Re: typeof and operands in named address spaces

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 09, 2020 at 01:38:51PM -0600, Segher Boessenkool wrote:
> On Mon, Nov 09, 2020 at 01:47:13PM +0100, Peter Zijlstra wrote:
> > 
> > + lots of people and linux-toolchains
> > 
> > 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.
> 
> Most ways you can try to use the result are undefined behaviour, even.
> 
> > 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?
> 
> What do you want to do with it?  It may be more feasible to do a
> compiler extension for *that*.

Like with the parent use-case it's pretty much always declaring
temporaries in macros. We don't want the temporaries to be volatile, or
as the parent post points out, to have a segment qualifier.