I plan to replace all AttributeDescription pointer references with array references instead. It makes back-mdb a lot more portable, and it also saves us 4 bytes for every attribute on 64 bit machines.
At a microscopic level it imposes some extra overhead on each reference (instead of a direct pointer deref, you have to do pointer+offset arithmetic) but on modern CPUs simple address offsets like this come for free anyway.
A change of this nature will necessarily touch the majority of slapd code as well as 3rd party overlays. Making this switch will mean that patches for RE24 will almost always be incompatible with patches for HEAD/RE25.
Very likely the same change should be made for the rest of the common data types in slapd. (E.g., ObjectClasses, AttributeTypes, Syntaxes, etc.) But since AttributeDescriptions are used in every Entry these will have the most impact on slapd's memory footprint, the others are inconsequential.