[Bug c++/85057] GCC fails to vectorize code unless dummy loop is added

[mokreutzer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85057

--- Comment #5 from Moritz Kreutzer <mokreutzer at gmail dot com> ---
(In reply to Richard Biener from comment #4)
> The issue lies in dependence analysis which faces
> 
>   _21 = (sizetype) i_24;
>   _22 = _21 * 8;
>   _2 = &a + _22;
>   _13 = MEM[(const Type_t &)&a][i_24].v[0];
>   _14 = _13 * 5.0e-1;
>   MEM[(double &)_2] = _14;
> 
> marks the two refs for a runtime alias test and then when doing that
> figures they always alias (but doesn't handle the distance == 0 case
> specially).

But then I still don't understand how adding the dummy loop helps GCC in
determining the independence of loop iterations.

> 
> This is a dup of another existing bug that dependence analysis doesn't
> cope very well with a mix of pointer vs. array accesses.

Are you talking about 65206? Seems like it's not an easy bug to fix? Anyways, I
hope it helps to have proof of another manifestation of this bug...