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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113441] [14 Regression] Fail to fold the last
 element with multiple loop
Date: Tue, 23 Jan 2024 06:42:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113441

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Tamar Christina from comment #9)
> There is a weird costing going on in the PHI nodes though:
>=20
> m_108 =3D PHI <m_92(16), m_111(5)> 1 times vector_stmt costs 0 in body=20
> m_108 =3D PHI <m_92(16), m_111(5)> 2 times scalar_to_vec costs 0 in prolo=
gue
>=20
> they have collapsed to 0. which can't be right..

Note this is likely because of the backend going wrong.

bool
vectorizable_phi (vec_info *,
                  stmt_vec_info stmt_info, gimple **vec_stmt,
                  slp_tree slp_node, stmt_vector_for_cost *cost_vec)
{
..

      /* For single-argument PHIs assume coalescing which means zero cost
         for the scalar and the vector PHIs.  This avoids artificially
         favoring the vector path (but may pessimize it in some cases).  */
      if (gimple_phi_num_args (as_a <gphi *> (stmt_info->stmt)) > 1)
        record_stmt_cost (cost_vec, SLP_TREE_NUMBER_OF_VEC_STMTS (slp_node),
                          vector_stmt, stmt_info, vectype, 0, vect_body);

You could check if we call this with sane values.=