Re: [patch] Fix PR tree-optimization/49771

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Richard Guenther wrote:
> On Sun, Jul 24, 2011 at 2:02 PM, Ira Rosen <ira.rosen@linaro.org> wrote:
> > On 21 July 2011 15:19, Ira Rosen <ira.rosen@linaro.org> wrote:
> >> I reproduced the failure. It occurs without Richard's
> >> (http://gcc.gnu.org/ml/gcc-patches/2011-07/msg01022.html) and this
> >> patches too. Obviously the vectorized loop is executed, but at the
> >> moment I don't understand why. I'll have a better look on Sunday.
> >
> > Actually it doesn't choose the vectorized code. But the scalar version
> > gets optimized in a harmful way for SPU, AFAIU.
> > Here is the scalar loop after vrp2
> >
> > <bb 8>:
> >  # ivtmp.42_50 = PHI <ivtmp.42_59(3), ivtmp.42_45(10)>
> >  D.4593_42 = (void *) ivtmp.53_32;
> >  D.4520_33 = MEM[base: D.4593_42, offset: 0B];
> >  D.4521_34 = D.4520_33 + 1;
> >  MEM[symbol: a, index: ivtmp.42_50, offset: 0B] = D.4521_34;
> >  ivtmp.42_45 = ivtmp.42_50 + 4;
> >  if (ivtmp.42_45 != 16)
> >    goto <bb 10>;
> >  else
> >    goto <bb 5>;
> >
> > and the load is changed by dom2 to:
> >
> > <bb 4>:
> >  ...
> >  D.4520_33 = MEM[base: vect_pa.9_19, offset: 0B];
> >   ...
> >
> > where vector(4) int * vect_pa.9;
> >
> > And the scalar loop has no rotate for that load:
> 
> Hum.  This smells like we are hiding sth from the tree optimizers?

Well, the back-end assumes a pointer to vector type is always
naturally aligned, and therefore the data it points to can be
accessed via a simple load, with no extra rotate needed.

It seems what happened here is that somehow, a pointer to int
gets replaced by a pointer to vector, even though their alignment
properties are different.

This vector pointer must originate somehow in the vectorizer,
however, since the original C source does not contain any
vector types at all ...

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com