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Subject: [Bug bootstrap/102681] [12 Regression] AArch64 bootstrap failure
Date: Fri, 22 Oct 2021 07:30:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102681

--- Comment #19 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #16)
> (In reply to Andrew Pinski from comment #15)
> > We totally missed the jump threading of 3->5->7 path and the 4->5->8 pa=
th.
>=20
>   FAIL: path through PHI in bb8 (incoming bb:6) crosses loop
>=20
> But but, it does not exactly cross the loop as 5 (6) is not part of the l=
oop
> but rather just 8.

Interesting.  The restriction that tickles this is old legacy code in place
from way before I touched any of 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.
      bool profitable_p =3D m_path[0]->loop_father =3D=3D e->src->loop_fath=
er;
      if (!profitable_p && 0)
        {
          if (dump_file && (dump_flags & TDF_DETAILS))
            fprintf (dump_file,
                     "  FAIL: path through PHI in bb%d (incoming bb:%d) cro=
sses
loop\n",
                     e->dest->index, e->src->index);
          continue;
        }

I even annotated it because it seemed strange that it was more restrictive =
than
the generic restrictions in the backward threader.

It is very possible that we can remove this, as we have much more thorough =
loop
restrictions in place in the shared registry.

If you remove the above chunk, does it work?  If so, I may have to test and
benchmark the change.=