[Bug middle-end/69921] New: Switch OpenACC kernels number of gangs from "decide at run time" to "decide at compile time"

[tschwinge at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69921

            Bug ID: 69921
           Summary: Switch OpenACC kernels number of gangs from "decide at
                    run time" to "decide at compile time"
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: openacc
          Severity: minor
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tschwinge at gcc dot gnu.org
                CC: nathan at gcc dot gnu.org, vries at gcc dot gnu.org
  Target Milestone: ---

Follow-up to r233634.  See
<http://news.gmane.org/find-root.php?message_id=%3C87bn7v4b0m.fsf%40kepler.schwinge.homeip.net%3E>.

For OpenACC kernels, gcc/tree-parloops.c:create_parallel_loop currently sets:

    OMP_CLAUSE_NUM_GANGS_EXPR (clause) = n_threads;

..., so to zero ("decide at run time").

| Originally, I want to use:
| 
|     OMP_CLAUSE_NUM_GANGS_EXPR (clause) = build_int_cst (integer_type_node,
n_threads == 0 ? -1 : n_threads);
| 
| ... to store -1 "have the compiler decidew" (instead of now 0 "have the
| run-time decide", which might prevent some code optimizations, as I
| understand it) for the n_threads == 0 case, but it seems that for an
| offloaded OpenACC kernels region, gcc/omp-low.c:oacc_validate_dims is
| called with the parameter "used" set to 0 instead of "gang", and then the
| "Default anything left to 1 or a partitioned default" logic will default
| dims["gang"] to oacc_min_dims["gang"] (that is, 1) instead of the
| oacc_default_dims["gang"] (that is, 32).  Nathan, does that smell like a
| bug (and could you look into that)?