Re: FORCING GCC TO UNROLL LOOPS

[dibstar <dibyam4 at gmail dot com>]

I am using gcc 4.4.5. I have looked into unroll_loop_runtime_iterations. Can
you please tell me how to change this code to enable unrolling by all
factors?


Ian Lance Taylor-3 wrote:
> 
> 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
> 
> 

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