n3294 - The restrict function attribute as a replacement of the restrict qualifier

[Alejandro Colomar]

Hi Joseph,

On Fri, Jul 26, 2024 at 08:30:33PM GMT, Joseph Myers wrote:
> On Fri, 26 Jul 2024, Alejandro Colomar via Gcc wrote:
> 
> > I don't see why it should not apply to void*.  memcpy(3) should get this
> > attribute:
> > 
> > 	[[alx::restrict(1)]]
> > 	[[alx::restrict(2)]]
> > 	void *memcpy(void *dst, const void *src, size_t n);
> 
> That would disallow copying between disjoint subarrays within the same 
> toplevel object (and there's no way to specify an array size for void *), 
> which hardly seems right.

Hmmm, I sometimes forget that ISO C is so painful about void.

Has WG14 discussed in the past about the GNU extension that defines
sizeof(void) == 1?

Maybe wording that also considers compiler-specific attributes and
extensions would allow for the following:

	[[gnu::access(write_only, 1, 3)]]
	[[gnu::access(read_only, 2, 3)]]
	[[alx::restrict(1)]]
	[[alx::restrict(2)]]
	void *memcpy(void *dst, const void *src, size_t n);

The GNU attribute specifies the number of elements of the subarrays, and
the GNU extension sizeof(void)==1 specifies the size of each element.
That gives us the size of the subarrays to be considered for the
restrictness.

So, ISO C wouldn't be allowed to mark malloc(3) as [[alx::restrict]]
(unless they add these GNU extensions), but GNU C could.

> > BTW, the author of n2529 didn't follow up, right?  I'd like that in, so
> > I'll prepare something after n2906 is merged.  Martin, would you mind
> > pinging me about it?
> 
> See reflector message SC22WG14.18575, 17 Nov 2020 (the former convenor 
> replying when I asked about just that paper).

Where can I find reflector messages?

>  As far as I know the author 
> has not yet provided an updated version / asked for it to be added to a 
> meeting agenda.

I think you mentioned that to me some time ago.  I guess I'll take over
then.  I'll ask for a number to propose _Nitems().

And another one to propose that [n] means the same as [static n] except
for the nonnull property of static.

Have a lovely night!
Alex

> 
> -- 
> Joseph S. Myers
> josmyers@redhat.com
> 
> 

-- 
<https://www.alejandro-colomar.es/>
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