What to remove after tree-ssa is merged?

[Richard Kenner]

    Of course.  But somewhen you _do_ apply it to an object.  Otherwise it
    wouldn't have any observable effect and you couldn't lower it to RTL.  
    And then it can be equally well lowered to trees.

Yes.  As I said, this can be done in trees that are part of *statement*,
but not in trees that are part of *types* and the mechanisms that compute
offsets and sizes (and hence likely fold) will need to know what to do 
about it, albeit transiently.

    Sure, all expressions involving any P_E recursively would have to be
    expanded in the frontend, including such expressions (probably by
    using temporaries in order not to have to lower the whole tree).  If
    that's feasible I don't know.

You won't need temporaries within statements and it can't be done at all
for the expressions within types.