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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103690] [12 Regression] ICE: in build2, at
 tree.c:4985 with -g -O2 -fno-tree-dce -fno-tree-dse -fno-tree-fre
 --param=max-jump-thread-duplication-stmts=94 since r12-2591-g2e96b5f14e402569
Date: Tue, 04 Jan 2022 10:21:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103690

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P1
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot =
gnu.org

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Value numbering stmt =3D __trans_tmp_1_14 =3D _24 + 4;
Setting value number of __trans_tmp_1_14 to __trans_tmp_1_22 (changed)

what we do is avoiding useless work during elimination, not visiting if (0)
guarded code - those regions will have not up-to-date SSA form.

Block 14: BB5 found not executable

the issue is that PRE does

  /* Because we don't follow exactly the standard PRE algorithm, and decide=
 not
     to insert PHI nodes sometimes, and because value numbering of casts is=
n't
     perfect, we sometimes end up inserting dead code.   This simple DCE-li=
ke
     pass removes any insertions we made that weren't actually used.  */
  simple_dce_from_worklist (inserted_exprs);

which eventually runs into those uses for expressions we inserted into those
unreachable blocks.

The issue is long latent I think.=