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)?
