Re: typeof and operands in named address spaces

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 09, 2020 at 11:50:15AM -0800, Nick Desaulniers wrote:
> On Mon, Nov 9, 2020 at 11:46 AM Segher Boessenkool
> <segher@kernel.crashing.org> 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.
> 
> Yes, removing `const` from a `const` declared variable (via cast) then
> expecting to use the result is a great way to have clang omit the use
> from the final program.  This has bitten us in the past getting MIPS
> support up and running, and one of the MTK gfx drivers.

Stripping const to delcare another variable is useful though. Sure C has
sharp edges, esp. if you cast stuff, but since when did that stop anyone
;-)

The point is, C++ has these very nice template helpers that can strip
qualifiers, I want that too, for much of the same reasons. We might not
have templates :-(, but we've become very creative with our
pre-processor.

Surely our __unqual_scalar_typeof() cries for a better solution.