On Tue, Jul 09, 2024 at 12:28:18PM GMT, Alejandro Colomar wrote:

Hi Jakub,

On Tue, Jul 09, 2024 at 11:18:11AM GMT, Jakub Jelinek wrote:

quoted

On Tue, Jul 09, 2024 at 11:07:59AM +0200, Alejandro Colomar wrote:

quoted

Yup, I was thinking that maybe noalias is a better name.

Name is one thing, but you'd also need to clearly define what it means.
When restrict is access based, it is clear what it means.

If you want something else which is not based on accesses and which should
allow warnings in the callers, I suppose you need to specify not just the
pointer but the extent as well (and maybe stride) or that it is an '\0'

Agree.  Here's how I'd define it as an attribute:

noalias

	The noalias function attribute specifies that the pointer to
	which it applies is the only reference to the array object that
	it points to (except that a pointer to one past the last
	element may overlap another object).

	If the number of elements is specified with array notation, the
	array object to be considered is a subobject of the original
	array object, which is limited to the number of elements
	specified in the function prototype.

	Example:

	[[alx::noalias(1)]] [[alx::noalias(2)]]
	[[gnu::access(read_write, 1)]] [[gnu::access(read_only, 2)]]
	void add_inplace(int a[n], const int b[n], size_t n);

Ooops, I meant 'n' to be the first parameter.

	char arr[100] = ...;

	add_inplace(arr, arr + 50, 50);

	In the example above, the parameters a and b don't alias inside
	the function, since the subobjects of 50 elements do not overlap
	eachother, even though they are one single array object to the
	outer function.

It may need some adjustment, to avoid conflicts with other parts of
ISO C, but this is the idea I have in mind.

quoted

terminated string, because if you want to say that for
void foo (char *, const char *, int);
the 2 pointers don't really alias, the size information is missing.  So,
shall the new warning warn on
struct S { char a[1024]; char b[1024]; } s;
foo (s.a, s.b, 512);

This does not need clarification of bounds.  You're passing separate
objects, and thus cannot alias (except that maybe you're able to cast
to the struct type, and then access s.b from a pointer derived from
s.a; I never know that rule too well).

quoted

or not?  Or foo (s.a, s.a + 512, 512);

According to the definition I provide in this email, the above is just
fine.

Thanks!

Have a lovely day!
Alex

quoted

	Jakub

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