Re: [PATCH 00/22] add support for Clang LTO

Paul E. McKenney
2020-07-03T14:51:52Z

On Fri, Jul 03, 2020 at 03:25:23PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 03, 2020 at 03:13:30PM +0200, Peter Zijlstra wrote:
> > > The prototype for GCC is here: https://github.com/AKG001/gcc/
> >
> > Thanks! Those test cases are somewhat over qualified though:
> >
> > static volatile _Atomic (TYPE) * _Dependent_ptr a; \
>
> One question though; since its a qualifier, and we've recently spend a
> whole lot of effort to strip qualifiers in say READ_ONCE(), how does,
> and how do we want, this qualifier to behave.

Dereferencing a _Dependent_ptr pointer gives you something that is not
_Dependent_ptr, unless the declaration was like this:

_Dependent_ptr _Atomic (TYPE) * _Dependent_ptr a;

And if I recall correctly, the current state is that assigning a
_Dependent_ptr variable to a non-_Dependent_ptr variable strips this
marking (though the thought was to be able to ask for a warning).

So, yes, it would be nice to be able to explicitly strip the
_Dependent_ptr, perhaps the kill_dependency() macro, which is already
in the C standard.

> C++ has very convenient means of manipulating qualifiers, so it's not
> much of a problem there, but for C it is, as we've found, really quite
> cumbersome. Even with _Generic() we can't manipulate individual
> qualifiers afaict.

Fair point, and in C++ this is a templated class, at least in the same
sense that std::atomic<> is a templated class.

But in this case, would kill_dependency do what you want?

Thanx, Paul