Availability of Expressions
The expression &alpha &larr a b is available at a point p if the value of the variable &alpha is the same as the value of a b computed at the point p.
The expression &alpha is available on entry to block b iff &alpha is available on exit from all immediate predecessors of b .
availentry(b) = &capx &isin &Gamma^-1 b availexit(x)
The expression &alpha is available on exit from block b iff &alpha is available at the last point of b .
availexit(b) = (availentry(b) - kill(b)) &cup 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.