[gcc r16-5849] vect: delay identification of update_e until after skip_epilog [PR122959]

[Tamar Christina]

https://gcc.gnu.org/g:4a1247a3b070cdb02910826a5effb505457a6e73

commit r16-5849-g4a1247a3b070cdb02910826a5effb505457a6e73
Author: Tamar Christina <tamar.christina@arm.com>
Date:   Wed Dec 3 10:55:40 2025 +0000

    vect: delay identification of update_e until after skip_epilog [PR122959]
    
    The testcase attached in the PR shows that for some reason
    the test openmp_vv.sum when doing OpenMP offloading creates
    an intermediate empty block after the skip_epilog split.
    
    This means we should just simply delay setting update_e for
    the non-early break case.  For early break we have to do it
    early still otherwise the skip_epilog edge would make us find
    the wrong edge.
    
    I haven't been able to replicate this on a C testcase and the
    attached reduction works fine on AArch64 and x86_64 but have
    been able to verify the fixed code with
    
    ./gcc/f951 -fopenmp test2.f90 -O3 -o - -march=sm_30 on an
    
     --target=nvptx-none --enable-as-accelerator-for=x86_64-pc-linux-gnu
    
    configured cc1.
    
    If I manage to create a testcase will push it too.
    
    gcc/ChangeLog:
    
            PR middle-end/122959
            * tree-vect-loop-manip.cc (vect_do_peeling): Delay setting update_e.

Diff:
---
 gcc/tree-vect-loop-manip.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 43847c4c3fbd..18af6a1811ac 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -3580,7 +3580,6 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree niters, tree nitersm1,
       /* Update IVs of original loop as if they were advanced by
 	 niters_vector_mult_vf steps.  */
       gcc_checking_assert (vect_can_advance_ivs_p (loop_vinfo));
-      update_e = skip_vector ? e : loop_preheader_edge (epilog);
       if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
 	update_e = single_succ_edge (LOOP_VINFO_IV_EXIT (loop_vinfo)->dest);
 
@@ -3639,6 +3638,11 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree niters, tree nitersm1,
 	  scale_loop_profile (epilog, prob_epilog, -1);
 	}
 
+      /* Identify the right foward edge for the non-early-break case which must
+	 be done after splitting the epilog edge.  */
+      if (!LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
+	update_e = skip_vector ? e : loop_preheader_edge (epilog);
+
       /* 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.