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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, 24 Feb 2021 14:20:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99101

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
Another fix for the finite-loop issue would be to avoid considering loops n=
ot
post-dominated by the exit block as finite.  Like with the imperfect

diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
index 3817ec423e7..388c03ee6fc 100644
--- a/gcc/tree-ssa-loop-niter.c
+++ b/gcc/tree-ssa-loop-niter.c
@@ -2865,6 +2865,16 @@ finite_loop_p (class loop *loop)
       FOR_EACH_VEC_ELT (exits, i, ex)
        if (!(ex->flags & (EDGE_EH | EDGE_ABNORMAL | EDGE_FAKE)))
          {
+           /* But do not count noreturn regions as exit.
+              ???  This is a to simple check, we'd have to either
+              use post-dominance by the exit block (which isn't
+              computed) or a DFS walk.  See PR99101 where the
+              the fact that control dependence (and post-dominance)
+              is not well-defined for not backward reachable regions
+              causes issues otherwise.  */
+           if (ex->dest->index !=3D EXIT_BLOCK
+               && EDGE_COUNT (ex->dest->succs) =3D=3D 0)
+             continue;
            if (dump_file)
              fprintf (dump_file, "Assume loop %i to be finite: it has an e=
xit
"
                       "and -ffinite-loops is on.\n", loop->num);

but that still leaves post-dominance and thus control dependence not
well-defined in not backwards reachable regions and thus possibly "bogus"
control-dependent DCE decisions in those.  Note even with the above
CD-DCE will then use control-dependence to make the loop control necessary:

      FOR_EACH_LOOP (loop, 0)
        if (!finite_loop_p (loop))
          {
            if (dump_file)
              fprintf (dump_file, "cannot prove finiteness of loop %i\n",
loop->num);
            mark_control_dependent_edges_necessary (loop->latch, false);
          }

Which means to be absolutely sure we could use non-CD DCE whenever there
are not backwards reachable blocks in the function.=