Re: typeof and operands in named address spaces

[Segher Boessenkool <segher@kernel.crashing.org>]

On Tue, Nov 10, 2020 at 09:11:08PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 10, 2020 at 10:42:58AM -0800, Nick Desaulniers wrote:
> > When I think of qualifiers, I think of const and volatile.  I'm not
> > sure why the first post I'm cc'ed on talks about "segment" qualifiers.
> > Maybe it's in reference to a variable attribute that the kernel
> > defines?  Looking at Clang's Qualifier class, I see const, volatile,
> > restrict (ah, right), some Objective-C stuff, and address space
> > (TR18037 is referenced, I haven't looked up what that is) though maybe
> > "segment" pseudo qualifiers the kernel defines expand to address space
> > variable attributes?
> 
> Right, x86 Named Address Space:
> 
>   https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Named-Address-Spaces.html#Named-Address-Spaces
> 
> Also, Google found me this:
> 
>   https://reviews.llvm.org/D64676
> 
> The basic problem seems to be they act exactly like qualifiers in that
> typeof() preserves them, so if you have:

GCC has the four standard type qualifiers (const, volatile, restrict,
and _Atomic), but also the address space things yes.

> > Maybe stripping all qualifiers is fine since you can add them back in
> > if necessary?
> 
> So far that seems sufficient. Although the Devil's advocate in me is
> trying to construct a case where we need to preserve const but strip
> volatile and that's then means we need to detect if the original has
> const or not, because unconditionally adding it will be wrong.

If you want to drop all qualifiers, you only need a way to convert
something to an rvalue (which always has an unqualified type).  So maybe
make syntax for just *that*?  __builtin_unqualified() perhaps?  Which
could be useful in more places than just doing an unqualified_typeof.


Segher