> -----Original Message-----
> From: Tamar Christina <tamar.christ...@arm.com>
> Sent: Wednesday, December 6, 2023 8:48 AM
> To: Richard Biener <rguent...@suse.de>
> Cc: gcc-patches@gcc.gnu.org; nd <n...@arm.com>; j...@ventanamicro.com
> Subject: RE: [PATCH]middle-end: Fix peeled vect loop IV values.
>
> > > Hi All,
> > >
> > > While waiting for reviews I found this case where both loop exit needs to
> > > go to
> > > epilogue loop, but there was an IV related variable that was used in the
> > > scalar
> > > iteration as well.
> > >
> > > vect_update_ivs_after_vectorizer then blew the value away and replaced it
> > > with
> > > the value if it took the normal exit.
> > >
> > > For these cases where we've peeled an a vector iteration, we should skip
> > > vect_update_ivs_after_vectorizer since all exits are "alternate" exits.
> > >
> > > For this to be correct we have peeling put the right LCSSA variables so
> > > vectorable_live_operations takes care of it.
> > >
> > > This is triggered by new testcases 79 and 80 in early break testsuite
> > > and I'll merge this commit in the main one.
> > >
> > > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> > >
> > > Ok for master?
> > >
> > > Thanks,
> > > Tamar
> > >
> > > gcc/ChangeLog:
> > >
> > > * tree-vect-loop-manip.cc (slpeel_tree_duplicate_loop_to_edge_cfg):
> > > Put right LCSSA var for peeled vect loops.
> > > (vect_do_peeling): Skip vect_update_ivs_after_vectorizer.
> > >
> > > --- inline copy of patch --
> > > diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
> > > index
> >
> 7d48502e2e46240553509dfa6d75fcab7fea36d3..bfdbeb7faaba29aad51c0561d
> > ace680c96759484 100644
> > > --- a/gcc/tree-vect-loop-manip.cc
> > > +++ b/gcc/tree-vect-loop-manip.cc
> > > @@ -1668,6 +1668,7 @@ slpeel_tree_duplicate_loop_to_edge_cfg (class loop
> > *loop, edge loop_exit,
> > > edge loop_entry = single_succ_edge (new_preheader);
> > > if (flow_loops)
> > > {
> > > + bool peeled_iters = single_pred (loop->latch) != loop_exit->src;
> > > /* Link through the main exit first. */
> > > for (auto gsi_from = gsi_start_phis (loop->header),
> > > gsi_to = gsi_start_phis (new_loop->header);
> > > @@ -1692,11 +1693,19 @@ slpeel_tree_duplicate_loop_to_edge_cfg (class
> > loop *loop, edge loop_exit,
> > > continue;
> > > }
> > > }
> > > + /* If we have multiple exits and the vector loop is peeled then we
> > > + need to use the value at start of loop. */
> >
> > This comment doesn't really match 'peeled_iters'? Iff the main IV exit
> > source isn't loop->latch then won't we miscompute? I realize the
> > complication is that slpeel_tree_duplicate_loop_to_edge_cfg is used from
> > elsewhere as well (so we can't check
> LOOP_VINFO_EARLY_BREAKS_VECT_PEELED).
> >
>
> No, because in both exits we restart the scalar iteration at the start of the
> last
> vector iteration.
> Normally, the counted main exit would be updated by vect_iters_bound_vf - vf.
> Which is the same
> As the induction value should we get to the final iteration.
>
> More on it in your question below.
>
> > > + if (peeled_iters)
> > > + {
> > > + tree tmp_arg = gimple_phi_result (from_phi);
> > > + if (!new_phi_args.get (tmp_arg))
> > > + new_arg = tmp_arg;
> > > + }
> > >
> > > tree new_res = copy_ssa_name (gimple_phi_result (from_phi));
> > > gphi *lcssa_phi = create_phi_node (new_res, new_preheader);
> > >
> > > - /* Main loop exit should use the final iter value. */
> > > + /* Otherwise, main loop exit should use the final iter value. */
> > > SET_PHI_ARG_DEF (lcssa_phi, loop_exit->dest_idx, new_arg);
> > >
> > >
> > adjust_phi_and_debug_stmts
> > (to_phi, loop_entry, new_res);
> > > @@ -3394,9 +3403,13 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree
> > niters, tree nitersm1,
> > > if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > update_e = single_succ_edge (e->dest);
> > >
> > > - /* Update the main exit. */
> > > - vect_update_ivs_after_vectorizer (loop_vinfo,
> > > niters_vector_mult_vf,
> > > - update_e);
> > > + /* If we have a peeled vector iteration, all exits are the same,
> > > leave it
> > > + and so the main exit needs to be treated the same as the alternative
> > > + exits in that we leave their updates to vectorizable_live_operations.
> > > + */
> > > + if (!LOOP_VINFO_EARLY_BREAKS_VECT_PEELED (loop_vinfo))
> > > + vect_update_ivs_after_vectorizer (loop_vinfo, niters_vector_mult_vf,
> > > + update_e);
> >
> > and now we don't update the main exit? What's
> > LOOP_VINFO_EARLY_BREAKS_VECT_PEELED again vs.
> > LOOP_VINFO_EARLY_BREAKS?
>
> So LOOP_VINFO_EARLY_BREAKS essentially says that there are multiple exits
> That we can vectorize.
>
> LOOP_VINFO_EARLY_BREAKS_VECT_PEELED is saying that in this loop, we've
> picked as the main exit not the loops latch connected exit. This means when
> we
> exit from the "main" exit in the final iteration we may still have side
> effects to
> perform and so the final iteration should be restarted.
>
> Similarly exiting from an early exit in this case also means having to
> restart the
> Loop at the same place to apply any partial side-effects. This is because in
> these
> Cases the loop exits at n - 1 iterations, the IV is updated and checked
> before the
> final exit, so taking an early exit means we still have to apply partial
> side-effects.
>
As an example, the testcase is
__attribute__((noinline, noipa))
int f(x) {
int i;
for (i = 0; i < 8 && (x & 1) == 0; i++)
x >>= 1;
return i;
}
main() {
if (f(4) != 2)
abort();
exit(0);
}
Where the value of x also needs to be preserved when we take either exit.
Cheers,
Tamar
> Regards,
> Tamar
> >
> > > if (skip_epilog || LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > {
> > >
> > >
> > >
> > >
> > >
> >
> > --
> > Richard Biener <rguent...@suse.de>
> > SUSE Software Solutions Germany GmbH,
> > Frankenstrasse 146, 90461 Nuernberg, Germany;
> > GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
