What to remove after tree-ssa is merged?

[Richard Kenner]

    I fail to see how this is a problem. The frontend passes complete
    functions to the optimizer. I agree that PLACEHOLDER_EXPR is a very
    useful in the frontend when building trees. However I can't see how it
    is of any use once the tree has been built.

    Once the optimizers see a function, haven't we already determined all
    uses of these expressions? Do you have any examples of the when an
    optimier would unexpectedly decide it needs to use one of these
    expressions?

I suspect we're saying the same thing: you won't find PLACEHOLDER_EXPR
in anything that comes from *statements*, but they will remain in the
trees describing *types*.  The optimizer wouldn't see them, but the
Dwarf output routines would, for example.