nvptx: Cache stacks block for OpenMP kernel launch

nvptx: Cache stacks block for OpenMP kernel launch

2021-01-05  Julian Brown  <julian@codesourcery.com>

libgomp/
	* plugin/plugin-nvptx.c (SOFTSTACK_CACHE_LIMIT): New define.
	(struct ptx_device): Add omp_stacks struct.
	(nvptx_open_device): Initialise cached-stacks housekeeping info.
	(nvptx_close_device): Free cached stacks block and mutex.
	(nvptx_stacks_free): New function.
	(nvptx_alloc): Add SUPPRESS_ERRORS parameter.
	(GOMP_OFFLOAD_alloc): Add strategies for freeing soft-stacks block.
	(nvptx_stacks_alloc): Rename to...
	(nvptx_stacks_acquire): This.  Cache stacks block between runs if same
	size or smaller is required.
	(nvptx_stacks_free): Remove.
	(GOMP_OFFLOAD_run): Call nvptx_stacks_acquire and lock stacks block
	during kernel execution.