Tell gcc to specifically unroll a loop

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GCC gives you a few different ways of handling this:

  • 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

  • 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;
    }
}

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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