Re: local data flow

[Kenneth Zadeck <zadeck at naturalbridge dot com>]

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