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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/102155] New: LIM fill_always_executed_in
 handles contains_call incorrectly
Date: Wed, 01 Sep 2021 07:09:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102155

            Bug ID: 102155
           Summary: LIM fill_always_executed_in handles contains_call
                    incorrectly
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

fill_always_executed_in walks loop bodies in dominator order
(get_loop_body_in_dom_order) but that does not run into BB3 before marking =
BB4
as always executed:

  do
    {
      BB1:
      if (x)
        BB2:
        if (y)
          BB3:
          bar();
      BB4;
    }
  while (++i < n);


the dominator children of BB1 are BB2 and BB4, BB3 with the call to bar()
is only a dominator child of BB2.

Since LIM gives up on "unanalyzable" refs like calls a testcase that is
miscompiled is a bit difficult to construct - it requires a 'pure' (or cons=
t)
call but whether it's valid for those to exit the program could be subject
to discussion.  The following uses a looping const function to show the
effect and the testcase segfaults with -O1+ while it runs into the infinite
loop at -O0.  Note we'd technically need to set looping_const but there's
now way to do this and with just -fno-inline and letting IPA figure out
looping-const dominator children are ordered in a "lucky" way for
fill_sons_in_loop to compute an order that happens to work.

extern void abort (void);
int flag[32];
int __attribute__((noinline,const)) bar (int i) { if (i) return i; while (1=
); }
int __attribute__((noipa)) foo (int *p, int n)
{
  int i =3D 0, res =3D 0;
  do
    {
      if (!flag[i])
        {
          if (i % 3 =3D=3D 0)
            res +=3D bar (i);
          else
            res++;
        }
      res +=3D *p;
    }
  while (++i < n);
  return res;
}

int main()
{
  if (foo ((int *)0, 5) !=3D 0)
    abort ();
  return 0;
}=