Two things stopped us using SLP reductions with variable-length vectors:

(1) We didn't have a way of constructing the initial vector.
    This patch does it by creating a vector full of the neutral
    identity value and then using a shift-and-insert function
    to insert any non-identity inputs into the low-numbered elements.
    (The non-identity values are needed for double reductions.)
    Alternatively, for unchained MIN/MAX reductions that have no neutral
    value, we instead use the same duplicate-and-interleave approach as
    for SLP constant and external definitions (added by a previous
    patch).

(2) The epilogue for constant-length vectors would extract the vector
    elements associated with each SLP statement and do scalar arithmetic
    on these individual elements.  For variable-length vectors, the patch
    instead creates a reduction vector for each SLP statement, replacing
    the elements for other SLP statements with the identity value.
    It then uses a hardware reduction instruction on each vector.

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

gcc/
	* doc/md.texi (vec_shl_insert_@var{m}): New optab.
	* internal-fn.def (VEC_SHL_INSERT): New internal function.
	* optabs.def (vec_shl_insert_optab): New optab.
	* tree-vectorizer.h (can_duplicate_and_interleave_p): Declare.
	(duplicate_and_interleave): Likewise.
	* tree-vect-loop.c: Include internal-fn.h.
	(neutral_op_for_slp_reduction): New function, split out from
	get_initial_defs_for_reduction.
	(get_initial_def_for_reduction): Handle option 2 for variable-length
	vectors by loading the neutral value into a vector and then shifting
	the initial value into element 0.
	(get_initial_defs_for_reduction): Replace the code argument with
	the neutral value calculated by neutral_op_for_slp_reduction.
	Use gimple_build_vector for constant-length vectors.
	Use IFN_VEC_SHL_INSERT for variable-length vectors if all
	but the first group_size elements have a neutral value.
	Use duplicate_and_interleave otherwise.
	(vect_create_epilog_for_reduction): Take a neutral_op parameter.
	Update call to get_initial_defs_for_reduction.  Handle SLP
	reductions for variable-length vectors by creating one vector
	result for each scalar result, with the elements associated
	with other scalar results stubbed out with the neutral value.
	(vectorizable_reduction): Call neutral_op_for_slp_reduction.
	Require IFN_VEC_SHL_INSERT for double reductions on
	variable-length vectors, or SLP reductions that have
	a neutral value.  Require can_duplicate_and_interleave_p
	support for variable-length unchained SLP reductions if there
	is no neutral value, such as for MIN/MAX reductions.  Also require
	the number of vector elements to be a multiple of the number of
	SLP statements when doing variable-length unchained SLP reductions.
	Update call to vect_create_epilog_for_reduction.
	* tree-vect-slp.c (can_duplicate_and_interleave_p): Make public
	and remove initial values.
	(duplicate_and_interleave): Make public.
	* config/aarch64/aarch64.md (UNSPEC_INSR): New unspec.
	* config/aarch64/aarch64-sve.md (vec_shl_insert_<mode>): New insn.

gcc/testsuite/
	* gcc.dg/vect/pr37027.c: Remove XFAIL for variable-length vectors.
	* gcc.dg/vect/pr67790.c: Likewise.
	* gcc.dg/vect/slp-reduc-1.c: Likewise.
	* gcc.dg/vect/slp-reduc-2.c: Likewise.
	* gcc.dg/vect/slp-reduc-3.c: Likewise.
	* gcc.dg/vect/slp-reduc-5.c: Likewise.
	* gcc.target/aarch64/sve/slp_5.c: New test.
	* gcc.target/aarch64/sve/slp_5_run.c: Likewise.
	* gcc.target/aarch64/sve/slp_6.c: Likewise.
	* gcc.target/aarch64/sve/slp_6_run.c: Likewise.
	* gcc.target/aarch64/sve/slp_7.c: Likewise.
	* gcc.target/aarch64/sve/slp_7_run.c: Likewise.

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

From-SVN: r256623