This API is a generalization of compare-and-swap which allows for multiple predicate-based preconditions, as well as multiple effects when those preconditions are met.
To clarify, think of the cmpxchg instruction as a combination of one precondition and one effect:
[precondition] The value at address A is expected to equal X
[if preconditions satisfied....]
[effect] Store Y at address A.
The Generalized CAS API is similar, except that it allows multiple preconditions and effects to be combined. This means that the following can be performed as one atomic operation:
[precondition] The value at address A is expected to equal 10
[precondition] The value at address B is expected to be greater than 100
[precondition] The value at address B is expected to be less than 200
[precondition] The value at address C is expected not to equal 17
[precondition] The value at address D is expected to be greater than 1000
[precondition] The value at adresss E is greater than 0
[if preconditions satisfied...]
[effect] Store the value 20 at address A
[effect] Atomically increment the value at B by 5
[effect] Store the value 17 at address C
[effect] Atomically decrement the value at D by 1
This interface does have limitations: it doesn't allow for more general logic or branching, and it can't currently model dependencies between target memory locations well. Despite this, it's still significantly more expressive than a single compare-and-swap.
This API was developed pretty recently - better docs will be up shortly. Currently, the unit tests are the best documentation.