Tell gcc to specifically unroll a loop

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.

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

#pragma GCC unroll n

Quoting from the manual:

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.

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