[Polly] Isl AST reduction pragmas [V2]

[Tobias Grosser]

On 17/06/2014 02:52, Johannes Doerfert wrote:
> Hey Tobias,
>
>
>
> I attached a updated version of the Isl AST reduction patch.
>
> I did not check for loop reversals (one of your comments) but afaik. the isl
> scheduler won't come up with them anyway [as it is based on Uday's paper
> which describes it disallows them].

Unfortunately, this is one of the differences between the isl scheduler 
and the Pluto scheduler. The isl scheduler does not disallow loop 
reversal. Similarly, newer versions of Pluto have also been changed to 
allow loop reversal:

http://www.csa.iisc.ernet.in/TR/2014/1/tr-2014-1.pdf

I just tried it myself. The code will be reversed:

void foo(float *A, long n) {

         for (long i = 0; i < n; i++)
                 A[i] = 1;
         for (long i = 0; i < n; i++)
                 A[n-i-1] += 1;


}

with the option '-polly-opt-fusion=max'. The result is:

   for (c1=1;c1<=n;c1++) {
     Stmt_for_body((c1-1));
     Stmt_for_body4((-c1+n));
   }

> We can in the future change this if necessary.

I am afraid we can not take a shortcut here.

> What do you think of this version?

I think we should find a test case where we both reverse the loops and 
where we get reduction dependences and write a separate patch for the 
scheduler that updates them if necessary. You should be able to use the 
same

>     /// @brief The type of the dependences.
> +  ///
> +  /// Reduction dependences are not included in "TYPE_ALL" dependences
> +  /// as they are no "validity" dependences in the classical sense.

'classical sense' is not well defined.

> +  /// Instead, they can be seen as dependences we wan't to fulfill but
                                                      want

Otherwise, I think the patch is fine. It is in fact a perfect example of 
the small, targeted patches I am looking for.

Tobias