volatile registers (was: The world is not ready for 'volatile')

[Liber]

[followups to comp.std.c]

In article <15171 at mimsy.UUCP> chris at mimsy.UUCP (Chris Torek) writes:
>In article <15166 at mimsy.UUCP> I suggested that
>... ["register"] implies not-volatile and not-aliased; and it does so
>in a way that the compiler can reasonably enforce.

>You mean to say that a register could be aliased?  Not in C!

This seems to be very similar to the argument about const.  You mean to
say that a (non-volatile) const-qualified object could be modified?  Not
in C!

>How about a volatile register?

I can ask the same question about volatile const.  All the volatile
register means is that the alias to the variable cannot occur within
the conforming C program.

[Side note:  I was skimming through the dpANS C, and the only place I
could find restricting register to being non-aliased is in footnote
#55, section 3.5.1.  Since footnotes aren't an official part of the
Standard, could someone please email me an official reference to this?
If it is not official, then I am asking why not?]

>It is a possibility, but it seems entirely unnecessary.

I agree that I don't see this as being particularly useful; that
doesn't make it illegal, however.

>>A STORAGE CLASS by no means obviates the need for type qualifiers.

>I did not say that.  I said `register implies not-volatile, not-
>aliased': that `volatile register' and `aliased register' are
>nonsensical combinations.

Register only implies non-aliased when not qualified by volatile, in
much the same way that const implies non-modifiable when not qualified
by volatile.  Register is allowed to be aliased by hardware or by a
different thread of execution, but a conforming C program cannot alias
it.

[Another side note:  is it even possible to have a strictly conforming
C program whose behavior would change by declaring any of its
variables volatile?  I cannot think of any examples without having to
escape to having some library routines around that 'cheat'.]
-- 
NEVIN ":-)" LIBER  AT&T Bell Laboratories  nevin1 at ihlpb.ATT.COM  (312) 979-4751