Re: [PATCH] Avoid peeling for gaps if accesses are aligned

[Richard Biener <rguenther at suse dot de>]

On Wed, 1 Mar 2017, Richard Sandiford wrote:

> Richard Biener <rguenther@suse.de> writes:
> > On Wed, 1 Mar 2017, Richard Sandiford wrote:
> >
> >> Richard Biener <rguenther@suse.de> writes:
> >> > On Wed, 1 Mar 2017, Richard Sandiford wrote:
> >> >
> >> >> Sorry for the late reply, but:
> >> >> 
> >> >> Richard Biener <rguenther@suse.de> writes:
> >> >> > On Mon, 7 Nov 2016, Richard Biener wrote:
> >> >> >
> >> >> >> 
> >> >> >> Currently we force peeling for gaps whenever element overrun can occur
> >> >> >> but for aligned accesses we know that the loads won't trap and thus
> >> >> >> we can avoid this.
> >> >> >> 
> >> >> >> Bootstrap and regtest running on x86_64-unknown-linux-gnu (I expect
> >> >> >> some testsuite fallout here so didn't bother to invent a new testcase).
> >> >> >> 
> >> >> >> Just in case somebody thinks the overrun is a bad idea in general
> >> >> >> (even when not trapping).  Like for ASAN or valgrind.
> >> >> >
> >> >> > This is what I applied.
> >> >> >
> >> >> > Bootstrapped and tested on x86_64-unknown-linux-gnu.
> >> >> >
> >> >> > Richard.
> >> >> [...]
> >> >> > diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
> >> >> > index 15aec21..c29e73d 100644
> >> >> > --- a/gcc/tree-vect-stmts.c
> >> >> > +++ b/gcc/tree-vect-stmts.c
> >> >> > @@ -1789,6 +1794,10 @@ get_group_load_store_type (gimple *stmt, tree vectype, bool slp,
> >> >> >        /* If there is a gap at the end of the group then these optimizations
> >> >> >  	 would access excess elements in the last iteration.  */
> >> >> >        bool would_overrun_p = (gap != 0);
> >> >> > +      /* If the access is aligned an overrun is fine.  */
> >> >> > +      if (would_overrun_p
> >> >> > +	  && aligned_access_p (STMT_VINFO_DATA_REF (stmt_info)))
> >> >> > +	would_overrun_p = false;
> >> >> >        if (!STMT_VINFO_STRIDED_P (stmt_info)
> >> >> >  	  && (can_overrun_p || !would_overrun_p)
> >> >> >  	  && compare_step_with_zero (stmt) > 0)
> >> >> 
> >> >> ...is this right for all cases?  I think it only looks for single-vector
> >> >> alignment, but the gap can in principle be vector-sized or larger,
> >> >> at least for load-lanes.
> >> >>
> >> >> E.g. say we have a 128-bit vector of doubles in a group of size 4
> >> >> and a gap of 2 or 3.  Even if the access itself is aligned, the group
> >> >> spans two vectors and we have no guarantee that the second one
> >> >> is mapped.
> >> >
> >> > The check assumes that if aligned_access_p () returns true then the
> >> > whole access is aligned in a way that it can't cross page boundaries.
> >> > That's of course not the case if alignment is 16 bytes but the access
> >> > will be a multiple of that.
> >> >  
> >> >> I haven't been able to come up with a testcase though.  We seem to be
> >> >> overly conservative when computing alignments.
> >> >
> >> > Not sure if we can run into this with load-lanes given that bumps the
> >> > vectorization factor.  Also does load-lane work with gaps?
> >> >
> >> > I think that gap can never be larger than nunits-1 so it is by definition
> >> > in the last "vector" independent of the VF.
> >> >
> >> > Classical gap case is
> >> >
> >> > for (i=0; i<n; ++i)
> >> >  {
> >> >    y[3*i + 0] = x[4*i + 0];
> >> >    y[3*i + 1] = x[4*i + 1];
> >> >    y[3*i + 2] = x[4*i + 2];
> >> >  }
> >> >
> >> > where x has a gap of 1.  You'll get VF of 12 for the above.  Make
> >> > the y's different streams and you should get the perfect case for
> >> > load-lane:
> >> >
> >> > for (i=0; i<n; ++i)
> >> >  {
> >> >    y[i] = x[4*i + 0];
> >> >    z[i] = x[4*i + 1];
> >> >    w[i] = x[4*i + 2];
> >> >  } 
> >> >
> >> > previously we'd peel at least 4 iterations into the epilogue for
> >> > the fear of accessing x[4*i + 3].  When x is V4SI aligned that's
> >> > ok.
> >> 
> >> The case I was thinking of was like the second, but with the
> >> element type being DI or DF and with the + 2 statement removed.
> >> E.g.:
> >> 
> >> double __attribute__((noinline))
> >> foo (double *a)
> >> {
> >>   double res = 0.0;
> >>   for (int n = 0; n < 256; n += 4)
> >>     res += a[n] + a[n + 1];
> >>   return res;
> >> }
> >> 
> >> (with -ffast-math).  We do use LD4 for this, and having "a" aligned
> >> to V2DF isn't enough to guarantee that we can access a[n + 2]
> >> and a[n + 3].
> >
> > Yes, indeed.  It's safe when peeling for gaps would remove
> > N < alignof (ref) / sizeof (ref) scalar iterations.
> >
> > Peeling for gaps simply subtracts one from the niter of the vectorized 
> > loop.
> 
> I think subtracting one is enough in all cases.  It's only the final
> iteration of the scalar loop that can't access a[n + 2] and a[n + 3].
> 
> (Of course, subtracting one happens before peeling for niters, so it
> only makes a difference if the original niters was a multiple of the VF,
> in which case we peel a full vector's worth of iterations instead of
> peeling none.)

I think one could extend the gcc.dg/vect/group-no-gaps-1.c testcase
to covert the case with bigger VF, for example by having different
types for a and b.

I can try playing with this later this week but if you can come up
with a testcase that exercises load-/store-lanes that would be great.
See also gcc.dg/vect/pr49038.c for a less convoluted testcase to copy
from.

Richard.