> -----Original Message-----
> From: Richard Sandiford <richard.sandif...@arm.com>
> Sent: Tuesday, April 15, 2025 10:52 AM
> To: Tamar Christina <tamar.christ...@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <n...@arm.com>; rguent...@suse.de
> Subject: Re: [PATCH]middle-end: Fix incorrect codegen with PFA and VLS
> [PR119351]
>
> Tamar Christina <tamar.christ...@arm.com> writes:
> > diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
> > index
> 56a4e9a8b63f3cae0bf596bf5d22893887dc80e8..0722679d6e66e5dd5af4ec1c
> e591f7c38b76d07f 100644
> > --- a/gcc/tree-vect-loop-manip.cc
> > +++ b/gcc/tree-vect-loop-manip.cc
> > @@ -2195,6 +2195,22 @@ vect_can_peel_nonlinear_iv_p (loop_vec_info
> loop_vinfo,
> > return false;
> > }
> >
> > + /* With early break vectorization we don't know whether the accesses
> > will stay
> > + inside the loop or not. TODO: The early break adjustment code can be
> > + implemented the same way for vectorizable_linear_induction. However
> > we
> > + can't test this today so reject it. */
> > + if (niters_skip != NULL_TREE
> > + && vect_use_loop_mask_for_alignment_p (loop_vinfo)
> > + && LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo)
> > + && LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > + {
> > + if (dump_enabled_p ())
> > + dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> > + "Peeling for alignement using masking is not supported"
> > + " for nonlinear induction when using early breaks.\n");
> > + return false;
> > + }
> > +
> > return true;
> > }
>
> FTR, I was wondering here whether we should predict this in advance and
> instead drop down to peeling for alignment without masks. It probably
> isn't worth the effort though.
We could move the check into vect_use_loop_mask_for_alignment_p where
rejecting it there would get it to fall back to scalar peeling. That seems
simple enough
if that's preferrable.
Cheers,
Tamar
>
> > diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> > index
> 9413dcef702597ab27165e676546b190e2bd36ba..6dcdee19bb250993d8cc6b0
> 057d2fa46245d04d9 100644
> > --- a/gcc/tree-vect-loop.cc
> > +++ b/gcc/tree-vect-loop.cc
> > @@ -10678,6 +10678,104 @@ vectorizable_induction (loop_vec_info
> loop_vinfo,
> > LOOP_VINFO_MASK_SKIP_NITERS
> (loop_vinfo));
> > peel_mul = gimple_build_vector_from_val (&init_stmts,
> > step_vectype, peel_mul);
> > +
> > + /* If early break then we have to create a new PHI which we can use as
> > + an offset to adjust the induction reduction in early exits. */
> > + if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
> > + {
> > + auto skip_niters = LOOP_VINFO_MASK_SKIP_NITERS (loop_vinfo);
> > + tree ty_skip_niters = TREE_TYPE (skip_niters);
> > + tree break_lhs_phi = NULL_TREE;
> > + break_lhs_phi = vect_get_new_vect_var (ty_skip_niters,
> > + vect_scalar_var,
> > + "pfa_iv_offset");
> > + gphi *nphi = create_phi_node (break_lhs_phi, bb);
> > + add_phi_arg (nphi, skip_niters, pe, UNKNOWN_LOCATION);
> > + add_phi_arg (nphi, build_zero_cst (ty_skip_niters),
> > + loop_latch_edge (iv_loop), UNKNOWN_LOCATION);
> > +
> > + /* Rewrite all the early exit usages. */
> > + tree phi_lhs = PHI_RESULT (phi);
> > + imm_use_iterator iter;
> > + use_operand_p use_p;
> > + gimple *use_stmt;
> > +
> > + FOR_EACH_IMM_USE_FAST (use_p, iter, phi_lhs)
> > + {
> > + use_stmt = USE_STMT (use_p);
> > + if (!flow_bb_inside_loop_p (iv_loop, gimple_bb (use_stmt))
> > + && is_a <gphi *> (use_stmt))
> > + {
> > + auto gsi = gsi_last_bb (use_stmt->bb);
> > + for (auto exit : get_loop_exit_edges (iv_loop))
> > + if (exit != LOOP_VINFO_IV_EXIT (loop_vinfo)
> > + && bb == exit->src)
> > + {
> > + /* Now create the PHI for the outside loop usage to
> > + retrieve the value for the offset counter. */
> > + tree rphi_lhs = make_ssa_name (ty_skip_niters);
> > + gphi *rphi
> > + = create_phi_node (rphi_lhs, use_stmt->bb);
> > + for (unsigned i = 0; i < gimple_phi_num_args (rphi);
> > + i++)
> > + SET_PHI_ARG_DEF (rphi, i, PHI_RESULT (nphi));
> > +
> > + tree tmp = make_ssa_name (TREE_TYPE (phi_lhs));
> > + tree stmt_lhs = PHI_RESULT (use_stmt);
> > + imm_use_iterator iter2;
> > + gimple *use_stmt2;
> > + use_operand_p use2_p;
> > +
> > + /* Now rewrite all the usages first. */
> > + FOR_EACH_IMM_USE_STMT (use_stmt2, iter2,
> stmt_lhs)
> > + FOR_EACH_IMM_USE_ON_STMT (use2_p, iter2)
> > + SET_USE (use2_p, tmp);
> > +
> > + /* And then generate the adjustment to avoid the
> > + update code from updating this new usage. The
> > + multiplicaiton is to get the original IV and the
> > + downwards counting IV correct. */
>
> typo: multiplication
>
> But I don't think it's just upcounting vs downcounting. An upcounting iv
> with step 2 would also need the multiplication. That is, we're applying
> PHI_RESULT (rphi) iv updates, and so need to add the iv step that many times.
>
> So IMO it would be clearer to drop the reference specifically to downcounting
> here.
>
> The patch LGTM with the comment nit fixed, but Richi should have the
> final say.
>
> Thanks,
> Richard
>
> > + gimple_seq iv_stmts = NULL;
> > + tree rphi_step
> > + = gimple_convert (&iv_stmts, ty_skip_niters,
> > + step_expr);
> > + tree tmp2
> > + = gimple_build (&iv_stmts, MULT_EXPR,
> > + ty_skip_niters, rphi_step,
> > + PHI_RESULT (rphi));
> > +
> > + if (POINTER_TYPE_P (TREE_TYPE (stmt_lhs)))
> > + tmp2
> > + = gimple_build (&iv_stmts, POINTER_PLUS_EXPR,
> > + TREE_TYPE (stmt_lhs), stmt_lhs,
> > + tmp2);
> > + else
> > + {
> > + tmp2
> > + = gimple_convert (&iv_stmts,
> > + TREE_TYPE (stmt_lhs),
> > + tmp2);
> > + tmp2
> > + = gimple_build (&iv_stmts, PLUS_EXPR,
> > + TREE_TYPE (stmt_lhs),
> > stmt_lhs,
> > + tmp2);
> > + }
> > +
> > + gsi_insert_seq_before (&gsi, iv_stmts,
> > + GSI_SAME_STMT);
> > + gimple *cvt_stmt =
> > + gimple_build_assign (tmp, VIEW_CONVERT_EXPR,
> > + build1 (VIEW_CONVERT_EXPR,
> > + TREE_TYPE (phi_lhs),
> > + tmp2));
> > + gsi_insert_before (&gsi, cvt_stmt, GSI_SAME_STMT);
> > + }
> > + /* All early exits point to the same common block, so we
> > + only have to find the first one. */
> > + break;
> > + }
> > + }
> > + }
> > }
> > tree step_mul = NULL_TREE;
> > unsigned ivn;
