RE: Turning off unrolling to certain loops

["Bingfeng Mei" <bmei at broadcom dot com>]

Jc,
How did you implement #pragma unroll?  I checked other compilers. The
pragma should govern the next immediate loop. It took me a while to 
find a not-so-elegant way to do that. I also implemented #pragma ivdep.
These information are supposed to be passed through both tree and RTL
levels and suvive all GCC optimization. I still have problem in handling
combination of unroll and ivdep.

Bingfeng

> -----Original Message-----
> From: fearyourself@gmail.com [mailto:fearyourself@gmail.com] 
> On Behalf Of Jean Christophe Beyler
> Sent: 15 October 2009 16:34
> To: Zdenek Dvorak
> Cc: Bingfeng Mei; gcc@gcc.gnu.org
> Subject: Re: Turning off unrolling to certain loops
> 
> 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
> >
> 
>