Similar to http://en.wikipedia.org/wiki/Liveness_analysis, with three additions:

•	With SSA, it is not sufficient to have a single live-in set per block. There is also an edge-live-in set per edge, consisting of phi inputs from each predecessor.
•	Liveness analysis annotates call sites with GC maps indicating the spill slots in the stack frame that contain tagged pointers, and thus have to be visited if a GC occurs inside the call.
•	GC maps can contain derived pointers. A derived pointer is a pointer into the middle of a data heap object. Each derived pointer has a base pointer, to keep it up to date when objects are moved by the garbage collector. This extends live intervals and inserts new ##phi instructions.

Querying liveness data:

live-in ( bb -- set )

live-in? ( vreg bb -- ? )

live-ins

live-out ( bb -- set )

live-out? ( vreg bb -- ? )

live-outs

Filling GC maps:

lookup-base-pointer ( vreg -- vreg/f )

visit-gc-root ( vreg derived-roots gc-roots -- )