[Bug tree-optimization/88398] vectorization failure for a small loop to do byte comparison

[guojiufu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88398

--- Comment #42 from Jiu Fu Guo <guojiufu at gcc dot gnu.org> ---
(In reply to Jiu Fu Guo from comment #41)
> (In reply to Wilco from comment #40)
> > (In reply to Jiu Fu Guo from comment #39)
> > > I’m thinking to draft a patch for this optimization.  If any suggestions,
> > > please point out, thanks.
> > 
> > Which optimization to be precise? Besides unrolling I haven't seen a
> > proposal for an optimization which is both safe and generally applicable.
> 
> 1. For unroll, there are still branches in the loop. And then need careful
> merge on those reading and comparison.  Another thing about unroll would be
> that, if we prefer to optimize this early in GIMPLE, we still not GIMPLE
> unroll on it.
> while (len != max)
> {
>     if (p[len] != cur[len])
>       break; ++len;
>     if (p[len] != cur[len])
>       break; ++len;
>     if (p[len] != cur[len])
>       break; ++len;
> ....
> }

Besides we do not have GIMPLE unroll for this kind of code, this may also
likely be transformed to vectorization.