This patch adds support for aligning vectors by using a partial
first iteration.  E.g. if the start pointer is 3 elements beyond
an aligned address, the first iteration will have a mask in which
the first three elements are false.

On SVE, the optimisation is only useful for vector-length-specific
code.  Vector-length-agnostic code doesn't try to align vectors
since the vector length might not be a power of 2.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vectorizer.h (_loop_vec_info::mask_skip_niters): New field.
	(LOOP_VINFO_MASK_SKIP_NITERS): New macro.
	(vect_use_loop_mask_for_alignment_p): New function.
	(vect_prepare_for_masked_peels, vect_gen_while_not): Declare.
	* tree-vect-loop-manip.c (vect_set_loop_masks_directly): Add an
	niters_skip argument.  Make sure that the first niters_skip elements
	of the first iteration are inactive.
	(vect_set_loop_condition_masked): Handle LOOP_VINFO_MASK_SKIP_NITERS.
	Update call to vect_set_loop_masks_directly.
	(get_misalign_in_elems): New function, split out from...
	(vect_gen_prolog_loop_niters): ...here.
	(vect_update_init_of_dr): Take a code argument that specifies whether
	the adjustment should be added or subtracted.
	(vect_update_init_of_drs): Likewise.
	(vect_prepare_for_masked_peels): New function.
	(vect_do_peeling): Skip prologue peeling if we're using a mask
	instead.  Update call to vect_update_inits_of_drs.
	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Initialize
	mask_skip_niters.
	(vect_analyze_loop_2): Allow fully-masked loops with peeling for
	alignment.  Do not include the number of peeled iterations in
	the minimum threshold in that case.
	(vectorizable_induction): Adjust the start value down by
	LOOP_VINFO_MASK_SKIP_NITERS iterations.
	(vect_transform_loop): Call vect_prepare_for_masked_peels.
	Take the number of skipped iterations into account when calculating
	the loop bounds.
	* tree-vect-stmts.c (vect_gen_while_not): New function.

gcc/testsuite/
	* gcc.target/aarch64/sve/nopeel_1.c: New test.
	* gcc.target/aarch64/sve/peel_ind_1.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_1_run.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_2.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_2_run.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_3.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_3_run.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_4.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_4_run.c: Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256630