register unions

[James Jones]

In article <7258 at brl-smoke.ARPA>, gwyn at brl-smoke.ARPA (Doug Gwyn ) writes:
> On the other hand, I don't think this is nearly as useful as
> you seem to think it would be.  Why would you want to twiddle
> specific registers when not having to be concerned with such
> matters is the whole point of using a higher-level language?

The place I can see people wanting unions to live in registers is something
like

register union {
	woof	*wp;
	char	*cp;
	...
}	mumble;

(Actually, this isn't great insight on my part; I've heard people say,
"Gee, it would be nice if a compiler let you do this...")  There are some
low-level cases in which one would like to add a byte offset to a pointer
that usually points at something bigger than one byte, and still keep the
pointer in a register. 

(Of course, compilers should all be smart enough to figure out what should go
in registers themselves, right?  We don't need no steenkin' register declara-
tions! :-)

(I *do* recognize the advantages of letting the compiler figure out what can
go into a register, what the lifetimes of such variables are, etc.--it's
just that if the compiler *can* really put such a union in a register itself,
it seems to me that the programmer should be able to specify explicitly that
he thinks it's important that the union be in a register.)

		James Jones