[og7, libgomp, nvptx, committed] Fix too-many-resources fatal error condition and message

[Tom de Vries]

Hi,

atm parallel-dims.c fails on Titan-V, with a cuda launch failure:
...
libgomp: cuLaunchKernel error: too many resources requested for launch
...

We've got a check in the libgomp nvptx plugin to prevent the cuda launch 
failure and give a more informative error message:
...
  /* Check if the accelerator has sufficient hardware resources to 

      launch the offloaded kernel.  */
   if (dims[GOMP_DIM_WORKER] > 1)
     {
       if (reg_granularity > 0
           && dims[GOMP_DIM_WORKER] > threads_per_block)
         GOMP_PLUGIN_fatal
           ("The Nvidia accelerator has insufficient resources "
            "to launch '%s'; recompile the program with "
            "'num_workers = %d' on that offloaded region or "
            "'-fopenacc-dim=-:%d'.\n",
            targ_fn->launch->fn, threads_per_block,
            threads_per_block);
     }
...

The message doesn't trigger, because reg_granularity == -1.
This value comes from dev->register_allocation_granularity which 
defaults to -1 because libgomp does not have a hardcoded constant for 
sm_70. The hardcoded constants that are present match 'Warp Allocation 
Granularity' in the GPU Data table in CUDA_Occupancy_calculator.xls, but 
AFAICT there's no column published yet for sm_70.

Furthermore, the comparison to threads_per_block is not correct. What we 
want here is the maximum amount of threads per block, while the 
threads_per_block variable contains an approximation of that, and the 
exact amount required is already available from the CUDA runtime and 
stored at targ_fn->max_threads_per_block.

Then, the comparison to dims[GOMP_DIM_WORKER] is incorrect. It used to 
be correct before "[nvptx] Handle large vectors in libgomp" when we used 
to do "threads_per_block /= warp_size", but now we need to compare 
against dims[GOMP_DIM_WORKER] * dims[GOMP_DIM_VECTOR].

Finally, the message has not been updated to reflect that vector length 
can be larger than 32.

The patch addresses these issues.

Committed to og7.

Thanks,
- Tom
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