On Wed, 6 Dec 2023, Tamar Christina wrote:
> > -----Original Message-----
> > From: Richard Biener <[email protected]>
> > Sent: Wednesday, December 6, 2023 8:32 AM
> > To: Tamar Christina <[email protected]>
> > Cc: [email protected]; nd <[email protected]>; [email protected]
> > Subject: Re: [PATCH 12/21]middle-end: Add remaining changes to peeling and
> > vectorizer to support early breaks
> >
> > On Mon, 6 Nov 2023, Tamar Christina wrote:
> >
> > > Hi All,
> > >
> > > This finishes wiring that didn't fit in any of the other patches.
> > > Essentially just adding related changes so peeling for early break works.
> > >
> > > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> > >
> > > Ok for master?
> > >
> > > Thanks,
> > > Tamar
> > >
> > > gcc/ChangeLog:
> > >
> > > * tree-vect-loop-manip.cc (vect_set_loop_condition_normal,
> > > vect_do_peeling): Support early breaks.
> > > * tree-vect-loop.cc (vect_need_peeling_or_partial_vectors_p): Likewise.
> > > * tree-vectorizer.cc (pass_vectorize::execute): Check all exits.
> > >
> > > --- inline copy of patch --
> > > diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
> > > index
> > eef2bb50c1505f5cf802d5d80300affc2cbe69f6..9c1405d79fd8fe8689007df3b7
> > 605b7a3d3ecdd7 100644
> > > --- a/gcc/tree-vect-loop-manip.cc
> > > +++ b/gcc/tree-vect-loop-manip.cc
> > > @@ -1187,7 +1187,7 @@ vect_set_loop_condition_partial_vectors_avx512
> > (class loop *loop,
> > > loop handles exactly VF scalars per iteration. */
> > >
> > > static gcond *
> > > -vect_set_loop_condition_normal (loop_vec_info /* loop_vinfo */, edge
> > exit_edge,
> > > +vect_set_loop_condition_normal (loop_vec_info loop_vinfo, edge exit_edge,
> > > class loop *loop, tree niters, tree step,
> > > tree final_iv, bool niters_maybe_zero,
> > > gimple_stmt_iterator loop_cond_gsi)
> > > @@ -1296,7 +1296,8 @@ vect_set_loop_condition_normal (loop_vec_info /*
> > loop_vinfo */, edge exit_edge,
> > > gsi_insert_before (&loop_cond_gsi, cond_stmt, GSI_SAME_STMT);
> > >
> > > /* Record the number of latch iterations. */
> > > - if (limit == niters)
> > > + if (limit == niters
> > > + || LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > /* Case A: the loop iterates NITERS times. Subtract one to get the
> > > latch count. */
> > > loop->nb_iterations = fold_build2 (MINUS_EXPR, niters_type, niters,
> > > @@ -3242,6 +3243,16 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree
> > niters, tree nitersm1,
> > > bound_epilog += vf - 1;
> > > if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo))
> > > bound_epilog += 1;
> > > +
> > > + /* For early breaks the scalar loop needs to execute at most VF times
> > > + to find the element that caused the break. */
> > > + if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > + {
> > > + bound_epilog = vf;
> > > + /* Force a scalar epilogue as we can't vectorize the index
> > > finding. */
> > > + vect_epilogues = false;
> >
> > This is originally initialized with
> >
> > bool vect_epilogues = loop_vinfo->epilogue_vinfos.length () > 0;
> >
> > so I think we should avoid filling that with LOOP_VINFO_EARLY_BREAKS
> > rather than fixing up after the fact? That is in vect_analyze_loop
> > adjust
> >
> > /* Only vectorize epilogues if PARAM_VECT_EPILOGUES_NOMASK is
> > enabled, SIMDUID is not set, it is the innermost loop and we have
> > either already found the loop's SIMDLEN or there was no SIMDLEN to
> > begin with.
> > TODO: Enable epilogue vectorization for loops with SIMDUID set. */
> > bool vect_epilogues = (!simdlen
> > && loop->inner == NULL
> > && param_vect_epilogues_nomask
> > && LOOP_VINFO_PEELING_FOR_NITER
> > (first_loop_vinfo)
> > && !loop->simduid);
> >
> > and add !LOOP_VINFO_EARLY_BREAKS?
> >
> > > + }
> > > +
> > > bool epilog_peeling = maybe_ne (bound_epilog, 0U);
> > > poly_uint64 bound_scalar = bound_epilog;
> > >
> > > @@ -3376,14 +3387,23 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree
> > niters, tree nitersm1,
> > > bound_prolog + bound_epilog)
> > > : (!LOOP_REQUIRES_VERSIONING (loop_vinfo)
> > > || vect_epilogues));
> > > +
> > > + /* We only support early break vectorization on known bounds at this
> > > time.
