So far, I now think that the solution of the issue would be to extend gen_lowpart and gen_highpart so that they are able to handle also subreg inputs and use them at all places that emmit RTL (i.e. expand and split).
Question is whether I should try to simultaneously implement support for the case of paradoxial subregs as input operand, once I am working on the code?
You were never very clear about what was wrong with gen_highpart and gen_lowpart with respect to subregs. rtl examples are always helpful, e.g. showing RTL input and RTL output and pointing out what is wrong. gen_lowpart already has support for subreg input and presumably should work.
gen_lowpart is already pretty involved. If you need something other than a trivial fix, it might be better to try to solve the problem in your md file.
I would suggest only trying to fix things that you have testcases for. So if you don't have a paradoxical subreg testcase, then don't try to fix it. You might accidentally make things worse than they already are.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
