openmp: Add middle-end support for metadirectives

openmp: Add middle-end support for metadirectives

This adds a new Gimple statement type GIMPLE_OMP_METADIRECTIVE, which
represents the metadirective in Gimple. In high Gimple, the statement
contains the body of the directive variants, whereas in low Gimple, it
only contains labels to the bodies.

This patch adds support for converting metadirectives from tree to Gimple
form, and handling of the Gimple form (Gimple lowering, OpenMP lowering
and expansion, inlining, SSA handling etc).

Metadirectives should be resolved before they reach the back-end, otherwise
the compiler will crash as GCC does not know how to convert metadirective
Gimple statements to RTX.

2022-01-25  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* gimple-low.c (lower_omp_metadirective): New.
	(lower_stmt): Handle GIMPLE_OMP_METADIRECTIVE.
	* gimple-pretty-print.c (dump_gimple_omp_metadirective): New.
	(pp_gimple_stmt_1): Handle GIMPLE_OMP_METADIRECTIVE.
	* gimple-walk.c (walk_gimple_op): Handle GIMPLE_OMP_METADIRECTIVE.
	(walk_gimple_stmt): Likewise.
	* gimple.c (gimple_alloc_omp_metadirective): New.
	(gimple_build_omp_metadirective): New.
	(gimple_build_omp_metadirective_variant): New.
	* gimple.def (GIMPLE_OMP_METADIRECTIVE): New.
	(GIMPLE_OMP_METADIRECTIVE_VARIANT): New.
	* gimple.h (gomp_metadirective_variant): New.
	(gomp_metadirective): New.
	(is_a_helper <gomp_metadirective *>::test): New.
	(is_a_helper <gomp_metadirective_variant *>::test): New.
	(is_a_helper <const gomp_metadirective *>::test): New.
	(is_a_helper <const gomp_metadirective_variant *>::test): New.
	(gimple_alloc_omp_metadirective): New prototype.
	(gimple_build_omp_metadirective): New prototype.
	(gimple_build_omp_metadirective_variant): New prototype.
	(gimple_has_substatements): Add GIMPLE_OMP_METADIRECTIVE case.
	(gimple_has_ops): Add GIMPLE_OMP_METADIRECTIVE.
	(gimple_omp_metadirective_label): New.
	(gimple_omp_metadirective_set_label): New.
	(gimple_omp_metadirective_variants): New.
	(gimple_omp_metadirective_set_variants): New.
	(CASE_GIMPLE_OMP): Add GIMPLE_OMP_METADIRECTIVE.
	* gimplify.c (is_gimple_stmt): Add OMP_METADIRECTIVE.
	(expand_omp_metadirective): New.
	(gimplify_omp_metadirective): New.
	(gimplify_expr): Add case for OMP_METADIRECTIVE.
	* gsstruct.def (GSS_OMP_METADIRECTIVE): New.
	(GSS_OMP_METADIRECTIVE_VARIANT): New.
	* omp-expand.c (build_omp_regions_1): Handle GIMPLE_OMP_METADIRECTIVE.
	(omp_make_gimple_edges): Likewise.
	* omp-low.c (struct omp_context): Add next_clone field.
	(new_omp_context): Initialize next_clone field.
	(clone_omp_context): New.
	(delete_omp_context): Delete clone contexts.
	(scan_omp_metadirective): New.
	(scan_omp_1_stmt): Handle GIMPLE_OMP_METADIRECTIVE.
	(lower_omp_metadirective): New.
	(lower_omp_1): Handle GIMPLE_OMP_METADIRECTIVE.
	* tree-cfg.c (cleanup_dead_labels): Handle GIMPLE_OMP_METADIRECTIVE.
	(gimple_redirect_edge_and_branch): Likewise.
	* tree-inline.c (remap_gimple_stmt): Handle GIMPLE_OMP_METADIRECTIVE.
	(estimate_num_insns): Likewise.
	* tree-pretty-print.c (dump_generic_node): Handle OMP_METADIRECTIVE.
	* tree-ssa-operands.c (parse_ssa_operands): Handle
	GIMPLE_OMP_METADIRECTIVE.