[Bug middle-end/88490] Missed autovectorization when indices are different

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88490

--- Comment #6 from rguenther at suse dot de <rguenther at suse dot de> ---
On Fri, 14 Dec 2018, joseph at codesourcery dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88490
> 
> --- Comment #5 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
> On Fri, 14 Dec 2018, rguenther at suse dot de wrote:
> 
> > Note I do not think the C standard is sufficiently clear with regarding 
> > to restrict qualified pointers loaded from memory.
> 
> I think this is where "Every access that modifies X shall be considered 
> also to modify P, for the purposes of this subclause." comes in.  (See 
> what I said at <https://gcc.gnu.org/ml/gcc-bugs/2005-01/msg03394.html>.)  
> Modifying s->d[n][0] is considered to modify s->d[n], and so considered to 
> modify s->d, and so considered to modify s.  (It's still perfectly valid 
> to have n == k; what's not valid is aliasing between objects accessed via 
> s->d[0] and s->d[1], for example.)

OK, thanks for clarifying.  I think we are fine implementation-wise
and I have a hard time thinking of a way to improve here given we
have to compute sth conservatively correct.