vect: Enforce dr_with_seg_len::align precondition [PR116125]

tree-data-refs.cc uses alignment information to try to optimise
the code generated for alias checks.  The assumption for "normal"
non-grouped, full-width scalar accesses was that the access size
would be a multiple of the alignment.  As Richi notes in the PR,
this is a documented precondition of dr_with_seg_len:

  /* The minimum common alignment of DR's start address, SEG_LEN and
     ACCESS_SIZE.  */
  unsigned int align;

PR115192 was a case in which this assumption didn't hold.  The access
was part of an aligned 4-element group, but only the first 2 elements
of the group were accessed.  The alignment was therefore double the
access size.

In r15-820-ga0fe4fb1c8d78045 I'd "fixed" that by capping the
alignment in one of the output routines.  But I think that was
misconceived.  The precondition means that we should cap the
alignment at source instead.

Failure to do that caused a similar wrong code bug in this PR,
where the alignment comes from a short bitfield access rather
than from a group access.

gcc/
	PR tree-optimization/116125
	* tree-vect-data-refs.cc (vect_prune_runtime_alias_test_list): Make
	the dr_with_seg_len alignment fields describe tha access sizes as
	well as the pointer alignment.
	* tree-data-ref.cc (create_intersect_range_checks): Don't compensate
	for invalid alignment fields here.

gcc/testsuite/
	PR tree-optimization/116125
	* gcc.dg/vect/pr116125.c: New test.

gcc/testsuite/gcc.dg/vect/pr116125.c

@@ -0,0 +1,30 @@
+#include "tree-vect.h"
+
+struct st
+{
+  unsigned int num : 8;
+};
+
+void __attribute__((noipa))
+mem_overlap (struct st *a, struct st *b)
+{
+  for (int i = 0; i < 9; i++)
+    a[i].num = b[i].num + 1;
+}
+
+int
+main (void)
+{
+  check_vect ();
+
+  struct st a[9] = {};
+
+  // input a = 0, 0, 0, 0, 0, 0, 0, 0, 0
+  mem_overlap (&a[1], a);
+
+  // output a = 0, 1, 2, 3, 4, 5, 6, 7, 8
+  if (a[2].num == 2)
+    return 0;
+  else
+    __builtin_abort ();
+}

gcc/tree-data-ref.cc

@@ -2647,8 +2647,6 @@ create_intersect_range_checks (class loop *loop, tree *cond_expr,
 	 if the maximum value of one segment is equal to the minimum
 	 value of the other.  */
       min_align = std::min (dr_a.align, dr_b.align);
-      min_align = std::min (min_align, known_alignment (dr_a.access_size));
-      min_align = std::min (min_align, known_alignment (dr_b.access_size));
       cmp_code = LT_EXPR;
     }
 

gcc/tree-vect-data-refs.cc

@@ -4074,7 +4074,7 @@ vect_prune_runtime_alias_test_list (loop_vec_info loop_vinfo)
       poly_uint64 lower_bound;
       tree segment_length_a, segment_length_b;
       unsigned HOST_WIDE_INT access_size_a, access_size_b;
-      unsigned int align_a, align_b;
+      unsigned HOST_WIDE_INT align_a, align_b;
 
       /* Ignore the alias if the VF we chose ended up being no greater
 	 than the dependence distance.  */
@@ -4231,6 +4231,13 @@ vect_prune_runtime_alias_test_list (loop_vec_info loop_vinfo)
 					   stmt_info_b->stmt);
 	}
 
+      /* dr_with_seg_len requires the alignment to apply to the segment length
+	 and access size, not just the start address.  The access size can be
+	 smaller than the pointer alignment for grouped accesses and bitfield
+	 references; see PR115192 and PR116125 respectively.  */
+      align_a = std::min (align_a, least_bit_hwi (access_size_a));
+      align_b = std::min (align_b, least_bit_hwi (access_size_b));
+
       dr_with_seg_len dr_a (dr_info_a->dr, segment_length_a,
 			    access_size_a, align_a);
       dr_with_seg_len dr_b (dr_info_b->dr, segment_length_b,