>>> I don't know about the rest of it, but it seems to me that this has
>>> order-of-evaluation problems: it evaluates its arguments *after*
>>> performing the write barrier, not *before*.
> Hmm, I'm not sure what you are trying to achieve here.

> All bus_space* functions are explicitly allowed (by the manual) to be
> implemented as macros.  Which in my world means that the user is not
> allowed to rely on arguments with side effects to be evaluated.  Or
> for that matter, to be evaluated only once.

The documentation really needs to explicitly spell out the interface
contract - in particular, any ways in which a macro implementation is
permitted to deviate from function-like semantics.  It's possible all
the arguments-cast-to-void can simply be deleted if there is no promise
that the arguments are evaluated at all.

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