PGI User Forum

TheMatt · Joined: 06 Jul 2009 Posts: 263 Location: Greenbelt, MD

As I'm trying to learn to rewire my brain for parallel thinking, I've been trying various things to reduce the number of "loop carried dependence", "loop carried reuse" and other issues reported by -Minfo=accel. One particular loop has been stymieing me, so I'm coming here to try and figure it out.

To wit, the loop:

mkcolg · Posted: Thu Sep 03, 2009 9:45 am Post subject:

Hi Matt,

The outer "i" loop is being parallelized. However the inner loop is not because for each iteration of the k loop, the same element of fsdir is being assigned to (i.e. loop carried re-use). So if the k loop were to be parallelized, all "k" threads would be trying to assign their values to the same spot, leading to nod-deterministic results. To parallelize the k loop, you'll need to make fsidr a two dimensional array.

Note that we are working on adding support for reductions within accelerator regions. My guess is that your code is more like "fsdir(i) = fsdir(i) + tda(i,k,2)", in which case we should be able to parallelize the inner loop once this support has been added.

- Mat

TheMatt · Joined: 06 Jul 2009 Posts: 263 Location: Greenbelt, MD

You know, you are right and that is actually what the code is doing (in some ways). Guess I've found a place to redo a bit of coding!

Thanks,
Matt