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