On Wed, 6 Dec 2023, Tamar Christina wrote:
> > -----Original Message-----
> > From: Richard Biener <rguent...@suse.de>
> > Sent: Wednesday, December 6, 2023 8:18 AM
> > To: Tamar Christina <tamar.christ...@arm.com>
> > Cc: gcc-patches@gcc.gnu.org; nd <n...@arm.com>; j...@ventanamicro.com
> > Subject: Re: [PATCH 13/21]middle-end: Update loop form analysis to support
> > early
> > break
> >
> > On Mon, 6 Nov 2023, Tamar Christina wrote:
> >
> > > Hi All,
> > >
> > > This sets LOOP_VINFO_EARLY_BREAKS and does some misc changes so the other
> > > patches are self contained.
> > >
> > > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> > >
> > > Ok for master?
> > >
> > > Thanks,
> > > Tamar
> > >
> > > gcc/ChangeLog:
> > >
> > > * tree-vect-loop.cc (vect_analyze_loop_form): Analyse all exits.
> > > (vect_create_loop_vinfo): Set LOOP_VINFO_EARLY_BREAKS.
> > > (vect_transform_loop): Use it.
> > >
> > > --- inline copy of patch --
> > > diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> > > index
> > 51a054c5b035ac80dfbbf3b5ba2f6da82fda91f6..f9483eff6e9606e835906fb991
> > f07cd6052491d0 100644
> > > --- a/gcc/tree-vect-loop.cc
> > > +++ b/gcc/tree-vect-loop.cc
> > > @@ -1700,12 +1700,12 @@ vect_compute_single_scalar_iteration_cost
> > (loop_vec_info loop_vinfo)
> > > loop_vinfo->scalar_costs->finish_cost (nullptr);
> > > }
> > >
> > > -
> > > /* Function vect_analyze_loop_form.
> > >
> > > Verify that certain CFG restrictions hold, including:
> > > - the loop has a pre-header
> > > - - the loop has a single entry and exit
> > > + - the loop has a single entry
> > > + - nested loops can have only a single exit.
> > > - the loop exit condition is simple enough
> > > - the number of iterations can be analyzed, i.e, a countable loop.
> > > The
> > > niter could be analyzed under some assumptions. */
> > > @@ -1841,10 +1841,14 @@ vect_analyze_loop_form (class loop *loop,
> > vect_loop_form_info *info)
> > > "not vectorized: latch block not empty.\n");
> > >
> > > /* Make sure the exit is not abnormal. */
> > > - if (exit_e->flags & EDGE_ABNORMAL)
> > > - return opt_result::failure_at (vect_location,
> > > - "not vectorized:"
> > > - " abnormal loop exit edge.\n");
> > > + auto_vec<edge> exits = get_loop_exit_edges (loop);
> > > + for (edge e : exits)
> >
> > Seeing this multiple times, this isn't the most efficient way to
> > iterate over all exits with LOOPS_HAVE_RECORDED_EXITS.
> >
> > Note to myself: fix (add to) the API.
> >
> > > + {
> > > + if (e->flags & EDGE_ABNORMAL)
> > > + return opt_result::failure_at (vect_location,
> > > + "not vectorized:"
> > > + " abnormal loop exit edge.\n");
> > > + }
> > >
> > > info->conds
> > > = vect_get_loop_niters (loop, exit_e, &info->assumptions,
> > > @@ -1920,6 +1924,10 @@ vect_create_loop_vinfo (class loop *loop,
> > vec_info_shared *shared,
> > >
> > > LOOP_VINFO_IV_EXIT (loop_vinfo) = info->loop_exit;
> > >
> > > + /* Check to see if we're vectorizing multiple exits. */
> > > + LOOP_VINFO_EARLY_BREAKS (loop_vinfo)
> > > + = !LOOP_VINFO_LOOP_CONDS (loop_vinfo).is_empty ();
> > > +
> >
> > Seeing this, s/LOOP_VINFO_LOOP_CONDS/LOOP_VINFO_LOOP_EXIT_CONDS/g
> > might be good, if we in future avoid if-conversion in a separate
> > pass we will have other CONDs as well.
> >
> > > if (info->inner_loop_cond)
> > > {
> > > stmt_vec_info inner_loop_cond_info
> > > @@ -11577,7 +11585,7 @@ vect_transform_loop (loop_vec_info loop_vinfo,
> > gimple *loop_vectorized_call)
> > > /* Make sure there exists a single-predecessor exit bb. Do this before
> > > versioning. */
> > > edge e = LOOP_VINFO_IV_EXIT (loop_vinfo);
> > > - if (! single_pred_p (e->dest))
> > > + if (! single_pred_p (e->dest) && !LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > {
> > > split_loop_exit_edge (e, true);
> >
> > Note this splitting is done to fulfil versioning constraints on CFG
> > update. Do you have test coverage with alias versioning and early
> > breaks?
>
> No, only non-alias versioning. I don't believe we can alias in the current
> implementation because it's restricted to statically known objects with
> a fixed size.
Hm, if side-effects are all correctly in place do we still have that
restriction?
int x;
void foo (int *a, int *b)
{
int local_x = x;
for (int i = 0; i < 1024; ++i)
{
if (i % local_x == 13)
break;
a[i] = 2 * b[i];
}
}
the early exit isn't SCEV analyzable but doesn't depend on any
memory and all side-effects are after the exit already. But
vectorizing would require alias versioning.
Richard.
