Eth2.0-specs: SSZ: explicit ordering vs sorted-by-field

[arnetheduck]

ebc136a

When hashing containers using SSZTreeHash, it is currently specified that fields short be sorted lexicographically.

There are advantages:

• generic solution
• no mental overhead to decide where fields go, can focus on "logical" ordering in spec

There are a few downsides of this approach however:

• Variable-length fields make offsets non-predictable even for fixed-length fields
• Reduced possibility to group similar fields together, for example by data type, to exploit bulk operations

Alternative approaches:

• Manual ordering - enforce that hashing is done in same order as in spec, and control a good ordering manually
  
  
  
  • Often coupled with a restriction that new fields are added at end for upgrades, to maintain soft compatibility with previous versions
  
  
• Data-type based ordering - generally, these kinds of orderings group data by type, putting fixed-length fields before variable-length fields - this gives each field a fixed relative offset making it easy to look up. This kind of ordering also helps keep aliasing issues at a minimum.

[JustinDrake]

+1 for manual ordering

[djrtwo]

closed via #339