Re: [patch] for PR 18040

[kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)]

    Except that this is exactly what you are doing by making it so
    conversions can appear more or less anywhere in an expression, instead
    of just a single place.

Not "more or less anywhere", it's precisely just inside a chain of references.
Since the expander treats this as a single reference, I don't see why the
optimizers care about what's inside that except in special situations.

It *really* pessimizes the code to not allow them, plus, as I've said, it
means we can't convert all valid GENERIC to GIMPLE. 

    1. Don't allow casts except at the top level. Deal with consequences.

I'd argue exactly the other way around: having a cast at the top level
of a nest of references isn't necessary, so we can require that it be
in it's own statement. But inside is where it's useful.

    2. Make them mandatory inside component_refs, so that you *always* have
    them there.  This is what LLVM does with getelementptr. Each piece has a
    type specifier

I don't follow.  Since every tree node has a type, we always have it.
It seems that all you need is an access function that strips off the
conversion and you have the same form.