I can confirm this fixes my test case. Had I known about
highest_pow2_factor() I might have come up with this myself ;-)
> Index: tree-ssa-loop-ivopts.c
> ===================================================================
> --- tree-ssa-loop-ivopts.c (revision 158148)
> +++ tree-ssa-loop-ivopts.c (working copy)
> @@ -1537,16 +1537,18 @@ may_be_unaligned_p (tree ref, tree step)
>
> if (mode != BLKmode)
> {
> - double_int mul;
> - tree al = build_int_cst (TREE_TYPE (step),
> - GET_MODE_ALIGNMENT (mode) / BITS_PER_UNIT);
> + unsigned mode_align = GET_MODE_ALIGNMENT (mode);
>
> - if (base_align < GET_MODE_ALIGNMENT (mode)
> - || bitpos % GET_MODE_ALIGNMENT (mode) != 0
> - || bitpos % BITS_PER_UNIT != 0)
> + if (base_align < mode_align
> + || (bitpos % mode_align) != 0
> + || (bitpos % BITS_PER_UNIT) != 0)
> return true;
>
> - if (!constant_multiple_of (step, al, &mul))
> + if (toffset
> + && (highest_pow2_factor (toffset) * BITS_PER_UNIT) < mode_align)
> + return true;
> +
> + if ((highest_pow2_factor (step) * BITS_PER_UNIT) < mode_align)
> return true;
> }
>
