What to remove after tree-ssa is merged?

[Richard Kenner]

    I've read the docu and think know when you need it (although I'm not sure
    about the specifics of how they are evaluated to RTL), but I don't see a
    reason why this can't be in the front-end, and lowered to trees which do
    not contain PLACEHOLDER_EXPR or WITH_RECORD_EXPR.  After all the functions
    in expr.c even manage to lower such trees to RTL, so there must be some
    way to lower them to GIMPLE which are higher level, isn't there?

There are two separate issues:

(1) The usage of PLACEHOLDER_EXPR in statements.
(2) The usage in types.

For (1), as I said, it is probably possible to fold the PLACEHOLDER_EXPR
into the appropriate object at some point precisely because, as you say,
this can be done when it's lowered to RTL.  So this usage indeed isn't
a problem.

But I don't see how to eliminate the usage in (2).  So the node can't
be eliminated.