PGI User Forum

[dcwarren]

In the code that follows, I have a set of nested loops. The outer loop is the one I'd like to parallelize on the GPU with OpenACC. The inner loop should be run sequentially by each individual thread.

When I try to compile this code using

[mkcolg]

Hi dcwarren,

For many reasons, but mostly due to the lack of a linker for device code, OpenACC does not allow for the calling of routines from within a compute region. While this is changing with the addition of the proposed OpenACC 2.0 standard's "routine" directive, (See: http://www.openacc.org/sites/default/files/Proposed%20Additions%20for%20OpenACC%202.pdf), until this has been implemented, all routines must be inlined, either implicitly by the compiler or explicitly by the user.

In this case, if you change your code so that "m" is not a contained function, "kiss" can be automatically inlined by the compiler by adding the "-Minline" flag. (routines that have contained functions can't be automatically inlined)

However, your code wont accelerate due to the loop dependency on the w,x,y, and z variables. You can work around this by making these variables public in your module and then using the "private" clause to make it so that each thread has it's own copy of the variables. The problem with this is that each thread will have the same initial seed and therefore generate the same random values. Hence, the better solution is to make w, x, y, and z arrays, one for each iteration of the "i" loop, and then randomly generate and array of seeds.

Hope this helps,
Mat

[dcwarren]

Thanks for the response. I knew about the necessity of inlining, but not about contained functions. Where could I have found that information?

As to generating the random seed, I changed kisset to a subroutine and called kisset at the top of each iteration of outer_loop as follows:

[mkcolg]

[dcwarren]

Making those changes (and giving up the fight against warning messages) makes that little test code run spectacularly. Thanks for the insights.

However, when I apply those lessons to the actual production code, I still get errors about unsupported calls to certain subroutines. Given what you mentioned before, I think the issue is inability to inline said subroutines. In the PGI Fortran Compiler Manual, there are a few reasons given why subprograms wouldn't be inlined: