[cfe-dev] address_space pragma

[Leonard Chan via cfe-dev]

On Wed, Aug 1, 2018 at 4:27 PM John McCall <rjmccall at apple.com> wrote:
>
> > On Aug 1, 2018, at 6:50 PM, Leonard Chan <leonardchan at google.com> wrote:
> >
> >> Is your expectation that these address spaces will be exclusively used for diagnostics?
> >
> > Yes. This will serve as a way of essentially shortening an attribute
> > list in the diagnostic for address_space since this is what's useful
> > to us for now, but we are open to expanding this to fit other use
> > cases as long as we get this.
> >
> >> Should they support promotions between address spaces?
> >
> > We weren't planning on this. The idea is that each new address space
> > created with this pragma will be a new address space disassociated
> > from any previous one.
>
> And these are supposed to work like actual language address spaces, right?
> Which is to say, they're not interconvertible at all?  I'm a little confused about how
> this fights into a code-analysis framework when it's actually a major semantic
> change.

These are meant to work like regular address spaces, but instead of
accepting an integer to represent a unique space, some unique
serializable ID is given instead. That is in your code, you should be
able to pass strings instead of integers to all instances you use the
attribute address_space, assuming you declare the pragma beforehand,
and you shouldn't have to change anything else.

I could be missing something about address spaces, but my
understanding was that they were for preventing usage of pointers from
one space in another and all address spaces were unique.

> >> Do you have an idea of what the "variable attributes" would look like?
> >
> > Should've clarified on this more. This is the syntax of regular
> > attributes that you would normally declare in an attribute list but
> > space separated. So something like
> >
> > ```
> > #define __iomem __attribute__((address_space(2), noderef, other_attr(arg)))
> > ```
> >
> > would be represented as
> >
> > ```
> > #pragma clang address_space "__iomem" noderef other_attr(arg)
> > #define __iomem __attribute__((address_space("__iomem")))
> > ```
> >
> > Though after experimenting for a bit, I'm running into a few
> > unexpected errors and may change the syntax. I've been looking into
> > arphaman's patch for the pragma clang attribute and may borrow his
> > syntax.
>
> The diagnostic goals could probably be achieved by just making a better effort to
> recognize and preserve that an address_space attribute was written with a macro.

I'll look into this also.