[committed][nvptx, libgomp] Don't launch with num_workers == 0

[Tom de Vries]

Hi,

When using a compiler build with:
...
+#define PTX_DEFAULT_VECTOR_LENGTH PTX_CTA_SIZE
+#define PTX_MAX_VECTOR_LENGTH PTX_CTA_SIZE
...
and running the libgomp testsuite, we run into an execution failure in
parallel-loop-1.c, due to a cuda launch failure:
...
  nvptx_exec: kernel f6_none_none$_omp_fn$0: launch gangs=480, workers=0, \
    vectors=1024

libgomp: cuLaunchKernel error: invalid argument
...
because workers == 0.

The workers variable is set to 0 here in nvptx_exec:
...
                workers = blocks / actual_vectors;
...
because actual_vectors is 1024, and blocks is 768:
...
cuOccupancyMaxPotentialBlockSize: grid = 10, block = 768
...

Fix this by ensuring that workers is at least one.

Committed to trunk.

Thanks,
- Tom

[nvptx, libgomp] Don't launch with num_workers == 0

2019-01-08  Tom de Vries  <tdevr...@suse.de>

        * plugin/plugin-nvptx.c (nvptx_exec): Make sure to launch with at least
        one worker.

---
 libgomp/plugin/plugin-nvptx.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libgomp/plugin/plugin-nvptx.c b/libgomp/plugin/plugin-nvptx.c
index 572d9ef8d5c..60553bdf3bd 100644
--- a/libgomp/plugin/plugin-nvptx.c
+++ b/libgomp/plugin/plugin-nvptx.c
@@ -1272,6 +1272,7 @@ nvptx_exec (void (*fn), size_t mapnum, void **hostaddrs, 
void **devaddrs,
                                      ? vectors
                                      : dims[GOMP_DIM_VECTOR]);
                workers = blocks / actual_vectors;
+               workers = MAX (workers, 1);
              }
 
            for (i = 0; i != GOMP_DIM_MAX; i++)