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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/99101] optimization bug with -ffinite-loops
Date: Wed, 03 Mar 2021 14:37:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99101

--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
Just for the record we had the idea to apply the "bolt" of marking the latch
control dependence (as done for possibly infinite loops) for loops containi=
ng
stmts with side-effects.

diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c
index c027230acdc..c07b60bf25c 100644
--- a/gcc/tree-ssa-dce.c
+++ b/gcc/tree-ssa-dce.c
@@ -695,6 +695,12 @@ propagate_necessity (bool aggressive)
          if (bb !=3D ENTRY_BLOCK_PTR_FOR_FN (cfun)
              && !bitmap_bit_p (visited_control_parents, bb->index))
            mark_control_dependent_edges_necessary (bb, false);
+         /* If the stmt has side-effects the number of invocations matter.
+            In this case mark the containing loop control.  */
+         if (gimple_has_side_effects (stmt)
+             && bb->loop_father->num !=3D 0)
+           mark_control_dependent_edges_necessary (bb->loop_father->latch,
+                                                   false);
        }

       if (gimple_code (stmt) =3D=3D GIMPLE_PHI

But while that works for CDDCE1, CDDCE2 is presented a slightly altered CFG
that somehow prevents it from working.  Which also means that both loops
need to be considered infinite for the present bolting to work.=