Availability of Expressions
The expression α ← a ° b is available at a point p if the value of the variable α is the same as the value of a ° b computed at the point p.
The expression α is available on entry to block b iff α is available on exit from all immediate predecessors of b .
availentry(b) = ∩x ∈ Γ^-1 b availexit(x)
The expression α is available on exit from block b iff α is available at the last point of b .
availexit(b) = (availentry(b) - kill(b)) ∪ comp(b)
In general, a system of simultaneous boolean equations may have multiple consistent solutions. It is necessary to compute the maximal solution of the set of boolean equations for intervals at all levels of the derived graph.