common: concat: make non-dim-1-over-axis md to decide on dst format

src/common/concat_pd.hpp

@@ -199,8 +199,18 @@ struct concat_pd_t : public primitive_desc_t {
         if (status != status::success) {
             for (int i = 0; i < n_; ++i) {
                 const memory_desc_wrapper src_d(src_mds_[i]);
-                if (src_d.is_blocking_desc() && src_d.is_plain()
-                        && src_d.nelems() > 0) {
+                // Dim of `1` may tweak a destination format leading to
+                // sub-optimal performance. Limit it to an axis case to allow
+                // case like a:a->ab or a:ab->ab to work properly.
+                // TODO: update the whole logic to getting string tags of
+                // sources but discarding dims of one. If ndims of any source
+                // coincides with dst ndims, use that tag (if they are same).
+                // If dst has +1 ndim (due to concat dim), use slices as dense
+                // layers inside a dst, which means axis should be the least
+                // dense dimension.
+                const bool axis_dim_has_one = src_d.dims()[concat_dim()] == 1;
+                if (!axis_dim_has_one && src_d.is_blocking_desc()
+                        && src_d.is_plain() && src_d.nelems() > 0) {
                     status = memory_desc_init_by_blocking_desc(dst_md_,
                             memory_desc_wrapper(src_mds_[i]).blocking_desc());
                     if (status == status::success) return status;