re PR target/80846 (auto-vectorized AVX2 horizontal sum should narrow to 128b right away, to be more efficient for Ryzen and Intel)

2018-01-12  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/80846
	* target.def (split_reduction): New target hook.
	* targhooks.c (default_split_reduction): New function.
	* targhooks.h (default_split_reduction): Declare.
	* tree-vect-loop.c (vect_create_epilog_for_reduction): If the
	target requests first reduce vectors by combining low and high
	parts.
	* tree-vect-stmts.c (vect_gen_perm_mask_any): Adjust.
	(get_vectype_for_scalar_type_and_size): Export.
	* tree-vectorizer.h (get_vectype_for_scalar_type_and_size): Declare.

	* doc/tm.texi.in (TARGET_VECTORIZE_SPLIT_REDUCTION): Document.
	* doc/tm.texi: Regenerate.

	i386/
	* config/i386/i386.c (ix86_split_reduction): Implement
	TARGET_VECTORIZE_SPLIT_REDUCTION.

	* gcc.target/i386/pr80846-1.c: New testcase.
	* gcc.target/i386/pr80846-2.c: Likewise.

From-SVN: r256576