OpenMP: Re-work and extend context selector resolution

This patch reimplements the middle-end support for "declare variant"
and extends the resolution mechanism to also handle metadirectives
(PR112779).  It also adds partial support for dynamic selectors
(PR113904) and fixes a selector scoring bug reported as PR114596.  I hope
this rewrite also improves the engineering aspect of the code, e.g. more
comments to explain what it is doing.

In most cases, variant constructs can be resolved either in the front
end or during gimplification; if the variant with the highest score
has a static selector, then only that one is emitted.  In the case
where it has a dynamic selector, it is resolved into a (possibly nested)
if/then/else construct, testing the run-time predicate for each selector
sorted by decreasing order of score until a static selector is found.

In some cases, notably a variant construct in a "declare simd"
function which may or may not expand into a simd clone, it may not be
possible to score or sort the variants until later in compilation (the
ompdevlow pass).  In this case the gimplifier emits a loop containing
a switch statement with the variants in arbitrary order and uses the
OMP_NEXT_VARIANT tree node as a placeholder to control which variant
is tested on each iteration of the loop.  It looks something like:

     switch_var = OMP_NEXT_VARIANT (0, state);
     loop_label:
     switch (switch_var)
       {
       case 1:
        if (dynamic_selector_predicate_1)
          {
            alternative_1;
            goto end_label;
          }
        else
          {
            switch_var = OMP_NEXT_VARIANT (1, state);
            goto loop_label;
          }
       case 2:
         ...
       }
      end_label:

Note that when there are no dynamic selectors, the loop is unnecessary
and only the switch is emitted.

Finally, in the ompdevlow pass, the OMP_NEXT_VARIANT magic cookies are
resolved and replaced with constants.  When compiling with -O we can
expect that the loop and switch will be discarded by subsequent
optimizations and replaced with direct jumps between the cases,
eventually arriving at code with similar control flow to the
early-resolution cases.

This approach is somewhat simpler than the one currently used for
handling declare variant in that all possible code paths are already
included in the output of the gimplifier, so it is not necessary to
maintain hidden references or data structures pointing to expansions of
not-yet-resolved variant constructs and special logic for passing them
through LTO (see PR lto/96680).

A possible disadvantage of this expansion strategy is that dead code
for unused variants in the switch can remain when compiling without
-O.  If this turns out to be a critical problem (e.g., an unused case
includes calls to functions not available to the linker) perhaps some
further processing could be performed by default after ompdevlow to
simplify such constructs.

In order to make this patch more readable for review purposes, it
leaves the existing code for "declare variant" resolution (including
the above-mentioned LTO hack) in place, in some cases just ifdef-ing
out functions that won't compile due to changed interfaces for
dependencies.  The next patch in the series will delete all the
now-unused code.

