Re: Re: typeof and operands in named address spaces

[Richard Biener <richard.guenther@gmail.com>]

On Sun, Nov 15, 2020 at 11:53 AM Uecker, Martin
<Martin.Uecker@med.uni-goettingen.de> wrote:
>
>
> > On Wed, Nov 04, 2020 at 07:31:42PM +0100, Uros Bizjak wrote:
> > > Hello!
> > >
> > > I was looking at the recent linux patch series [1] where segment
> > > qualifiers (named address spaces) were introduced to handle percpu
> > > variables. In the patch [2], the author mentions that:
> > >
> > > --q--
> > > Unfortunately, gcc does not provide a way to remove segment
> > > qualifiers, which is needed to use typeof() to create local instances
> > > of the per-cpu variable. For this reason, do not use the segment
> > > qualifier for per-cpu variables, and do casting using the segment
> > > qualifier instead.
> > > --/q--
> >
> > C in general does not provide means to strip qualifiers. We recently had
> > a _lot_ of 'fun' trying to strip volatile from a type, see here:
> >
> >   https://lore.kernel.org/lkml/875zimp0ay.fsf@mpe.ellerman.id.au
> >
> > which resulted in the current __unqual_scalar_typeof() hack.
> >
> > If we're going to do compiler extentions here, can we pretty please have
> > a sane means of modifying qualifiers in general?
>
> Another way to drop qualifiers is using a cast. So you
> can use typeof twice:
>
> typeof((typeof(_var))_var) tmp__;
>
> This also works for non-scalars but this is a GCC extension.
>
>
> WG14 plans to standardize typeof. I would like to hear opinion
> whether we should have typeof drop qualifiers or not.
>
> Currently, it does not do this on all compilers I tested
> (except _Atomic on GCC) and there are also use cases for
> keeping qualifiers. This is an argument for keeping qualifiers
> should we standardize it, but then we need a way to drop
> qualifiers.
>
>
> lvalue conversion drops qualifers in C.  In GCC, this is not
> implemented correctly as it is unobvervable in standard C
> (but it using typeof).
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97702
>
> A have a working patch in preparation to change this. Then you
> could use
>
> typeof( ((void)0, x) )
>
> to drop qualifiers. But this would then
> also do array-to-pointer conversion. I am not sure
> whether this is a problem.
>
>
> For fun, I tried to come up with a standard+typeof-compliant
> macro that drops qualifiers for all types without doing
> array-to-pointer conversion
>
> https://github.com/uecker/unqual/blob/main/unqual.c
>
> but recursing into multi-dim. array types causes
> a macro-explosion.... (but maybe multi-dim arrays are
> also not too important)
>
>
> Of course, we could also introduce a new feature for
> dropping qualifiers. Thoughts?

Just add a new qualifier that un-qualifies?

_Unqual volatile T x;

is T with volatile (evenually) removed.  Or just a way to drop
all using _Unqual?

_Unqual T x;

removing all qualifiers from T.  Or add a special _Unqual_all
to achieve that.  I think removing a specific qualification is
useful.  Leaves cases like

_Unqual volatile volatile T x;

to be specified (that is ordering and cancellation of the
unqual and qual variants of qualifiers).

Richard.

>
> Best,
> Martin
>
>
>
> _
>
>