https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101173
--- Comment #5 from bin cheng <amker at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> So we're exchanging the inner two loops
>
> a[1][3] = 8;
> for (int b = 1; b <= 5; b++)
> for (int d = 0; d <= 5; d++)
> for (c = 0; c <= 5; c++)
> a[b][c] = a[b][c + 2] & 216;
>
> to
>
> a[1][3] = 8;
> for (int b = 1; b <= 5; b++)
> for (c = 0; c <= 5; c++)
> for (int d = 0; d <= 5; d++)
> a[b][c] = a[b][c + 2] & 216;
>
> but that looks wrong from a dependence analysis perspective. We have
>
> (compute_affine_dependence
> ref_a: a[b_33][_1], stmt_a: _2 = a[b_33][_1];
> ref_b: a[b_33][c.3_32], stmt_b: a[b_33][c.3_32] = _3;
> (analyze_overlapping_iterations
> (chrec_a = {2, +, 1}_5)
> (chrec_b = {0, +, 1}_5)
> (analyze_siv_subscript
> (analyze_subscript_affine_affine
> (overlaps_a = [0 + 1 * x_1])
> (overlaps_b = [2 + 1 * x_1]))
> )
> (overlap_iterations_a = [0 + 1 * x_1])
> (overlap_iterations_b = [2 + 1 * x_1]))
> (analyze_overlapping_iterations
> (chrec_a = {1, +, 1}_1)
> (chrec_b = {1, +, 1}_1)
> (overlap_iterations_a = [0])
> (overlap_iterations_b = [0]))
> (analyze_overlapping_iterations
> (chrec_a = {0, +, 1}_5)
> (chrec_b = {2, +, 1}_5)
> (analyze_siv_subscript
> (analyze_subscript_affine_affine
> (overlaps_a = [2 + 1 * x_1])
> (overlaps_b = [0 + 1 * x_1]))
> )
> (overlap_iterations_a = [2 + 1 * x_1])
> (overlap_iterations_b = [0 + 1 * x_1]))
> (analyze_overlapping_iterations
> (chrec_a = {1, +, 1}_1)
> (chrec_b = {1, +, 1}_1)
> (overlap_iterations_a = [0])
> (overlap_iterations_b = [0]))
> (build_classic_dist_vector
> dist_vector = ( 0 0 2
> )
> )
> )
>
> I don't see anything wrong with that at a first glance so the bug must be in
> tree_loop_interchange::valid_data_dependences it checks
>
> /* Be conservative, skip case if either direction at i_idx/o_idx
> levels is not '=' or '<'. */
> if (dist_vect[i_idx] < 0 || dist_vect[o_idx] < 0)
> return false;
>
> dist_vect is [0 0 2], i_idx 2 and o_idx 1 but I think that dist_vect[o_idx]
> should exclude zero, thus
>
> diff --git a/gcc/gimple-loop-interchange.cc b/gcc/gimple-loop-interchange.cc
> index f45b9364644..265e36c48d4 100644
> --- a/gcc/gimple-loop-interchange.cc
> +++ b/gcc/gimple-loop-interchange.cc
> @@ -1043,8 +1043,8 @@ tree_loop_interchange::valid_data_dependences
> (unsigned i_idx, unsigned o_idx,
> continue;
>
> /* Be conservative, skip case if either direction at i_idx/o_idx
> - levels is not '=' or '<'. */
> - if (dist_vect[i_idx] < 0 || dist_vect[o_idx] < 0)
> + levels is not '=' (for the inner loop) or '<'. */
> + if (dist_vect[i_idx] < 0 || dist_vect[o_idx] <= 0)
> return false;
> }
> }
>
> Bin - does this analysis look sound?
Hi Richard,
Thanks very much for helping on this. Sorry I would need a bit more time to
answer this question. Thanks again.
