处理这几种不同的方式：

GCC gives you a few different ways of handling this:

• 使用 #pragma指令，如的#pragma GCC优化（串......），因为看到在<一个href=\"http://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas\">GCC文档。请注意，编译使得优化全球的余下的功能。如果你使用的#pragma push_options 和 pop_options 宏巧妙，你很可能定义这个周围像这样只有一个功能：

• Use #pragma directives, like #pragma GCC optimize ("string"...), as seen in the GCC docs. Note that the pragma makes the optimizations global for the remaining functions. If you used #pragma push_options and pop_options macros 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

批注各个功能是 GCC的属性语法：查看 GCC功能属性文档的关于这个问题的更详细的论文。举个例子：

Annotate individual functions with GCC's attribute syntax: check the GCC function attribute docs 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;
    }
}

注意：我不知道GCC有多好是展开反反复循环（我这样做是为了让降价打我的code不错）。这些例子应编译正常，虽然。

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.

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