Re: [PATCH] "volatile considered harmful", take 3

["Satyam Sharma" <satyam.sharma@gmail.com>]

On 5/12/07, H. Peter Anvin <hpa@zytor.com> wrote:
> Satyam Sharma wrote:
> >
> >> +  - Pointers to data structures in coherent memory which might be
> >> modified
> >> +    by I/O devices can, sometimes, legitimately be volatile.  A ring
> >> buffer
> >> +    used by a network adapter, where that adapter changes pointers to
> >> +    indicate which descriptors have been processed, is an example of
> >> this
> >> +    type of situation.
> >
> > is a legitimate use case for volatile is still not clear to me (I
> > agree with Alan's
> > comment in a previous thread that this seems to be a case where a memory
> > barrier would be applicable^Wbetter, actually). I could be wrong here, so
> > would be nice if Peter explains why volatile is legitimate here.
> >
> > Otherwise, it's fine with me.
> >
>
> I don't see why Alan's way is necessarily better;

Because volatile is ill-defined? Or actually, *undefined* (well,
implementation-defined is as good as that)? It's *so* _vague_,
one doesn't _feel_ like using it at all!

We already have a complete API containing optimization barriers,
load/store/full memory barriers. With well-defined and
well-understood semantics. Just ... _why_ use volatile?

> it should work but is

It will _always_ work. In fact you can't really say the same for
volatile. We already assume the compiler _actually_ took some
pains to stuff meaning into C's (lack of) definition of volatile and
implement it -- but in what sense, nobody knows (the C standard
doesn't, so what are we).

> more heavy-handed as it's disabling *all* optimization such as loop
> invariants across the barrier.

This is a legitimate criticism, I agree.

Thanks,
Satyam
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