c - 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.