Re: [PATCH] Fix mixed input kind permute optimization

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener <rguenther@suse.de> writes:
> When change_vec_perm_layout runs into a permute combining two
> nodes where one is invariant and one internal the partition of
> one input can be -1 but the other might not be.  The following
> supports this case by simply ignoring inputs with input partiton -1.
>
> I'm not sure this is correct but it avoids ICEing when accessing
> that partitions layout for gcc.target/i386/pr98928.c with the
> change to avoid splitting store dataref groups during SLP discovery.
>
> Bootstrap and regtest running on x86_64-unknown-linux-gnu (ontop of
> the SLP series).  The change can't break anything that's already
> broken but I'm not sure this does the right thing - the testcase
> has an uniform constant.  I'll try to come up with a better runtime
> testcase tomorrow.  Hints as to where to correctly fix such case
> appreciated.

Famous last words, but yeah, it looks correct to me.  I think the
routine in principle should have a free choice of which layout to
choose for invariants (as long as it's consistent for all queries
about the same node).  So it should just be a question of whether
keeping the original layout is more likely to give a valid
permutation, or whether going with out_layout_i would be better.
I don't have a strong intuition either way.

Thanks,
Richard

>
> 	* tree-vect-slp.cc (change_vec_perm_layout): Ignore an
> 	input partition of -1.
> ---
>  gcc/tree-vect-slp.cc | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
> index 873748b0a72..f6ec1a81c96 100644
> --- a/gcc/tree-vect-slp.cc
> +++ b/gcc/tree-vect-slp.cc
> @@ -4828,6 +4828,8 @@ change_vec_perm_layout (slp_tree node, lane_permutation_t &perm,
>  	{
>  	  slp_tree in_node = SLP_TREE_CHILDREN (node)[entry.first];
>  	  unsigned int in_partition_i = m_vertices[in_node->vertex].partition;
> +	  if (in_partition_i == -1u)
> +	    continue;
>  	  this_in_layout_i = m_partitions[in_partition_i].layout;
>  	}
>        if (this_in_layout_i > 0)