On Wed, 1 Mar 2017, Richard Sandiford wrote:
> Richard Biener <rguent...@suse.de> writes:
> > On Wed, 1 Mar 2017, Richard Sandiford wrote:
> >
> >> Sorry for the late reply, but:
> >>
> >> Richard Biener <rguent...@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.
One should be able to construct a testcase w/o load-lanes by ensuring
a high enough VF.
Richard.
> Thanks,
> Richard
>
>
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
