[Bug tree-optimization/125396] [16 Regression] ICE at -O1/2/3 during RTL pass: expand since r16-5258

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125396

--- Comment #10 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-16 branch has been updated by Andrea Pinski
<[email protected]>:

https://gcc.gnu.org/g:6933427282a2f1a22d0e967e16f41b419f83c11e

commit r16-9412-g6933427282a2f1a22d0e967e16f41b419f83c11e
Author: Andrew Pinski <[email protected]>
Date:   Wed May 20 13:20:15 2026 -0700

    tree-cfgcleanup: Don't remove forwarder blocks (with phis) with phis that
have abnormal uses [PR125396]

    This was a latent bug in the checks for removing of a forwarder block which
has a phi.

    Take:
          <bb 15> [local count: 182536112]:
          # arr1$0_32 = PHI <arr1$0_36(ab)(11)>

          <bb 14> [local count: 206998870]:
          # arr1$0_43(ab) = PHI <arr1$0_25(ab)(13), 9(15)>
          # sj12_44(ab) = PHI <sj12_31(ab)(13), arr1$0_32(15)>
          f8 ();

    Here bb 15 predecessor is a normal edge.
    So when merging the forwarder bb 15 into bb14 we end up with:
          <bb 12> [local count: 206998870]:
          # arr1$0_42(ab) = PHI <arr1$0_24(ab)(11), 9(9)>
          # sj12_43(ab) = PHI <sj12_30(ab)(11), arr1$0_35(ab)(9)>
          f8 ();
    and now there is an overlap of live range of arr1$0_35 and arr1$0_42.
    So we need to reject the case where we have phis and the phi arguments that
    use abnormal uses.

    Changes since v1:
    * v2: Look at phi arguments of the forwarder block rather than the dest bb
          having an abnormal edge out.
    * v3: Fix bb_phis_references_abnormal_uses to use the gimple_phi_num_args
to
          search over the phi arguments. Also fix the commit message which was
wrong.

    Bootstrapped and tested on x86_64-linux-gnu.

            PR tree-optimization/125396

    gcc/ChangeLog:

            * tree-cfgcleanup.cc (bb_phis_references_abnormal_uses): New
function.
            (maybe_remove_forwarder_block): Check to make sure the
            forwarder block does not have a phi that references ssa name that
has
            abnormal uses.

    gcc/testsuite/ChangeLog:

            * gcc.dg/torture/pr125396-1.c: New test.

    Signed-off-by: Andrew Pinski <[email protected]>
    (cherry picked from commit ec79b0779b6eeb9ef3b8a45f46a596cbb75a7844)