Tell gcc to specifically unroll a loop

Question

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.

Answer 1

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.

Answer 2

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.