[Bug tree-optimization/43423] gcc should vectorize this loop through if-conversion

["changpeng dot fang at amd dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #7 from changpeng dot fang at amd dot com  2010-05-07 21:41 -------
(In reply to comment #4)
> (In reply to comment #3)
> > Subject: Re:  gcc should vectorize this loop 
> >         through "iteration range splitting"
> > You mean that the problem is the if-conversion of the stores
> > "a[i] = ..."
> 
> If we rewrite the code like:
> int a[100], b[100], c[100];
> 
> void foo(int n, int mid)
> {
>   int i;
>   for(i=0; i<n; i++)
>     {
>       int t;
>       int ai = a[i], bi = b[i], ci = c[i];
>       if (i < mid)
>         t = ai + bi;
>       else
>         t = ai + ci;
>       a[i] = t;
>     }
> }
> 
> --- CUT ---
> This gets vectorized as we produce an if-cvt first.
> 

There are both correctness and performance issues in the re-written code.
b[i] or c[i] may not be executed in the original loop.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43423