Path: utzoo!utgpu!water!watmath!clyde!rutgers!cmcl2!brl-adm!husc6!mailrus!umix!uunet!mcrware!jejones
From: jejones@mcrware.UUCP (James Jones)
Newsgroups: comp.lang.c
Subject: Re: register unions
Summary: unions in registers
Keywords: registers, unions
Message-ID: <604@mcrware.UUCP>
Date: 20 Feb 88 14:59:10 GMT
References: <1229@eneevax.UUCP> <7258@brl-smoke.ARPA>
Distribution: comp.lang.c
Organization: Microware Systems Corp., Des Moines, Ia.
Lines: 32

In article <7258@brl-smoke.ARPA>, gwyn@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