[Bug tree-optimization/23855] loop header should also be pulled out of the inner loop too

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #17 from rguenther at suse dot de  2006-04-10 08:10 -------
Subject: Re:  loop header should also be pulled
 out of the inner loop too

On Mon, 9 Apr 2006, rakdver at gcc dot gnu dot org wrote:

> (In reply to comment #14)
> > (In reply to comment #11)
> > > I updated the patch for current mainline, but it still has issues for some
> > > common uses of loops:
> > > 
> > > void foo(int *ie, int *je, double *x)
> > > {
> > >   int i, j;
> > >   for (j=0; j<*je; ++j)
> > >     for (i=0; i<*ie; ++i)
> > >       x[i+j] = 0.0;
> > > }
> > > 
> > > After loop header copying we have
> > > 
> > >   if (*je > 0)
> > >     for (j=0; j<*je; ++j)
> > >       if (*ie > 0)
> > >         for (i=0; i<*ie; ++i)
> > >           x[i+j ] = 0.0;
> > > 
> > > note how in this form we see the condition *ie > 0 not invariant wrt the
> > > outer loop (because it does a memory load), but if we would run load-PRE
> > > between loop header copying and guard hoisting we would get
> > 
> > actually, thinking about it again, it should suffice to teach
> > invariant_without_guard_p about invariant memory loads, and this should just
> > work.
> 
> It basically does, the only other problem is that we are not able to determine
> that the outer loop is not infinite.

It that because we don't recognize the condition of the loop header?  
Otherwise we know that *je > 0 and the loop runs from 0 to *je (in fact,
I have loop determine the number of iterations of the loop to be *je).  Of
course this is with load-PRE moving the load before the loop-header and 
thus SCEV being presented with

   D.xxx = *je;
   if (D.xxx > 0)
     for (j=0; j<D.xxx; ++j)
      ...

which it just handles fine.  It looks like a chicken-and-egg problem ;)

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23855