gcc/ChangeLog
	PR middle-end/114596
	PR middle-end/112779
	PR middle-end/113904

	* Makefile.in (GTFILES): Move omp-general.h earlier; required
	because of moving score_wide_int declaration to that file.
	* cgraph.h (struct cgraph_node): Add has_omp_variant_constructs flag.
	* cgraphclones.cc (cgraph_node::create_clone): Propagate
	has_omp_variant_constructs flag.
	* gimplify.cc (omp_resolved_variant_calls): New.
	(expand_late_variant_directive): New.
	(find_supercontext): New.
	(gimplify_variant_call_expr): New.
	(gimplify_call_expr): Adjust parameters to make fallback available.
	Update processing for "declare variant" substitution.
	(is_gimple_stmt): Add OMP_METADIRECTIVE.
	(omp_construct_selector_matches): Ifdef out unused function.
	(omp_get_construct_context): New.
	(gimplify_omp_dispatch): Replace call to deleted function
	omp_resolve_declare_variant with equivalent logic.
	(expand_omp_metadirective): New.
	(expand_late_variant_directive): New.
	(gimplify_omp_metadirective): New.
	(gimplify_expr): Adjust arguments to gimplify_call_expr.  Add
	cases for OMP_METADIRECTIVE, OMP_NEXT_VARIANT, and
	OMP_TARGET_DEVICE_MATCHES.
	(gimplify_function_tree): Initialize/clean up
	omp_resolved_variant_calls.
	* gimplify.h (omp_construct_selector_matches): Delete declaration.
	(omp_get_construct_context): Declare.
	* lto-cgraph.cc (lto_output_node): Write has_omp_variant_constructs.
	(input_overwrite_node): Read has_omp_variant_constructs.
	* omp-builtins.def (BUILT_IN_OMP_GET_NUM_DEVICES): New.
	* omp-expand.cc (expand_omp_taskreg): Propagate
	has_omp_variant_constructs.
	(expand_omp_target): Likewise.
	* omp-general.cc (omp_maybe_offloaded): Add construct_context
	parameter; use it instead of querying gimplifier state.  Add
	comments.
	(omp_context_name_list_prop): Do not test lang_GNU_Fortran in
	offload compiler, just use the string as-is.
	(expr_uses_parm_decl): New.
	(omp_check_context_selector): Add metadirective_p parameter.
	Remove sorry for target_device selector.  Add additional checks
	specific to metadirective or declare variant.
	(make_omp_metadirective_variant): New.
	(omp_construct_traits_match): New.
	(omp_context_selector_matches): Temporarily ifdef out the previous
	code, and add a new implementation based on the old one with
	different parameters, some unnecessary loops removed, and code
	re-indented.
	(omp_target_device_matches_on_host): New.
	(resolve_omp_target_device_matches): New.
	(omp_construct_simd_compare): Support matching of "simdlen" and
	"aligned" clauses.
	(omp_context_selector_set_compare): Make static.  Adjust call to
	omp_construct_simd_compare.
	(score_wide_int): Move declaration to omp-general.h.
	(omp_selector_is_dynamic): New.
	(omp_device_num_check): New.
	(omp_dynamic_cond): New.
	(omp_context_compute_score): Ifdef out the old version and
	re-implement with different parameters.
	(omp_complete_construct_context): New.
	(omp_resolve_late_declare_variant): Ifdef out.
	(omp_declare_variant_remove_hook): Likewise.
	(omp_resolve_declare_variant): Likewise.
	(sort_variant): New.
	(omp_get_dynamic_candidates): New.
	(omp_declare_variant_candidates): New.
	(omp_metadirective_candidates): New.
	(omp_early_resolve_metadirective): New.
	(omp_resolve_variant_construct): New.
	* omp-general.h (score_wide_int): Moved here from omp-general.cc.
	(struct omp_variant): New.
	(make_omp_metadirective_variant): Declare.
	(omp_construct_traits_to_codes): Delete declaration.
	(omp_check_context_selector): Adjust parameters.
	(omp_context_selector_matches): Likewise.
	(omp_context_selector_set_compare): Delete declaration.
	(omp_resolve_declare_variant): Likewise.
	(omp_declare_variant_candidates): Declare.
	(omp_metadirective_candidates): Declare.
	(omp_get_dynamic_candidates): Declare.
	(omp_early_resolve_metadirective): Declare.
	(omp_resolve_variant_construct): Declare.
	(omp_dynamic_cond): Declare.
	* omp-offload.cc (resolve_omp_variant_cookies): New.
	(execute_omp_device_lower): Call the above function to resolve
	variant directives.  Remove call to omp_resolve_declare_variant.
	(pass_omp_device_lower::gate): Check has_omp_variant_construct bit.
	* omp-simd-clone.cc (simd_clone_create): Propagate
	has_omp_variant_constructs bit.
	* tree-inline.cc (expand_call_inline): Likewise.
	(tree_function_versioning): Likewise.

gcc/c/ChangeLog
	PR middle-end/114596
	PR middle-end/112779
	PR middle-end/113904
	* c-parser.cc (c_finish_omp_declare_variant): Update for changes
	to omp-general.h interfaces.

gcc/cp/ChangeLog
	PR middle-end/114596
	PR middle-end/112779
	PR middle-end/113904
	* decl.cc (omp_declare_variant_finalize_one): Update for changes
	to omp-general.h interfaces.
	* parser.cc (cp_finish_omp_declare_variant): Likewise.

gcc/fortran/ChangeLog
	PR middle-end/114596
	PR middle-end/112779
	PR middle-end/113904
	* trans-openmp.cc (gfc_trans_omp_declare_variant): Update for changes
	to omp-general.h interfaces.

gcc/testsuite/
	PR middle-end/114596
	PR middle-end/112779
	PR middle-end/113904
	* c-c++-common/gomp/declare-variant-12.c: Adjust expected behavior
	per PR114596.
	* c-c++-common/gomp/declare-variant-13.c: Test that this is resolvable
	after gimplification, not just final resolution.
	* c-c++-common/gomp/declare-variant-14.c: Tweak testcase to ensure
	that -O causes dead code to be optimized away.
	* gfortran.dg/gomp/declare-variant-12.f90: Adjust expected behavior
	per PR114596.
	* gfortran.dg/gomp/declare-variant-13.f90: Test that this is resolvable
	after gimplification, not just final resolution.
	* gfortran.dg/gomp/declare-variant-14.f90: Tweak testcase to ensure
	that -O	causes dead code to be optimized away.

Co-Authored-By: Kwok Cheung Yeung <kcy@codesourcery.com>
Co-Authored-By: Sandra Loosemore <sandra@codesourcery.com>
Co-Authored-By: Marcel Vollweiler <marcel@codesourcery.com>