Re: Auto-vectorization with gcj

[Bryce McKinlay <mckinlay at redhat dot com>]

Andrew Haley wrote:

> Dietmar Lippold writes:
> > I tried to test auto-vectorization with the following java program:
> > 
> > public class Test {
> >   private static int[] a = new int[256];
> >   private static int[] b = new int[256];
> >   private static int[] c = new int[256];
> > 
> >   public static final void main(String[] args) {
> >     for (int i = 0; i < 256; i++) {
> >       b[i] = i;
> >       c[i] = i;
> >     }
> >     for (int i = 0; i < 256; i++) {
> >       a[i] = b[i] + c[i];
> >     }
> >   }
> > }
> > 
> > When I compile it with
> > 
> > gcj -msse2 -O2 -ftree-vectorize -ftree-vectorizer-verbose=5 -c Test.java
> > 
> > I get the following message:
> > 
> > Test.java:9: note: not vectorized: multiple exits.
> > Test.java:14: note: not vectorized: multiple exits.
> > Test.java:7: note: vectorized 0 loops in function.
> > 
> > Is it not possible to use auto-vectorization with gcj until now?
>
> Not as far as I am aware, no.  You may be the first person ever to
> try it.
>
> I don't know why the compiler believes the loops have multiple exits.

The possibility of throwing an ArrayBoundsException would be considered 
an exit. In order to vectorize this loop we first need value range 
propagation, so that the array bounds checks can be eliminated.

However, compiling with --no-bounds-check would work around the issue 
and should permit the loop to be vectorized.

Bryce