Re: [wwwdocs] Vectorizer's updates to 4.3.0 changes

[Jakub Jelinek <jakub at redhat dot com>]

On Tue, Mar 11, 2008 at 11:41:32AM +0100, Andi Kleen wrote:
> > +     <li>The <code>-ftree-vectorize</code> option is now on by default
> > under
> > +     <code>-O3</code>.
> 
> That seems awfully brief. I would recommend you mention at least
> e.g. that it needs sse2 or altivec enabled and roughly what the 
> vectorizer does (in very high level terms for users) and that it may 
> increase code size.

You don't need sse2 nor altivec enabled (though of course vectorization is
better if you do).  Consider say:
char buf[1024] __attribute__((aligned));
void foo (void)
{
  int i;
  for (i = 0; i < 1024; i++)
    buf[i] |= 2;
}
Even without sse this is (e.g. on i386) vectorized, with -O3 you get
following loop:
.L2:
        orl     $33686018, (%eax)
        addl    $4, %eax
        cmpl    $buf+1024, %eax
        jne     .L2
with -O2 or -O3 -fno-tree-vectorize
.L2:
        orb     $2, buf(%eax)
        addl    $1, %eax
        cmpl    $1024, %eax
        jne     .L2

	Jakub