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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103388] [12 Regression] missed optimization
 for dead code elimination at -O3 (vs. -O2)
Date: Tue, 23 Nov 2021 19:31:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103388

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org
         Resolution|DUPLICATE                   |---
             Status|RESOLVED                    |NEW
   Last reconfirmed|                            |2021-11-23
     Ever confirmed|0                           |1

--- Comment #2 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Aldy Hernandez from comment #1)
> By *.threadfull1 this is the path at 4->5->7.  It looks like:
>=20
> PREHEADER
> |
> v
> HEADER--------+
> |             |
> V
> UNREACHABLE   |
> |            /
> V           /
> return 0 <-+
>=20
> This is more or less PR102981.
>=20
> Is there any way we can stop reporting the same thing over and over?
>=20
> *** This bug has been marked as a duplicate of bug 102981 ***

Errr, wait a minute, that's not a loop.  My bad.

We're failing to thread 4->5->xxx because:

Checking profitability of path (backwards):  bb:5 (10 insns) bb:4
  Control statement insns: 2
  Overall: 8 insns
  FAIL: Did not thread around loop and would copy too many statements.

which is a limitation of the backward threader copier:

  /* The generic copier used by the backthreader does not re-use an
     existing threading path to reduce code duplication.  So for that
     case, drastically reduce the number of statements we are allowed
     to copy.  */
  if (!(threaded_through_latch && threaded_multiway_branch)
      && (n_insns * param_fsm_scale_path_stmts
          >=3D param_max_jump_thread_duplication_stmts))
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file,
                 "  FAIL: Did not thread around loop and would copy too "
                 "many statements.\n");
      return false;
    }

Confirmed.=