[libgomp, nvptx] Add error with recompilation hint for launch failure

Currently, when a kernel is lauched with too many workers, it results in a cuda
launch failure.  This is triggered f.i. for parallel-loop-1.c at -O0 on a Quadro
M1200.

This patch detects this situation, and errors out with a hint on how to fix it.

Build and reg-tested on x86_64 with nvptx accelerator.

2018-07-26  Cesar Philippidis  <cesar@codesourcery.com>
	    Tom de Vries  <tdevries@suse.de>

	* plugin/plugin-nvptx.c (nvptx_exec): Error if the hardware doesn't have
	sufficient resources to launch a kernel, and give a hint on how to fix
	it.

Co-Authored-By: Tom de Vries <tdevries@suse.de>

From-SVN: r262997

libgomp/ChangeLog

 2018-07-26  Cesar Philippidis  <cesar@codesourcery.com>
 	    Tom de Vries  <tdevries@suse.de>
 
+	* plugin/plugin-nvptx.c (nvptx_exec): Error if the hardware doesn't have
+	sufficient resources to launch a kernel, and give a hint on how to fix
+	it.
+
+2018-07-26  Cesar Philippidis  <cesar@codesourcery.com>
+	    Tom de Vries  <tdevries@suse.de>
+
 	* plugin/plugin-nvptx.c (struct ptx_device): Add warp_size,
 	max_threads_per_block and max_threads_per_multiprocessor fields.
 	(nvptx_open_device): Initialize new fields.

libgomp/plugin/plugin-nvptx.c

@@ -1204,6 +1204,21 @@ nvptx_exec (void (*fn), size_t mapnum, void **hostaddrs, void **devaddrs,
 	  dims[i] = default_dims[i];
     }
 
+  /* Check if the accelerator has sufficient hardware resources to
+     launch the offloaded kernel.  */
+  if (dims[GOMP_DIM_WORKER] * dims[GOMP_DIM_VECTOR]
+      > targ_fn->max_threads_per_block)
+    {
+      int suggest_workers
+	= targ_fn->max_threads_per_block / dims[GOMP_DIM_VECTOR];
+      GOMP_PLUGIN_fatal ("The Nvidia accelerator has insufficient resources to"
+			 " launch '%s' with num_workers = %d; recompile the"
+			 " program with 'num_workers = %d' on that offloaded"
+			 " region or '-fopenacc-dim=:%d'",
+			 targ_fn->launch->fn, dims[GOMP_DIM_WORKER],
+			 suggest_workers, suggest_workers);
+    }
+
   /* This reserves a chunk of a pre-allocated page of memory mapped on both
      the host and the device. HP is a host pointer to the new chunk, and DP is
      the corresponding device pointer.  */