Turning off unrolling to certain loops

[Jean Christophe Beyler]

You are entirely correct, I hadn't thought that through enough.

So I backtracked and have just merged what Bingfeng Mei has done with
your code and have now a corrected version of the loop unrolling.

What I did was directly modified tree_unroll_loop to handle the case
of a perfect unroll or not internally and then used something similar
to what you had done around that. I added what I think is needed to
stop unrolling of those loops in later passes.

I'll be starting my tests but I can port it to 4.5 if you are
interested to see what I did.
Jc

On Thu, Oct 15, 2009 at 6:00 AM, Zdenek Dvorak <rakdver@kam.mff.cuni.cz> wrote:
> Hi,
>
>> I faced a similar issue a while ago. I filed a bug report
>> (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36712) In the end,
>> I implemented a simple tree-level unrolling pass in our port
>> which uses all the existing infrastructure. It works quite well for
>> our purpose, but I hesitated to submit the patch because it contains
>> our not-very-elegannt #prgama unroll implementation.
>
> could you please do so anyway?  Even if there are some issues with the
> #prgama unroll implementation, it could serve as a basis of a usable
> patch.
>
>> /* Perfect unrolling of a loop */
>> static void tree_unroll_perfect_loop (struct loop *loop, unsigned factor,
>>                 edge exit)
>> {
>> ...
>> }
>>
>>
>>
>> /* Go through all the loops:
>>      1. Determine unrolling factor
>>      2. Unroll loops in different conditions
>>         -- perfect loop: no extra copy of original loop
>>         -- other loops: the original version of loops to execute the remaining iterations
>> */
>> static unsigned int rest_of_tree_unroll (void)
>> {
> ...
>>        tree niters = number_of_exit_cond_executions(loop);
>>
>>        bool perfect_unrolling = false;
>>        if(niters != NULL_TREE && niters!= chrec_dont_know && TREE_CODE(niters) == INTEGER_CST){
>>          int num_iters = tree_low_cst(niters, 1);
>>          if((num_iters % unroll_factor) == 0)
>>            perfect_unrolling = true;
>>        }
>>
>>        /* If no. of iterations can be divided by unrolling factor, we have perfect unrolling */
>>        if(perfect_unrolling){
>>          tree_unroll_perfect_loop(loop, unroll_factor, single_dom_exit(loop));
>>        }
>>        else{
>>          tree_unroll_loop (loop, unroll_factor, single_dom_exit (loop), &desc);
>>        }
>
> It would be better to move this test to tree_unroll_loop, and not
> duplicate its code in tree_unroll_perfect_loop.
>
> Zdenek
>