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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/109720] -Wmaybe-uninitialized triggering when
 I can see no path that would allow it
Date: Thu, 04 May 2023 11:28:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109720

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
We complain about the m_num_bits read.  While PRE isolates it to BB3 even
before PRE that happens unconditionally:

  <bb 2> [local count: 118111600]:
  nbits =3D{v} {CLOBBER};
  _25 =3D MEM[(const struct DynamicBitSet &)&set]._size;
  nbits._size =3D _25;
  if (_25 <=3D 64)
    goto <bb 3>; [67.00%]
  else
    goto <bb 4>; [33.00%]

  <bb 3> [local count: 79134772]:
  _162 =3D MEM[(const struct DynamicBitSet &)&set].D.151332._fixedSizeBitSe=
t;
  nbits.D.151332._fixedSizeBitSet =3D _162;
  goto <bb 13>; [100.00%]

...
  <bb 12> [local count: 56691384]:
  _46 =3D (long unsigned int) _40;
  _47 =3D iftmp.2_49 + _46;
  MEM[(struct vector *)&nbits + 8B].D.150855._M_impl.D.150172._M_finish =3D=
 _47;
  _29 =3D MEM[(const struct dynamic_bitset &)&set + 8].m_num_bits;
  MEM[(struct dynamic_bitset *)&nbits + 8B].m_num_bits =3D _29;
  _60 =3D boost::dynamic_bitset<>::m_do_find_from
(&nbits.D.151332._dynamicBitSet, 0);
  if (_60 !=3D 18446744073709551615)
    goto <bb 13>; [66.92%]
  else
    goto <bb 33>; [33.08%]

  <bb 13> [local count: 103222856]:
  # _63 =3D PHI <_60(12), _25(3)>
  _68 =3D MEM[(const struct dynamic_bitset *)&nbits + 8B].m_num_bits;

PRE just sees the partial redundancy on the edge from BB12.

The read is from dynamic_bitset<Block, Allocator>::to_ulong().

It _looks_ like that maybe the range-for handling code exposes this in a way
that doesn't properly short-cut this read (or that GCC short-circuits a
test there).  Difficult to identify in the preprocessed sources.=