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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/105219] [12 Regression] SVE: Wrong code with -O3
 -msve-vector-bits=128 -mtune=thunderx
Date: Wed, 13 Apr 2022 13:48:57 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105219

--- Comment #10 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
nb_iterations_upper_bound is indeed set incorrectly and tracked to this com=
mit,

commit 7ed1cd9665d8ca0fa07b2483e604c25e704584af
Author: Andre Vieira <andre.simoesdiasvieira@arm.com>
Date:   Thu Jun 3 13:55:24 2021 +0100

    vect: Use main loop's thresholds and VF to narrow upper_bound of epilog=
ue

    This patch uses the knowledge of the conditions to enter an epilogue lo=
op
to
    help come up with a potentially more restricive upper bound.

    gcc/ChangeLog:

            * tree-vect-loop.c (vect_transform_loop): Use main loop's vario=
us'
            thresholds to narrow the upper bound on epilogue iterations.

    gcc/testsuite/ChangeLog:

            * gcc.target/aarch64/sve/part_vect_single_iter_epilog.c: New te=
st.

I don't quite understand when comparing the two bounds there's a -1 in the =
min
comparison.

And indeed:

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 5c7b163f01c..19235ea79fe 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -9971,7 +9971,7 @@ vect_transform_loop (loop_vec_info loop_vinfo, gimple
*loop_vectorized_call)
                                        LOOP_VINFO_VECT_FACTOR (loop_vinfo),
                                        &bound))
            loop->nb_iterations_upper_bound
-             =3D wi::umin ((widest_int) (bound - 1),
+             =3D wi::umin ((widest_int)bound,
                          loop->nb_iterations_upper_bound);
       }
   }

fixes it. It looks to me that when comparing the bounds of the main loop and
epilogue you shouldn't subtract 1 again. But need to ask why this is there.=