Re: [PATCH, OpenACC] Make reduction arguments addressable

[Thomas Schwinge <thomas@codesourcery.com>]

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Hi!

On Mon, 30 May 2016 18:53:41 +0200, Jakub Jelinek <jakub@redhat.com> wrote:
> On Mon, May 30, 2016 at 10:38:59PM +0800, Chung-Lin Tang wrote:
> > Hi, a previous patch of Cesar's has made the middle-end omp-lowering
> > automatically create and insert a tofrom (i.e. present_or_copy) map for
> > parallel reductions.  This allowed the user to not need explicit
> > clauses to copy out the reduction result, but because reduction arguments
> > are not marked addressable, async does not work as expected,
> > i.e. the asynchronous copy-out results are not used in the compiler generated code.
> 
> If you need it only for async parallel/kernels? regions, can't you do that
> only for those and not for others?

Also, please add comments to the source code to document the need for
such special handling.

> > This patch fixes this in the front-ends, I've tested this patch without
> > new regressions, and fixes some C++ OpenACC tests that regressed after
> > my last OpenACC async patch.  Is this okay for trunk?
> 
> Testcases in the testsuite or others?  If the latter, we should add them.

The r236772 commit "[PATCH, libgomp] Rewire OpenACC async",
<http://news.gmane.org/find-root.php?message_id=%3C56FA4F69.1060101%40codesourcery.com%3E>
regressed (or, triggered/exposed the existing wrong behavior?)
libgomp.oacc-c++/template-reduction.C execution testing for nvptx
offloading.  (Please always send email about such known regressions, and
XFAIL them with your commit -- that would have saved me an hour
yesterday, when I bisected recent changes to figure out why that test
suddenly fails.)

For reference, here is a test case, a reduced C version of
libgomp/testsuite/libgomp.oacc-c++/template-reduction.C.  This test case
works (without Chung-Lin's "[PATCH, OpenACC] Make reduction arguments
addressable" patch) if I enable "POCs", which surprises me a bit, because
I thought after Cesar's recent changes, the gimplifier is doing the same
thing of adding a data clause next to the reduction clause.  Probably
it's not doing the exactly same thing, though.  Should it?  Cesar, do you
have any comments on this?  For example (just guessing), should
TREE_ADDRESSABLE be set where the gimplifier does its work, instead of in
the three front ends?

    // Reduced C version of libgomp/testsuite/libgomp.oacc-c++/template-reduction.C.
    
    const int n = 100;
    
    // Check present and async and an explicit firstprivate
    
    int
    async_sum (int c)
    {
      int s = 0;
    
    #define POCs //present_or_copy(s)
    #pragma acc parallel loop num_gangs (10) gang reduction (+:s) POCs firstprivate (c) async
      for (int i = 0; i < n; i++)
        s += i+c;
    
    #pragma acc wait
    
      return s;
    }
    
    int
    main()
    {
      int result = 0;
    
      for (int i = 0; i < n; i++)
        {
          result += i+1;
        }
    
      if (async_sum (1) != result)
        __builtin_abort ();
    
      return 0;
    }


Grüße
 Thomas

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