[nvptx] Don't allow vector_length 64 with num_workers 16

[nvptx] Don't allow vector_length 64 with num_workers 16

When using a compiler build with:
...
+#define PTX_DEFAULT_VECTOR_LENGTH PTX_CTA_SIZE
...

consider a test-case:
...
int
main (void)
{
  #pragma acc parallel vector_length (64)
    #pragma acc loop worker
    for (unsigned int i = 0; i < 32; i++)
      #pragma acc loop vector
      for (unsigned int j = 0; j < 64; j++)
        ;

  return 0;
}
...

If num_workers is 16, either because:
- we add a "num_workers (16)" clause on the parallel directive, or
- we set "GOMP_OPENACC_DIM=:16:", or
- the libgomp plugin chooses 16 num_workers
we run into an illegal instruction at runtime, because a bar.sync instruction
tries to use a barrier 16.  The instruction is illegal, because ptx supports
only 16 barriers per CTA, and the valid range is 0..15.

The problem is that with a warp-multiple vector length, we use a code generation
scheme with a per-worker barrier.  And because barrier zero is reserved for
per-cta barrier, only the remaining 15 barriers can be used as per-worker
barrier, and consequently we can't use num_workers larger than 15.

This problem occurs only for vector_length 64.  For vector_length 32, we use a
different code generation scheme, and for vector_length >= 96, the maximum
num_workers is not big enough not to trigger this problem.

Also, this problem only occurs for num_workers 16.  As explained above,
num_workers 15 is safe to use, and 16 is already the maximum num_workers for
vector_length 64.

This patch fixes the problem in both the compiler (handling "num_workers (16)")
and in the libgomp nvptx plugin (with and without "GOMP_OPENACC_DIM=:16:").

2019-01-11  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.c (PTX_CTA_NUM_BARRIERS, PTX_PER_CTA_BARRIER)
	(PTX_NUM_PER_CTA_BARRIER, PTX_FIRST_PER_WORKER_BARRIER)
	(PTX_NUM_PER_WORKER_BARRIERS): Define.
	(nvptx_apply_dim_limits): Prevent vector_length 64 and
	num_workers 16.

	* plugin/plugin-nvptx.c (nvptx_exec): Prevent vector_length 64 and
	num_workers 16.

From-SVN: r267838