Support for aliasing with variable strides

Support for aliasing with variable strides

This patch adds runtime alias checks for loops with variable strides,
so that we can vectorise them even without a restrict qualifier.
There are several parts to doing this:

1) For accesses like:

     x[i * n] += 1;

   we need to check whether n (and thus the DR_STEP) is nonzero.
   vect_analyze_data_ref_dependence records values that need to be
   checked in this way, then prune_runtime_alias_test_list records a
   bounds check on DR_STEP being outside the range [0, 0].

2) For accesses like:

     x[i * n] = x[i * n + 1] + 1;

   we simply need to test whether abs (n) >= 2.
   prune_runtime_alias_test_list looks for cases like this and tries
   to guess whether it is better to use this kind of check or a check
   for non-overlapping ranges.  (We could do an OR of the two conditions
   at runtime, but that isn't implemented yet.)

3) Checks for overlapping ranges need to cope with variable strides.
   At present the "length" of each segment in a range check is
   represented as an offset from the base that lies outside the
   touched range, in the same direction as DR_STEP.  The length
   can therefore be negative and is sometimes conservative.

   With variable steps it's easier to reaon about if we split
   this into two:

     seg_len:
       distance travelled from the first iteration of interest
       to the last, e.g. DR_STEP * (VF - 1)

     access_size:
       the number of bytes accessed in each iteration

   with access_size always being a positive constant and seg_len
   possibly being variable.  We can then combine alias checks
   for two accesses that are a constant number of bytes apart by
   adjusting the access size to account for the gap.  This leaves
   the segment length unchanged, which allows the check to be combined
   with further accesses.

   When seg_len is positive, the runtime alias check has the form:

        base_a >= base_b + seg_len_b + access_size_b
     || base_b >= base_a + seg_len_a + access_size_a

   In many accesses the base will be aligned to the access size, which
   allows us to skip the addition:

        base_a > base_b + seg_len_b
     || base_b > base_a + seg_len_a

   A similar saving is possible with "negative" lengths.

   The patch therefore tracks the alignment in addition to seg_len
   and access_size.

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 (vec_lower_bound): New structure.
	(_loop_vec_info): Add check_nonzero and lower_bounds.
	(LOOP_VINFO_CHECK_NONZERO): New macro.
	(LOOP_VINFO_LOWER_BOUNDS): Likewise.
	(LOOP_REQUIRES_VERSIONING_FOR_ALIAS): Check lower_bounds too.
	* tree-data-ref.h (dr_with_seg_len): Add access_size and align
	fields.  Make seg_len the distance travelled, not including the
	access size.
	(dr_direction_indicator): Declare.
	(dr_zero_step_indicator): Likewise.
	(dr_known_forward_stride_p): Likewise.
	* tree-data-ref.c: Include stringpool.h, tree-vrp.h and
	tree-ssanames.h.
	(runtime_alias_check_p): Allow runtime alias checks with
	variable strides.
	(operator ==): Compare access_size and align.
	(prune_runtime_alias_test_list): Rework for new distinction between
	the access_size and seg_len.
	(create_intersect_range_checks_index): Likewise.  Cope with polynomial
	segment lengths.
	(get_segment_min_max): New function.
	(create_intersect_range_checks): Use it.
	(dr_step_indicator): New function.
	(dr_direction_indicator): Likewise.
	(dr_zero_step_indicator): Likewise.
	(dr_known_forward_stride_p): Likewise.
	* tree-loop-distribution.c (data_ref_segment_size): Return
	DR_STEP * (niters - 1).
	(compute_alias_check_pairs): Update call to the dr_with_seg_len
	constructor.
	* tree-vect-data-refs.c (vect_check_nonzero_value): New function.
	(vect_preserves_scalar_order_p): New function, split out from...
	(vect_analyze_data_ref_dependence): ...here.  Check for zero steps.
	(vect_vfa_segment_size): Return DR_STEP * (length_factor - 1).
	(vect_vfa_access_size): New function.
	(vect_vfa_align): Likewise.
	(vect_compile_time_alias): Take access_size_a and access_b arguments.
	(dump_lower_bound): New function.
	(vect_check_lower_bound): Likewise.
	(vect_small_gap_p): Likewise.
	(vectorizable_with_step_bound_p): Likewise.
	(vect_prune_runtime_alias_test_list): Ignore cross-iteration
	depencies if the vectorization factor is 1.  Convert the checks
	for nonzero steps into checks on the bounds of DR_STEP.  Try using
	a bunds check for variable steps if the minimum required step is
	relatively small. Update calls to the dr_with_seg_len
	constructor and to vect_compile_time_alias.
	* tree-vect-loop-manip.c (vect_create_cond_for_lower_bounds): New
	function.
	(vect_loop_versioning): Call it.
	* tree-vect-loop.c (vect_analyze_loop_2): Clear LOOP_VINFO_LOWER_BOUNDS
	when retrying.
	(vect_estimate_min_profitable_iters): Account for any bounds checks.

gcc/testsuite/
	* gcc.dg/vect/bb-slp-cond-1.c: Expect loop vectorization rather
	than SLP vectorization.
	* gcc.dg/vect/vect-alias-check-10.c: New test.
	* gcc.dg/vect/vect-alias-check-11.c: Likewise.
	* gcc.dg/vect/vect-alias-check-12.c: Likewise.
	* gcc.dg/vect/vect-alias-check-8.c: Likewise.
	* gcc.dg/vect/vect-alias-check-9.c: Likewise.
	* gcc.target/aarch64/sve/strided_load_8.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_1.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_1.h: Likewise.
	* gcc.target/aarch64/sve/var_stride_1_run.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_2.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_2_run.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_3.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_3_run.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_4.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_4_run.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_5.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_5_run.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_6.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_6_run.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_7.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_7_run.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_8.c: Likewise.
	* gcc.target/aarch64/sve/var_stride_8_run.c: Likewise.
	* gfortran.dg/vect/vect-alias-check-1.F90: Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256644