Hi Jakub,
this patch provides more implementation of the requires directive, basically:

(1) The collection of the reverse_offload, unified_address, and unified_shared_memory
clauses into a .gnu.gomp_requires section

(2) libgomp checking of consistency across the entire .gnu.gomp_requires section,
and querying into the offload plugin to see if the offload target supports the required
features (as of now, the setting is that none of those features are supported by any
of the plugins).

We currently emit errors, but do not fatally cause exit of the program if those
are not met. We're still unsure if complete block-out of program execution is the right
thing for the user. This can be discussed later.

Is this okay for trunk after stage1 re-opens?

Thanks,
Chung-Lin

2021-01-13  Chung-Lin Tang  <cltang@codesourcery.com>

	gcc/c/
	* c-parser.c (c_parser_declaration_or_fndef): Set
	OMP_REQUIRES_TARGET_USED in omp_requires_mask if function has
	"omp declare target" attribute.
	(c_parser_omp_target_data): Set	OMP_REQUIRES_TARGET_USED in
	omp_requires_mask.
	(c_parser_omp_target_enter_data): Likewise.
	(c_parser_omp_target_exit_data): Likewise.
	(c_parser_omp_requires): Adjust to only mention "not implemented yet"
	for OMP_REQUIRES_DYNAMIC_ALLOCATORS.

	gcc/cp/
	* parser.c (cp_parser_simple_declaration): Set
	OMP_REQUIRES_TARGET_USED in omp_requires_mask if function has
	"omp declare target" attribute.
	(cp_parser_omp_target_data): Set OMP_REQUIRES_TARGET_USED in
	omp_requires_mask.
	(cp_parser_omp_target_enter_data): Likewise.
	(cp_parser_omp_target_exit_data): Likewise.
	(cp_parser_omp_requires): Adjust to only mention "not implemented yet"
	for OMP_REQUIRES_DYNAMIC_ALLOCATORS.

	gcc/fortran/
	* openmp.c (gfc_check_omp_requires): Fix REVERSE_OFFLOAD typo.
	(gfc_match_omp_requires): Adjust to only mention "not implemented yet"
	for OMP_REQUIRES_DYNAMIC_ALLOCATORS.
	* parse.c ("tree.h"): Add include.
	("omp-general.h"): Likewise.
	(gfc_parse_file): Add code to merge omp_requires to omp_requires_mask.

	gcc/
	* omp-offload.c (omp_finish_file): Add code to reate OpenMP requires
	mask variable in .gnu.gomp_requires section if needed.

	gcc/testsuite/
	* c-c++-common/gomp/requires-4.c: Remove prune of "not supported yet".
	* gcc/testsuite/gfortran.dg/gomp/requires-4.f90: Fix REVERSE_OFFLOAD typo.
	* gcc/testsuite/gfortran.dg/gomp/requires-8.f90: Likewise.

	include/
	* gomp-constants.h (GOMP_REQUIRES_UNIFIED_ADDRESS): New symbol.
	(GOMP_REQUIRES_UNIFIED_SHARED_MEMORY): Likewise.
	(GOMP_REQUIRES_REVERSE_OFFLOAD): Likewise.

	libgcc/
	* offloadstuff.c (__requires_mask_table): New symbol to mark start of
	.gnu.gomp_requires section.
	(__requires_mask_table_end): New symbol to mark end of
	.gnu.gomp_requires section.

	libgomp/
	* libgomp-plugin.h (GOMP_OFFLOAD_supported_features): New declaration.
	* libgomp.h (struct gomp_device_descr): New 'supported_features_func'
	plugin hook field.
	* oacc-host.c (host_supported_features): New host hook function.
	(host_dispatch): Initialize 'supported_features_func' host hook.
	* plugin/plugin-gcn.c (GOMP_OFFLOAD_supported_features): New function.
	* plugin/plugin-nvptx.c (GOMP_OFFLOAD_supported_features): Likewise.
	* target.c (<stdio.h>): Add include of standard header.
	(gomp_requires_mask): New static variable.
	(__requires_mask_table): New declaration.
	(__requires_mask_table_end): Likewise.
	(gomp_load_plugin_for_device): Add loading of 'supported_features' hook.
	(gomp_target_init): Add code to summarize .gnu._gomp_requires section
	mask values, emit error if inconsistency found.

	* testsuite/libgomp.c-c++-common/requires-1.c: New test.
	* testsuite/libgomp.c-c++-common/requires-1-aux.c: New file linked with
	above test.
	* testsuite/libgomp.c-c++-common/requires-2.c: New test.
	* testsuite/libgomp.c-c++-common/requires-2-aux.c: New file linked with
	above test.

	liboffloadmic/
	* plugin/libgomp-plugin-intelmic.cpp (GOMP_OFFLOAD_supported_features):
	New function.