Evaluation Order

Any topological sort of dependency graph gives a valid order in which semantic rules must be evaluated

a4 = real

a5 = a4

addtype(id3.entry, a5)

a7 = a5

addtype(id2.entry, a7 )

a9 := a7 addtype(id1.entry, a9 )

A topological sort of a directed acyclic graph is any ordering m1, m2, m3 ..... mk of the nodes of the graph such that edges go from nodes earlier in the ordering to later nodes. Thus if mi -> mj is an edge from mi to mj then mi appears before mj in the ordering. The order of the statements shown in the slide is obtained from the topological sort of the dependency graph in the previous slide. 'an' stands for the attribute associated with the node numbered n in the dependency graph. The numbering is as shown in the previous slide.