Re: FORCING GCC TO UNROLL LOOPS

[Ian Lance Taylor <iant at google dot com>]

dibstar <dibyam4@gmail.com> writes:

>         I wish to force gcc to unroll both the constant time iterating loops
> and the runtime iterating loops as outlined in loop-unroll.c. I modified the
> loop-unroll.c to place all unrolling decisions in the function
> decide-unroll-and-peel. I also made gcc interact with the user by asking the
> user the function name that which contains the desired loop and the loop
> number and the desired unroll factor. This seems to be fine for unrolling
> constant time iterating loops, i.e. i am able to unroll these loops by all
> factors from 1 to 8. However, for runtime loops, i get a segmentation fault.
> I analysed the code and found that gcc assumes that the unroll factor is a
> power of 2 for runtime loops. I wish to change this and force gcc to unroll
> a particular loop by all factors from 1 to 8. How can I do this?

What version of gcc?

I assume you mean decide_unrolling_and_peeling rather than
decide-unroll-and-peel.

I agree that the code assumes that the unroll factor is a power of 2.
If you want to use a different factor, you will have to change the code.
Are you asking how to do that?  Look in unroll_loop_runtime_iterations.
It uses AND; as the comment explains, that only works because it is
unrolling by a power of 2.  If you want something else to work, you will
need to change that.

Ian