linux-kernel - Re: [RFC] Use plan9 C extensions

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Date:   Thu, 10 Jan 2019 11:34:37 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Joel Stanley <joel@....id.au>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [RFC] Use plan9 C extensions

On 10/01/2019 00.55, Nick Desaulniers wrote:

> I agree that something like builtin_types_compatible_p() in a macro
> could help make these functions more "generic" in the sense of being
> able to accept either a `struct xarray*` or `struct xarray_cyclic*`.

An alternative to implementing all the generic functions as macros is
transparent_union:

https://godbolt.org/z/TK_SJD

If there is an interface that takes a "const xarray *" one would define
another union type

union __transparent_union const_xarray_any {
  const struct xarray *xa;
  const struct xarray_cyclic *xac;
};

The obvious implementation using _Generic fails since all expressions
must be valid:

https://godbolt.org/z/X0bvwO

and _Generic is gcc >= 4.9 anyway. I think an implementation based on
choose_expr/types_compatible might have the same problem, but maybe
there's some clever way around that.

Rasmus