Re: Re: typeof and operands in named address spaces

["Uecker, Martin" <Martin.Uecker@med.uni-goettingen.de>]

> 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?

Best,
Martin



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