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.

Heh can you do it using macros? Probably not, so just writing it out is the only thing left? — Nils
In all seriousness, I'd suggest looking into separate compilation of just that bit with -funroll-loops before using Duff's Device: it's a beautiful thing to study, but an ugly thing to have in your code. — dmckee --- ex-moderator kitten
You can wrap GCC's function attribute syntax in macros. I've done this once or twice for spot optimizations. — Philip Conrad
Upvoted @dmckee's comment but just have to add that "-funroll-loops" always sounds like the name of a cool roller-coaster at a theme park to me :-) — Vicky

Philip Conrad Philip Conrad · Accepted Answer · 2013-01-10T05:44:49

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.

Tell gcc to specifically unroll a loop

3 Answers