Joern RENNECKE wrote: > I 've been looking at the problem of converting the struct-equiv code > to use DEF-USE chains > instead of global dataflow information, but I have hit a snag. > We can find local registers as being registers that are defined > somewhere in the examined (partial) block, > and have all their uses within the (partial) block. However, there is > no feasible way to find dying inputs > with def-use / use-def chains. We could find the definition with > use-def chains, and examine all the uses > of this definition with def-use chains, but we can't easily tell if a > use could be reached through the examined > (partial) block. > > And this is really not an isolated problem. Generally, when we > examine REG_DEAD notes, there is no > equivalent information to be found in def-use chains. The problem is > that we are not really interested > in what uses can be reached from the definition, but which can be > reached from the use we are examining. > I.e. we'd like a list of uses of the same register after the current > use. In fact, we don't need an exhaustive list. > It is sufficient if we have a set of uses such that any use which is > reachable from the current use is dominated > by one of the uses in the set (a minimal set is desirable, but not > necessary). With these sets in place, we could > also restrict the def-use information to such a set. When we add back > pointers to each link, these data structures > should be reasonably easy to keep sufficiently up-to-date to remain > usable, i.e. such that every dependency is > represented, and that there are no dangling pointers. you can have def-use chains, you can have use-def chains or you can have both. It seems like what you are asking for are use-def chains, i.e. the list of defs that can reach each use. If the set is empty, this is equiv to a reg-dead note.
kenny