> > > + This means that if the vector loop can't be entered then we won't
> > > generate
> > > + it at all. So for now force skip_vector off because the additional
> > > control
> > > + flow messes with the BB exits and we've already analyzed them. */
> > > + skip_vector = skip_vector && !LOOP_VINFO_EARLY_BREAKS (loop_vinfo);
> > > +
> >
> > bool skip_vector = (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> > ? maybe_lt (LOOP_VINFO_INT_NITERS (loop_vinfo),
> > bound_prolog + bound_epilog)
> > : (!LOOP_REQUIRES_VERSIONING (loop_vinfo)
> > || vect_epilogues));
> >
> > to me that looks like
> >
> > gcc_assert (!skip_vector || !LOOP_VINFO_EARLY_BREAKS (loop_vinfo));
> >
> > should work? You are basically relying on cost modeling rejecting
> > vectorization that doesn't enter the vector loop.
> >
> > > /* Epilog loop must be executed if the number of iterations for epilog
> > > loop is known at compile time, otherwise we need to add a check at
> > > the end of vector loop and skip to the end of epilog loop. */
> > > bool skip_epilog = (prolog_peeling < 0
> > > || !LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> > > || !vf.is_constant ());
> > > - /* PEELING_FOR_GAPS is special because epilog loop must be executed.
> > > */
> > > - if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo))
> > > + /* PEELING_FOR_GAPS and peeling for early breaks are special because
> > > epilog
> > > + loop must be executed. */
> > > + if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo)
> > > + || LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > skip_epilog = false;
> > >
> > > class loop *scalar_loop = LOOP_VINFO_SCALAR_LOOP (loop_vinfo);
> > > diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> > > index
> > 55d6aee3d29151e6b528f6fdde15c693e5bdd847..51a054c5b035ac80dfbbf3b5
> > ba2f6da82fda91f6 100644
> > > --- a/gcc/tree-vect-loop.cc
> > > +++ b/gcc/tree-vect-loop.cc
> > > @@ -1236,6 +1236,14 @@ vect_need_peeling_or_partial_vectors_p
> > (loop_vec_info loop_vinfo)
> > > th = LOOP_VINFO_COST_MODEL_THRESHOLD
> > (LOOP_VINFO_ORIG_LOOP_INFO
> > > (loop_vinfo));
> > >
> > > + /* When we have multiple exits and VF is unknown, we must require
> > > partial
> > > + vectors because the loop bounds is not a minimum but a maximum.
> > > That is
> > to
> > > + say we cannot unpredicate the main loop unless we peel or use
> > > partial
> > > + vectors in the epilogue. */
> > > + if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo)
> > > + && !LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant ())
> > > + return true;
> > > +
> >
> > I don't quite understand the !.is_constant (), early breaks always
> > require peeling?
>
> It's mostly to force the use of an unpredicated main loop, since this
> forces LOOP_VINFO_PEELING_FOR_NITER. I can alternatively not
> set it and then mask the individual comparisons in
> vectorizable_early_break. Both work equally, I guess that may be
> better since the assumption is that the break is not taken.
That might be easier to understand, yes. But why not
if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
return true;
? Because even with constant VF we can predicate the main loop
(with AVX512, --param vect-partial-vector-usage=2)
> Any preference?
>
> Thanks,
> Tamar
>
> >
> > > if (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> > > && LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo) >= 0)
> > > {
> > > @@ -3149,7 +3157,8 @@ start_over:
> > >
> > > /* If an epilogue loop is required make sure we can create one. */
> > > if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo)
> > > - || LOOP_VINFO_PEELING_FOR_NITER (loop_vinfo))
> > > + || LOOP_VINFO_PEELING_FOR_NITER (loop_vinfo)
> > > + || LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > > {
> > > if (dump_enabled_p ())
> > > dump_printf_loc (MSG_NOTE, vect_location, "epilog loop
> > > required\n");
> > > diff --git a/gcc/tree-vectorizer.cc b/gcc/tree-vectorizer.cc
> > > index
> > d97e2b54c25ac60378935392aa7b73476efed74b..8b495fc7ca137109fc0097c7
> > 7837233c660e032a 100644
> > > --- a/gcc/tree-vectorizer.cc
> > > +++ b/gcc/tree-vectorizer.cc
> > > @@ -1381,7 +1381,9 @@ pass_vectorize::execute (function *fun)
> > > predicates that need to be shared for optimal predicate usage.
> > > However reassoc will re-order them and prevent CSE from working
> > > as it should. CSE only the loop body, not the entry. */
> > > - bitmap_set_bit (exit_bbs, single_exit (loop)->dest->index);
> > > + auto_vec<edge> exits = get_loop_exit_edges (loop);
> > > + for (edge exit : exits)
> > > + bitmap_set_bit (exit_bbs, exit->dest->index);
> > >
> > > edge entry = EDGE_PRED (loop_preheader_edge (loop)->src, 0);
> > > do_rpo_vn (fun, entry, exit_bbs);
> >
> > Otherwise OK.
> >
> > Richard.
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
