Using ChainRules in your AD system

This section is for authors of AD systems. It assumes a pretty solid understanding of both Julia and automatic differentiation. It explains how to make use of ChainRule's "rulesets" (frules, rrules,) to avoid having to code all your own AD primitives / custom sensitives.

There are 3 main ways to access ChainRules rule sets in your AutoDiff system.

1. Operation Overloading Generation
   • This is primarily intended for operator overloading based AD systems which will generate overloads for primal functions based for their overloaded types based on the existence of an rrule/frule.
   • A source code generation based AD can also use this by overloading their transform generating function directly so as not to recursively generate a transform but to just return the rule.
   • This does not play nice with Revise.jl, adding or modifying rules in loaded files will not be reflected until a manual refresh, and deleting rules will not be reflected at all.
2. Source code tranform based on inserting branches that check of rrule/frule return nothing
   • If the rrule/frule returns a rule result then use it, if it returns nothing then do normal AD path.
   • In theory type inference optimizes these branchs out; in practice it may not.
   • This is a fairly simple Cassette overdub (or similar) of all calls, and is suitable for overloading based AD or source code transformation.
3. Source code transform based on rrule/frule method-table
   • If an applicable rrule/frule exists in the method table then use it, else generate normal AD path.
   • This avoids having branches in your generated code.
   • This requires maintaining your own back-edges.
   • This is pretty hardcore even by the standard of source code tranformations.