On Wed, Feb 19, 2020 at 1:13 PM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> This PR is a regression caused by r256644, which added support for alias
> checks involving variable strides. One of the changes in that commit
> was to split the access size out of the segment length. The PR shows
> that I hadn't done that correctly for the handling of negative strides
> in vect_compile_time_alias. The old code was:
>
> const_length_a = (-wi::to_poly_wide (segment_length_a)).force_uhwi ();
> offset_a = (offset_a + vect_get_scalar_dr_size (a)) - const_length_a;
>
> where vect_get_scalar_dr_size (a) was cancelling out the subtraction
> of the access size inherent in "- const_length_a". Taking the access
> size out of the segment length meant that the addition was no longer
> needed/correct.
>
> Tested on aarch64-linux-gnu and x86_64-linux-gnu. OK to install?
OK.
Thanks,
Richard.
> Richard
>
>
> 2020-02-19 Richard Sandiford <richard.sandif...@arm.com>
>
> gcc/
> PR tree-optimization/93767
> * tree-vect-data-refs.c (vect_compile_time_alias): Remove the
> access-size bias from the offset calculations for negative strides.
>
> gcc/testsuite/
> PR tree-optimization/93767
> * gcc.dg/vect/pr93767.c: New test.
> ---
> gcc/testsuite/gcc.dg/vect/pr93767.c | 13 +++++++++++++
> gcc/tree-vect-data-refs.c | 4 ++--
> 2 files changed, 15 insertions(+), 2 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/vect/pr93767.c
>
> diff --git a/gcc/testsuite/gcc.dg/vect/pr93767.c
> b/gcc/testsuite/gcc.dg/vect/pr93767.c
> new file mode 100644
> index 00000000000..5f95d7bd35b
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/pr93767.c
> @@ -0,0 +1,13 @@
> +int
> +main ()
> +{
> + int a[10], b;
> + for (b = 6; b >= 3; b--)
> + {
> + a[b] = 1;
> + a[b + 2] = a[3];
> + }
> + if (a[5] != 1)
> + __builtin_abort ();
> + return 0;
> +}
> diff --git a/gcc/tree-vect-data-refs.c b/gcc/tree-vect-data-refs.c
> index 554ef892254..0192aa64636 100644
> --- a/gcc/tree-vect-data-refs.c
> +++ b/gcc/tree-vect-data-refs.c
> @@ -3261,14 +3261,14 @@ vect_compile_time_alias (dr_vec_info *a, dr_vec_info
> *b,
> if (tree_int_cst_compare (DR_STEP (a->dr), size_zero_node) < 0)
> {
> const_length_a = (-wi::to_poly_wide (segment_length_a)).force_uhwi ();
> - offset_a = (offset_a + access_size_a) - const_length_a;
> + offset_a -= const_length_a;
> }
> else
> const_length_a = tree_to_poly_uint64 (segment_length_a);
> if (tree_int_cst_compare (DR_STEP (b->dr), size_zero_node) < 0)
> {
> const_length_b = (-wi::to_poly_wide (segment_length_b)).force_uhwi ();
> - offset_b = (offset_b + access_size_b) - const_length_b;
> + offset_b -= const_length_b;
> }
> else
> const_length_b = tree_to_poly_uint64 (segment_length_b);
