Re: A few proposals from the C standards committee

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Jan 23, 2024 at 12:43:02PM -0800, Linus Torvalds wrote:
> On Tue, 23 Jan 2024 at 12:36, Jakub Jelinek <jakub@redhat.com> wrote:
> >
> > C++23 has [[assume (condition)]]; for this (see https://wg21.link/p1774r8)
> > and GCC supports it also as [[gnu::assume (condition)]] and
> > __attribute__((assume (condition)));, both in C (the former only in C23)
> > and C++.  Side-effects in condition aren't evaluated, so it has
> > different behavior from if (!(condition)) __builtin_unreachable ();
> 
> That's lovely, and exactly the kind of thing I'd think is the rigth model.
> 
> If you can also do it in a function declaration, so that it informs
> the caller, it's basically perfect.
> 
> IOW, something like
> 
>    size_t strlen(const char *s [[assume(s)]]);
> 
> would be the equivalent of "const char *_Nonnull s" in that callers
> could warn if not true.
> 
> Except it also would work for other things, not just NULL pointers.

None of the current compilers support this, but it should not be
hard to mechanically transform this to the form using static inlines,
presumably with a made-up name for one level or the other </handwaving>.
(Especially easy for all concerned if someone other than me does it,
of course...)

However, the possibility of pointers to these functions means that I must
ask if this assume() is part of the type.  There are a lot of reasons
to *not* want it to be part of the type, but that would mean that calls
through pointers would ignore that assume().

Thoughts?

							Thanx, Paul