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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102981] [12 Regression] Dead Code Elimination
 Regression at -O3 (trunk vs 11.2.0)
Date: Sat, 30 Oct 2021 07:03:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102981

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-10-30
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
In the pre-loop threaders (ethread, thread1, threadfull1), we can't touch
anything because it would cross loops, but by threadfull2 we should be able=
 to.

There's a threadable path starting at the 2->6 edge here:

<bb 6> [local count: 118111600]:
  # c_21 =3D PHI <c_30(5), 0(2)>
  # ivtmp.16_8 =3D PHI <ivtmp.16_13(5), ivtmp.16_25(2)>
  a.1_26 =3D a;
  if (a.1_26 < 0)
    goto <bb 12>; [89.00%]
  else
    goto <bb 10>; [11.00%]

but we don't because doing so would peel off an iteration.   Hmmm, this is
really old code.  I'm going to have to think about this:

      // This is like path_crosses_loops in profitable_path_p but more
      // restrictive, since profitable_path_p allows threading the
      // first block because it would be redirected anyhow.
      //
      // If we loosened the restriction and used profitable_path_p()
      // here instead, we would peel off the first iterations of loops
      // in places like tree-ssa/pr14341.c.=