Tell gcc to specifically unroll a loop

问题

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.

回答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.

回答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.

回答3:

-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...

来源：https://stackoverflow.com/questions/4071690/tell-gcc-to-specifically-unroll-a-loop

标签

gcc

pragma

unroll