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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: Wed, 17 Nov 2021 09:49:57 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102981

--- Comment #6 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
This looks like a class of problems we could easily get if we wanted.  The
pattern is:

PREHEADER
    |
    |
    V
  HEADER --> LOOPEXIT
    |
    |
    V
   SUCC
    |  \
    |   \
   DEAD  \
     |   /
     |  /
     | v
   XXXXXX

On the PREHEADER->HEADER->SUCC path we want to know if the edge out of SUCC=
 can
be statically determined.  The threader can't do this for a number of reaso=
ns.=20
First, we'd be essentially peeling an iteration.  Second, IIUC, we'd be
rotating the loop.

However, there's no reason we can't catch this in a loop optimizer like we =
did
with the loopch pass.  This is the exact type of problem that is trivially
handled by the path solver, which is quite cheap when you don't have to do =
full
path discovery like the threader has to do.

Something like:

gimple *control =3D gimple_outgoing_range_stmt_p (succ);
if (control) {
  auto_vec<basic_block> bbs (3);
  bbs.quick_push (preheader);
  bbs.quick_push (header);
  bbs.quick_push (succ);

  int_range<2> r;
  path_range_query query;
  query.compute_ranges (bbs);
  query.range_of_stmt (r, control);
  if (r =3D=3D desired_static_value...)
    peel();
...
}

If "dead code on the first iteration" is something we want to handle, I cou=
ld
help with the ranger bits if someone gives me a hand with the loop bits.=