[Bug tree-optimization/65930] Reduction with sign-change not handled

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65930

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #8)
> Note the tricky part is not so much the loop vectorization (teaching
> reduction path support about the conversion is not difficult with the twist
> that we
> need to handle conversions as reduction code...) but the vector epilogue
> which
> does the lane reduction in signed int exposing undefined overflow issues.
> 
> Of course the vectorizer doesn't seem to care here anyways and vectorizes
> signed integer reductions without caring for that specific issue...
> 
> In general feeding the conversion as reduction operation through the
> vectorizer
> is a bit awkward so pattern-recognizing this as
> 
>   int tem = (int)x[i];
>   sum += tem;
> 
> would be easier given we seem to ignore the undefined overflow issues...

but pattern recog runs after reduction detection (and that's not easy to
change).