> We first expand zero_extend:DI address to and:DI and then try > to restore zero_extend:DI. Why do we do this transformation > to begin with?
Suppose there were an outer AND that duplicated what this one did. Then when you combine those two, you merge it to one AND. Then make_compound_operation puts it back. The net result is to eliminate the outer AND. There are lots of similar sorts of things. As I said, the strategy there was to convert extractions and expansions into the corresponding logical and shift operations, see if they can merge with something outside (which is similarly converted), then convert the result (possibly merged) back. This, for example, is the code that will remove nested SIGN_EXTENDs.
