Warn about inconsistent OpenACC nested reduction clauses

Warn about inconsistent OpenACC nested reduction clauses

	OpenACC (cf. OpenACC 2.7, section 2.9.11. "reduction clause";
	this was first clarified by OpenACC 2.6) requires that, if a
	variable is used in reduction clauses on two nested loops, then
	there must be reduction clauses for that variable on all loops
	that are nested in between the two loops and all these reduction
	clauses must use the same operator.
	This commit introduces a check for that property which reports
	warnings if it is violated.

	2019-11-06  Gergö Barany  <gergo@codesourcery.com>
	            Frederik Harwath  <frederik@codesourcery.com>
	            Thomas Schwinge  <thomas@codesourcery.com>

	gcc/
	* omp-low.c (struct omp_context): New fields
	local_reduction_clauses, outer_reduction_clauses.
	(new_omp_context): Initialize these.
	(scan_sharing_clauses): Record reduction clauses on OpenACC constructs.
	(scan_omp_for): Check reduction clauses for incorrect nesting.
	gcc/testsuite/
	* c-c++-common/goacc/nested-reductions-warn.c: New test.
	* c-c++-common/goacc/nested-reductions.c: New test.
	* gfortran.dg/goacc/nested-reductions-warn.f90: New test.
	* gfortran.dg/goacc/nested-reductions.f90: New test.
	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/par-loop-comb-reduction-1.c:
	Add expected warnings about missing reduction clauses.
	* testsuite/libgomp.oacc-c-c++-common/par-loop-comb-reduction-2.c:
	Likewise.
	* testsuite/libgomp.oacc-c-c++-common/par-loop-comb-reduction-3.c:
	Likewise.
	* testsuite/libgomp.oacc-c-c++-common/par-loop-comb-reduction-4.c:
	Likewise.

	Reviewed-by: Thomas Schwinge <thomas@codesourcery.com>
From-SVN: r277875